INTERNET Background

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Basic Description

A Network of Networks

The simplest way of explaining the Internet is to call it "the network of networks."It's the connection of computer networks around the world into one entity, so tospeak. It's not one big computer, but rather numerous networked computersconnected together.

When you dial into your Internet service provider (AOL, Earthlink, etc) fromhome, you are essentially connecting your computer to a network. If you are oncampus you connect to the Internet through your school's network, which isconnected to the larger Internet network through Peachnet, which is theelectronic highway for all educational institutions and libraries throughout thestate of Georgia. The "backbone" of all these connections is what you might hearreferred to as the "information superhighway."

A Brief History of the Internet

Sharing Resources

The Internet started in the 1960s as a way for government researchers to shareinformation. Computers in the '60s were large and immobile and in order to makeuse of information stored in any one computer, one had to either travel to the siteof the computer or have magnetic computer tapes sent through the conventionalpostal system.

Another catalyst in the formation of the Internet was the heating up of the ColdWar. The Soviet Union's launch of the Sputnik satellite spurred the U.S. DefenseDepartment to consider ways information could still be disseminated even after anuclear attack. This eventually led to the formation of the ARPANET (AdvancedResearch Projects Agency Network), the network that ultimately evolved intowhat we now know as the Internet. ARPANET was a great success butmembership was limited to certain academic and research organizations whohad contracts with the Defense Department. In response to this, other networkswere created to provide information sharing.

January 1, 1983 is considered the official birthday of the Internet. Prior to this, thevarious computer networks did not have a standard way to communicate witheach other. A new communications protocol was established called TransferControl Protocol/Inter network Protocol (TCP/IP). This allowed different kinds ofcomputers on different networks to "talk" to each other. ARPANET and theDefense Data Network officially changed to the TCP/IP standard on January 1,1983, hence the birth of the Internet. All networks could now be connected by auniversal language.

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Applications of the Internet

Internet can be used for the following purposes

Banking

Marketing

Searching

Gaming

Mailing

Chatting &

Data sharing

Types of Internet servicesBefore you connect your network to the Internet, you need to decide whichInternet services you want to provide to both local users and external users.

Many types of Internet services are available. The following are among the mostpopular and useful:

WWW overview

Internet e-mail services

Gopher

FTP Telnet

Before you access or provide any Internet service, you also need to understandthe associated security risks. Understanding your risks enables you to develop asecurity policy to protect your network.

WWW overview

The World Wide Web (WWW) is the fastest growing application of the Internet.The WWW is a system of distributed servers that handle hypermedia documents.

Hypermedia authors use Hyper Text Markup Language (HTML) to createhypermedia documents. HTML is a method of presenting information in whichselected words or phrases in the text are hyperlinks to other, related information.Linked information can be in the form of other documents, graphics, audio files,or video files.

Web browsers access and display HTML documents on the WWW. Webbrowsers are applications that are specifically designed to access and display

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HTML documents. Web servers use Hyper Text Transfer Protocol (HTTP) to sendHTML documents to client Web browsers for display.

Hyper Text Transfer Protocol (HTTP) is a communications protocol that allowsthe transfer of Hyper Text Markup Language (HTML) documents from Web

servers to Web browsers. HTML is a tag language for creating documents thatcontain links to related information. You can click on a link to access otherdocuments, images, or multimedia objects and obtain additional informationabout the linked item.

Both the client and the server must support HTTP in order to send and receiveHTML documents and interact on the World Wide Web.

Internet e-mail services

Electronic mail (e-mail) is one of the most commonly used Internet services. It

allows users of the Internet to exchange electronic messages. Most electronicmail programs generate mail messages in a standard format. This allows userson one type of platform to send mail to someone across the Internet who may beon another type of platform.

Some of the newer mail applications allow you to include attachments with themail message. These attachments can be things such as PostScript documents,video clips, programs, and other files. An electronic mail program that can handlesuch attachments is called a MIME-enabled mailer.

To send and receive electronic mail across the Internet, your network must have

mail servers configured for this purpose. Usually, you must install and configure aSimple Mail Transfer Protocol (SMTP) server to handle mail between yournetwork and the Internet. To make e-mail easier for your users, you may alsowant to install and configure a post office protocol (POP) post server. POPservers store mail in individual accounts for each user to retrieve.

Simple Mail Transfer Protocol (SMTP)

Simple Mail Transfer Protocol (SMTP) is an electronic mail protocol with bothclient (sender) and server (receiver) functions. SMTP is the underlyingtransmission mechanism that most systems use to send mail between host

servers on the Internet. SMTP allows the transfer of electronic mail from onecomputer mail server to another. An SMTP server accepts mail from local usersto transmit to servers outside the network.

Most systems route e-mail locally through a local mail system server. A PostOffice Protocol (POP) server is one of the more popular choices. Lotus Notes isanother popular mail application for local storage and retrieval of incoming mail.

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These and other local mail system servers sort incoming mail and store it for theindividual users to whom it is addressed.

Post Office Protocol (POP)

Post Office Protocol (POP) is an electronic mail protocol with both client (senderand receiver) and server (storage) functions. POP allows storage in a centrallocation of mail for multiple users until a user's electronic mail program makes arequest for delivery.

An electronic mail server on the network accepts the incoming mail for a site.This is often a Simple Mail Transfer Protocol (SMTP) server. The SMTP serverthen forwards the mail to a POP server, which sorts the mail and stores it for theindividual users to whom it is addressed.

Gopher

A system that pre-dates the World Wide Web for organizing and displaying fileson Internet servers. A Gopher server presents its contents as a hierarchicallystructured list of files. With the ascendance of the Web, many gopher databaseswere converted to Web sites which can be more easily accessed via Web searchengines.

Gopher was developed at the University of Minnesota and named after theschool's mascot. Two systems, Veronica and Jughead, let you search globalindices of resources stored in Gopher systems.

FTP

You can set up your AS/400 to send, receive, and share files across networks byusing file transfer protocol (FTP). Before you set up your AS/400 to transfer files,you must have TCP/IP started and configured on your system.

These topics provide further details about FTP for your AS/400:

Print this topic. Download and print all the FTP information.

FTP functions with Operations Navigator: Instructions on how to access FTP

through Operations Navigator.

FTP administration: This section includes instructions on how to set up yourAS/400 to send, receive, and share files across networks by using file transferprotocol (FTP) with Operations Navigator, monitor FTP users, log FTP servererrors, and set up FTP servers for a graphical interface.

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File transfer with FTP: This section includes instructions on how to transfer filesbetween a remote computer and your AS/400.

File systems and naming conventions FTP supports: Each AS/400 file systemhas its own set of rules for naming files. The format used to name any file must

adhere to the naming conventions of the file system in which it resides.

FTP client subcommands: You can establish a connection with a remote FTPserver, navigate libraries and directories, and create, delete, and transfer fileswith these subcommands.

FTP server subcommands: Information about FTP server subcommands, theirsyntax, and their parameters.

FTP security controls: Information on how you can use FTP exit programs toincrease or change FTP access to your system.

Anonymous FTP: Anonymous FTP logon support enables remote users to log onto your FTP server and download files even though they do not have a useridand password.

Tip: A number of FTP procedures are available in the online help from AS/400Operations Navigator.

Telnet

Telnet allows you to log on to a remote computer and use it as though you are

connected directly to it within the local network. The machine, or system, that youare physically in front of is the client. The Telnet server is the remote computer towhich the client is attached. AS/400 TCP/IP supports both the Telnet client andserver.

One of the most important functions of Telnet is its ability to negotiate optionsbetween the client and the server. This type of open negotiation makes itpossible for either the client or the server to initiate or to honor a request.

In addition to supporting virtual display sessions, the AS/400 Telnet server alsosupports a virtual printer client.

Several different emulation types are available to you for negotiating requestsand converting them to output. For AS/400, the preferred type is 5250 emulation.The Telnet client/server information focuses solely on tasks related to the 5250full-screen mode.

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Introduction to TCP/IP

Summary: TCP and IP were developed by a Department of Defense (DOD)research project to connect a number different networks designed by differentvendors into a network of networks (the "Internet"). It was initially successful

because it delivered a few basic services that everyone needs (file transfer,electronic mail, remote logon) across a very large number of client and serversystems. Several computers in a small department can use TCP/IP (along withother protocols) on a single LAN. The IP component provides routing from thedepartment to the enterprise network, then to regional networks, and finally to theglobal Internet. On the battlefield a communications network will sustain damage,so the DOD designed TCP/IP to be robust and automatically recover from anynode or phone line failure. This design allows the construction of very largenetworks with less central management. However, because of the automaticrecovery, network problems can go undiagnosed and uncorrected for longperiods of time.

As with all other communications protocol, TCP/IP is composed of layers:

IP - is responsible for moving packet of data from node to node. IP forwards each

packet based on a four byte destination address (the IP number). The Internet

authorities assign ranges of numbers to different organizations. The organizationsassign groups of their numbers to departments. IP operates on gateway machines

that move data from department to organization to region and then around the

world.

TCP - is responsible for verifying the correct delivery of data from client to

server. Data can be lost in the intermediate network. TCP adds support to detect

errors or lost data and to trigger retransmission until the data is correctly andcompletely received.

Sockets - is a name given to the package of subroutines that provide access to

TCP/IP on most systems.

Network of Lowest Bidders

The Army puts out a bid on a computer and DEC wins the bid. The Air Force putsout a bid and IBM wins. The Navy bid is won by Unisys. Then the Presidentdecides to invade Grenada and the armed forces discover that their computerscannot talk to each other. The DOD must build a "network" out of systems each

of which, by law, was delivered by the lowest bidder on a single contract.

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The Internet Protocol was developed to create a Network of Networks (the"Internet"). Individual machines are first connected to a LAN (Ethernet or Token

Ring). TCP/IP shares the LAN with other uses (a Novell file server, Windows forWorkgroups peer systems). One device provides the TCP/IP connection betweenthe LAN and the rest of the world.

To insure that all types of systems from all vendors can communicate, TCP/IP isabsolutely standardized on the LAN. However, larger networks based on longdistances and phone lines are more volatile. In the US, many large corporationswould wish to reuse large internal networks based on IBM's SNA. In Europe, thenational phone companies traditionally standardize on X.25. However, thesudden explosion of high speed microprocessors, fiber optics, and digital phonesystems has created a burst of new options: ISDN, frame relay, FDDI,

Asynchronous Transfer Mode (ATM). New technologies arise and becomeobsolete within a few years. With cable TV and phone companies competing tobuild the National Information Superhighway, no single standard can governcitywide, nationwide, or worldwide communications.

The original design of TCP/IP as a Network of Networks fits nicely within thecurrent technological uncertainty. TCP/IP data can be sent across a LAN, or itcan be carried within an internal corporate SNA network, or it can piggyback onthe cable TV service. Furthermore, machines connected to any of these networkscan communicate to any other network through gateways supplied by thenetwork vendor.

Addresses

Each technology has its own convention for transmitting messages between twomachines within the same network. On a LAN, messages are sent betweenmachines by supplying the six byte unique identifier (the "MAC" address). In anSNA network, every machine has Logical Units with their own network address.

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DECNET, Apple talk, and Novell IPX all have a scheme for assigning numbers toeach local network and to each workstation attached to the network.

On top of these local or vendor specific network addresses, TCP/IP assigns aunique number to every workstation in the world. This "IP number" is a four byte

value that, by convention, is expressed by converting each byte into a decimalnumber (0 to 255) and separating the bytes with a period. For example, the PCLube and Tune server is 130.132.59.234.

An organization begins by sending electronic mail [email protected] requesting assignment of a network number. It isstill possible for almost anyone to get assignment of a number for a small "ClassC" network in which the first three bytes identify the network and the last byteidentifies the individual computer. The author followed this procedure and wasassigned the numbers 192.35.91.* for a network of computers at his house.Larger organizations can get a "Class B" network where the first two bytes

identify the network and the last two bytes identify each of up to 64 thousandindividual workstations. Yale's Class B network is 130.132, so all computers withIP address 130.132.*.* are connected through Yale.

The organization then connects to the Internet through one of a dozen regional orspecialized network suppliers. The network vendor is given the subscribernetwork number and adds it to the routing configuration in its own machines andthose of the other major network suppliers.

There is no mathematical formula that translates the numbers 192.35.91 or130.132 into "Yale University" or "New Haven, CT." The machines that manage

large regional networks or the central Internet routers managed by the NationalScience Foundation can only locate these networks by looking each networknumber up in a table. There are potentially thousands of Class B networks, andmillions of Class C networks, but computer memory costs are low, so the tablesare reasonable. Customers that connect to the Internet, even customers as largeas IBM, do not need to maintain any information on other networks. They send allexternal data to the regional carrier to which they subscribe, and the regionalcarrier maintains the tables and does the appropriate routing.

New Haven is in a border state, split 50-50 between the Yankees and the RedSox. In this spirit, Yale recently switched its connection from the Middle Atlantic

regional network to the New England carrier. When the switch occurred, tables inthe other regional areas and in the national spine had to be updated, so thattraffic for 130.132 was routed through Boston instead of New Jersey. The largenetwork carriers handle the paperwork and can perform such a switch givensufficient notice. During a conversion period, the university was connected toboth networks so that messages could arrive through either path.

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Subnets

Although the individual subscribers do not need to tabulate network numbers orprovide explicit routing, it is convenient for most Class B networks to be internallymanaged as a much smaller and simpler version of the larger network

organizations. It is common to subdivide the two bytes available for internalassignment into a one byte department number and a one byte workstation ID.

The enterprise network is built using commercially available TCP/IP router boxes.Each router has small tables with 255 entries to translate the one bytedepartment number into selection of a destination Ethernet connected to one of

the routers. Messages to the PC Lube and Tune server (130.132.59.234) aresent through the national and New England regional networks based on the130.132 part of the number. Arriving at Yale, the 59 department ID selects anEthernet connector in the C& IS building. The 234 selects a particular workstationon that LAN. The Yale network must be updated as new Ethernets anddepartments are added, but it is not effected by changes outside the university orthe movement of machines within the department.

A Uncertain Path

Every time a message arrives at an IP router, it makes an individual decision

about where to send it next. There is concept of a session with a pre selectedpath for all traffic. Consider a company with facilities in New York, Los Angeles,Chicago and Atlanta. It could build a network from four phone lines forming aloop (NY to Chicago to LA to Atlanta to NY). A message arriving at the NY routercould go to LA via either Chicago or Atlanta. The reply could come back the otherway.

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How does the router make a decision between routes? There is no correctanswer. Traffic could be routed by the "clockwise" algorithm (go NY to Atlanta, LAto Chicago). The routers could alternate, sending one message to Atlanta andthe next to Chicago. More sophisticated routing measures traffic patterns andsends data through the least busy link.

If one phone line in this network breaks down, traffic can still reach its destinationthrough a roundabout path. After losing the NY to Chicago line, data can be sentNY to Atlanta to LA to Chicago. This provides continued service though withdegraded performance. This kind of recovery is the primary design feature of IP.The loss of the line is immediately detected by the routers in NY and Chicago,but somehow this information must be sent to the other nodes. Otherwise, LAcould continue to send NY messages through Chicago, where they arrive at a"dead end." Each network adopts some Router Protocol which periodicallyupdates the routing tables throughout the network with information aboutchanges in route status.

If the size of the network grows, then the complexity of the routing updates willincrease as will the cost of transmitting them. Building a single network thatcovers the entire US would be unreasonably complicated. Fortunately, theInternet is designed as a Network of Networks. This means that loops andredundancy are built into each regional carrier. The regional network handles itsown problems and reroutes messages internally. Its Router Protocol updates thetables in its own routers, but no routing updates need to propagate from aregional carrier to the NSF spine or to the other regions (unless, of course, asubscriber switches permanently from one region to another).

Undiagnosed Problems

IBM designs its SNA networks to be centrally managed. If any error occurs, it isreported to the network authorities. By design, any error is a problem that shouldbe corrected or repaired. IP networks, however, were designed to be robust. Inbattlefield conditions, the loss of a node or line is a normal circumstance.Casualties can be sorted out later on, but the network must stay up. So IPnetworks are robust. They automatically (and silently) reconfigure themselveswhen something goes wrong. If there is enough redundancy built into the system,then communication is maintained.

In 1975 when SNA was designed, such redundancy would be prohibitivelyexpensive, or it might have been argued that only the Defense Department couldafford it. Today, however, simple routers cost no more than a PC. However, theTCP/IP design that, "Errors are normal and can be largely ignored," producesproblems of its own.

Data traffic is frequently organized around "hubs," much like airline traffic. Onecould imagine an IP router in Atlanta routing messages for smaller cities

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throughout the Southeast. The problem is that data arrives without a reservation.Airline companies experience the problem around major events, like the SuperBowl. Just before the game, everyone wants to fly into the city. After the game,everyone wants to fly out. Imbalance occurs on the network when something newgets advertised. Adam Curry announced the server at "mtv.com" and his regional

carrier was swamped with traffic the next day. The problem is that messagescome in from the entire world over high speed lines, but they go out to mtv.comover what was then a slow speed phone line.

Occasionally a snow storm cancels flights and airports fill up with strandedpassengers. Many go off to hotels in town. When data arrives at a congestedrouter, there is no place to send the overflow. Excess packets are simplydiscarded. It becomes the responsibility of the sender to retry the data a fewseconds later and to persist until it finally gets through. This recovery is providedby the TCP component of the Internet protocol.

TCP was designed to recover from node or line failures where the networkpropagates routing table changes to all router nodes. Since the update takessome time, TCP is slow to initiate recovery. The TCP algorithms are not tuned tooptimally handle packet loss due to traffic congestion. Instead, the traditionalInternet response to traffic problems has been to increase the speed of lines andequipment in order to say ahead of growth in demand.

TCP treats the data as a stream of bytes. It logically assigns a sequence numberto each byte. The TCP packet has a header that says, in effect, "This packetstarts with byte 379642 and contains 200 bytes of data." The receiver can detectmissing or incorrectly sequenced packets. TCP acknowledges data that has been

received and retransmits data that has been lost. The TCP design means thaterror recovery is done end-to-end between the Client and Server machine. Thereis no formal standard for tracking problems in the middle of the network, thougheach network has adopted some ad hoc tools.

Need to Know

There are three levels of TCP/IP knowledge. Those who administer a regional ornational network must design a system of long distance phone lines, dedicatedrouting devices, and very large configuration files. They must know the IPnumbers and physical locations of thousands of subscriber networks. They must

also have a formal network monitor strategy to detect problems and respondquickly.

Each large company or university that subscribes to the Internet must have anintermediate level of network organization and expertise. A half dozen routersmight be configured to connect several dozen departmental LANs in severalbuildings. All traffic outside the organization would typically be routed to a singleconnection to a regional network provider.

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However, the end user can install TCP/IP on a personal computer without anyknowledge of either the corporate or regional network. Three pieces ofinformation are required:

1. The IP address assigned to this personal computer

2. The part of the IP address (the subnet mask) that distinguishes other machines onthe same LAN (messages can be sent to them directly) from machines in other

departments or elsewhere in the world (which are sent to a router machine)3. The IP address of the router machine that connects this LAN to the rest of the

world.

In the case of the PCLT server, the IP address is 130.132.59.234. Since the firstthree bytes designate this department, a "subnet mask" is defined as255.255.255.0 (255 is the largest byte value and represents the number with allbits turned on). It is a Yale convention (which we recommend to everyone) thatthe router for each department have station number 1 within the department

network. Thus the PCLT router is 130.132.59.1. Thus the PCLT server isconfigured with the values:

My IP address: 130.132.59.234

Subnet mask: 255.255.255.0

Default router: 130.132.59.1

The subnet mask tells the server that any other machine with an IP addressbeginning 130.132.59.* is on the same department LAN, so messages are sentto it directly. Any IP address beginning with a different value is accessedindirectly by sending the message through the router at 130.132.59.1 (which is

on the departmental LAN).

Point To Point Protocol

PPP was designed somewhat after the original HDLC specifications. Thedesigners of PPP included many additional features that had been seen only invarious proprietary data-link protocols up to that time.

PPP is described by Internet Engineering Task Force (IETF) RFC 1661.

Automatic self configuration

Link Control Protocol (LCP) is an integral part of PPP, and defined in the samestandard specification. LCP provides automatic configuration of the interfaces ateach end (such as setting datagram size, escaped characters, and magicnumbers) and for selecting optional authentication. The LCP protocol runs atopPPP (with PPP protocol number 0xC021) and therefore a basic PPP connectionhas to be established before LCP is able to configure it.

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RFC 1994 describes Challenge-handshake authentication protocol (CHAP),preferred for establishing dial-up connections with ISPs. Although deprecated,Password authentication protocol (PAP) is often used.

After the link has been established, additional network (layer 3) configuration

may take place. Most commonly, the Internet Protocol Control Protocol (IPCP) isavailable, although Internetwork Packet Exchange Control Protocol (IPXCP) andAppleTalk Control Protocol (ATCP) were once very popular.

Both PPP and Dynamic Host Configuration Protocol (DHCP) offer support forautomatic configuration of interfaces. While DHCP is used primarily for multi-point access, it may also be used on point-to-point links but only after PPP hascompleted establishment of the essential link and network configuration.

Multiple network layer protocols

PPP permits multiple network layer protocols to operate on the samecommunications link. For every network layer protocol used, a separate NetworkControl Protocol (NCP) is provided in order to encapsulate and negotiate optionsfor the multiple network layer protocols.

For example, Internet Protocol (IP) uses the IP Control Protocol (IPCP), andInternetwork Packet Exchange (IPX) uses the Novell IPX Control Protocol(IPXCP). NCPs include fields containing standardized codes to indicate thenetwork layer protocol type that PPP encapsulates.

Looped link detection

PPP detects looped links using a feature involving magic numbers. When thenode sends PPP LCP messages, these messages may include a magic number.If a line is looped, the node receives an LCP message with its own magicnumber, instead of getting a message with the peer's magic number.

Most important features

Link Control Protocol initiates and terminates connections gracefully,

allowing hosts to negotiate connection options. It also supports both byte-and bit-oriented encodings

Network Control Protocol is used for negotiating network-layer information,

e.g. network address or compression options, after the connection hasbeen established.

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PPP frame

Name Number of bytes Description

Protocol 1 or 2 setting of protocol in data field

Information variable (0 or more) datagram

Padding variable (0 or more) optional padding

Name Number of bytes Description

Flag 1 indicates frame's begin or end

Address 1 broadcast address

Control 1 control byte

Protocol 1 or 2 setting of protocol in information field

Information variable (0 or more) datagram

Padding variable (0 or more) optional padding

FCS 2 (or 4) error correction sum

The Protocol field indicates the kind of payload packet (e.g. LCP, NCP, IP, IPX,AppleTalk, etc.).

The Information field contains the PPP payload; it has a variable length with anegotiated maximum. By default the maximum is 1500 octets. It might be padded

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on transmission; if the information for a particular protocol can be padded, thatprotocol must allow information to be distinguished from padding.

Encapsulation

PPP frames are encapsulated in a lower-layer protocol that provides framing andmay provide other functions such as a checksum to detect transmission errors.PPP on serial links is usually encapsulated in a framing similar to HDLC,described by IETF RFC 1662.

The Flag field is present when PPP with HDLC-like framing is used.

The Address and Control fields always have the value hex FF (for "all stations")and hex 03 (for "unnumbered information"), and can be omitted whenever PPPLCP Address-and-Control-Field-Compression (ACFC) is negotiated.

The Frame Check Sequence (FCS) field is used to determine whether anindividual frame has an error. It contains a checksum computed over the frame toprovide basic protection against errors in transmission. This is a CRC codesimilar to the one used for other layer two protocol error protection schemes suchas the one used in Ethernet. According to RFC 1662, it can be either 16 bits(2bytes) or 32 bits (4 bytes) in size (default is 16 bits - Polynomialx16 +x12 +x5 +1).

The FCS is calculated over the Address, Control, Protocol, Information andPadding fields.

Although these are not standard applications, PPP is also used over broadbandconnections. RFC 2516 describes Point-to-Point Protocol over Ethernet(PPPoE), a method for transmitting PPP over Ethernet that is sometimes usedwith DSL. RFC 2364 describes Point-to-Point Protocol over ATM (PPPoATM), amethod for transmitting PPP over ATM Adaptation Layer 5 (AAL5), which is alsosometimes used with DSL.

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PPP line activation and states

The states of the Point to Point Protocol are such:

Dead (or idle), no active connection or carrier.

Establish, where Link Control Protocol negotiation begins.

Authenticate, where two terminal points authenticate each other. Either

PAP, or CHAP protocol comes in use at this point.

Network, where the appropriate Network Control Protocol is invoked.

IPCP is used to establish IP service over the network

Open, where data transport takes place.

Terminate, where the connection requires termination

Multilink PPP

Multilink PPP can connect multiple links between two systems as needed to

provide extra bandwidth. Remotely accessing resources through PPP Multilinkallows for the increase in overall throughput by combining the bandwidth of twoor more physical communication links such as analog modems, ISDN, and otheranalog/digital links. PPP Multilink is based on Internet Engineering Task Force(IETF) standard RFC 1990 (obsoletes RFC 1717).

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Multiclass PPP

MP's monotonically increasing sequence numbering (contiguous numbers are needed forall fragments of a packet) does not allow suspension of the sending of a sequence of

fragments of one packet in order to send another packet. The obvious approach to

providing more than one level of suspension with PPP Multilink is to run Multilinkmultiple times over one link. Multilink as it is defined provides no way for more than one

instance to be active. Each class runs a separate copy of the mechanism defined i.e. uses a

separate sequence number space and reassembly buffer.

Post Office Protocol

Overview

POP3 has made earlier versions of the protocol obsolete, POP (informally calledPOP1 and POP2). In contemporary usage, the less precise term POPalmostalways means POP3 in the context of e-mail protocols.

The design of POP3 and its procedures supports end-users with intermittentconnections (such as dial-up connections), allowing these users to retrieve e-mailwhen connected and then to view and manipulate the retrieved messageswithout needing to stay connected. Although most clients have an option to leavemail on server, e-mail clients using POP3 generally connect, retrieve allmessages, store them on the user's PC as new messages, delete them from theserver, and then disconnect. In contrast, the newer, more capable InternetMessage Access Protocol (IMAP) supports both connectedand disconnectedmodes of operation. E-mail clients using IMAP generally leave messages on theserver until the user explicitly deletes them. This and other facets of IMAPoperation allow multiple clients to access the same mailbox. Most e-mail clientssupport either POP3 or IMAP to retrieve messages; however, fewer InternetService Providers (ISPs) support IMAP. The fundamental difference betweenPOP3 and IMAP4 is that POP3 offers access to a mail drop; the mail exists onthe server until it is collected by the client. Even if the client leaves some or allmessages on the server, the client's message store is considered authoritative. Incontrast, IMAP4 offers access to the mail store; the client may store local copiesof the messages, but these are considered to be a temporary cache; the server'sstore is authoritative.

Clients with a leave mail on serveroption generally use the POP3 UIDL (UniqueIDentification Listing) command. Most POP3 commands identify specificmessages by their ordinal number on the mail server. This creates a problem fora client intending to leave messages on the server, since these messagenumbers may change from one connection to the server to another. For exampleif a mailbox contains five messages at last connect, and a different client then

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deletes message #3, the next connecting user will find the last two messages'numbers decremented by one. UIDL provides a mechanism to avoid thesenumbering issues. The server assigns a string of characters as a permanent andunique ID for the message. When a POP3-compatible e-mail client connects tothe server, it can use the UIDL command to get the current mapping from these

message IDs to the ordinal message numbers. The client can then use thismapping to determine which messages it has yet to download, which saves timewhen downloading. IMAP has a similar mechanism, using a 32-bit UID (UniqueIDentifier) that is required to be strictly ascending. The advantage of the numericUID is with large mailboxes; a client can request just the UIDs greater than itspreviously stored "highest UID". In POP, the client must fetch the entire UIDLmap.

Whether using POP3 or IMAP to retrieve messages, e-mail clients typically usethe SMTP_Submit profile of the SMTP protocol to send messages. E-mail clientsare commonly categorized as eitherPOPorIMAPclients, but in both cases the

clients also use SMTP. There are extensions to POP3 that allow some clients totransmit outbound mail via POP3 - these are known as "XTND XMIT" extensions.The Qualcomm qpopper and CommuniGate Pro servers and Eudora clients areexamples of systems that optionally utilize the XTND XMIT methods ofauthenticated client-to-server e-mail transmission.

MIME serves as the standard for attachments and non-ASCII text in e-mail.Although neither POP3 nor SMTP require MIME-formatted e-mail, essentially allInternet e-mail comes MIME-formatted, so POP clients must also understand anduse MIME. IMAP, by design, assumes MIME-formatted e-mail.

Like many other older Internet protocols, POP3 originally supported only anunencrypted login mechanism. Although plain text transmission of passwords inPOP3 still commonly occurs, POP3 currently supports several authenticationmethods to provide varying levels of protection against illegitimate access to auser's e-mail. One such method, APOP, uses the MD5 hash function in anattempt to avoid replay attacks and disclosure of the shared secret. Clientsimplementing APOP include Mozilla Thunderbird, Opera, Eudora, KMail andNovell Evolution. POP3 clients can also support SASL authentication methodsvia the AUTH extension.

POP3 works over a TCP/IP connection using TCP on network port 110. E-mailclients can encrypt POP3 traffic using TLS or SSL. A TLS or SSL connection isnegotiated using the STLS command. Some clients and servers, like GoogleGmail, instead use the deprecated alternate-port method, which uses TCP port995.

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POP4

While not yet an official standardized mail protocol, a proposal has been outlinedfor a POP4 specification, complete with a working server implementation.

The proposed POP4 extension adds basic folder management, multipartmessage support, as well as message flag management, allowing for a lightprotocol which supports some popular IMAP features which POP3 currentlylacks.

No progress has been observed in the POP4 specification since 2003.

SDPS

Demon Internet introduced extensions to POP3 that allow multiple accounts perdomain, and has become known as Standard Dial-up POP3 Service (SDPS).

Simple Mail Transfer Protocol

Description

SMTP is a relatively simple, text-based protocol, where one or more recipients ofa message are specified (and in most cases verified to exist) and then themessage text is transferred. It is a client-server protocol, where the clienttransmits an email message to the server. Either an end-user's email client, a.k.a.MUA (Mail User Agent), or a relaying server's MTA (Mail Transfer Agents) can act

as an SMTP client.

An email client knows the outgoing mailSMTP server from its configuration. Arelaying server typically determines which SMTP server to connect to by lookingup the MX (Mail eXchange) DNS record for each recipient's domain name, thepart of the email address to the right of the at sign (@). Conformant MTAs (notall) fall back to a simple A record in the case of no MX. Some current mailtransfer agents will also use SRV records, a more general form of MX, thoughthese are not widely adopted. (Relaying servers can also be configured to use asmart host.)

The SMTP client initiates a TCP connection to server's port 25 (unless overriddenby configuration.) It is quite easy to test an SMTP server using the telnet program(see below).

SMTP is a "push" protocol that does not allow one to "pull" messages from aremote server on demand. To do this a mail client must use POP3 or IMAP.

Another SMTP server can trigger a delivery in SMTP using ETRN.

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History

SMTP developed out of Mail Box Protocol (ca. 1971), FTP Mail (ca. 1973) andMail Protocol The work continued throughout the 1970s, until the ARPANETconverted into the modern Internet around 1990. Jon Postel then proposed a

Mail Transfer Protocol in 1980 that began to remove the mail's reliance on FTPSMTP was published as RFC 821 in 1982, also by Jonathan Postel.

SMTP started becoming widely used in the early 1980s. At the time, it was acomplement to UUCP (Unix to Unix CoPy) which was better suited to handle e-mail transfers between machines that were intermittently connected. SMTP, onthe other hand, works best when both the sending and receiving machines areconnected to the network all the time. Both use a store and forward mechanismand are examples of push technology.

The article about sender rewriting contains technical background info about the

early SMTP history and source routing before RFC 1123.

SMTP-AUTH was introduced with RFC 2554 in 1998/99, more or less at thesame time as RFC 2476 that introduced Mail submission agent as a differentflavor of SMTP. Until then clients could only be recognized by their IP address.

Sendmail was one of the first (if not the first) mail transfer agent to implementSMTP. As of 2001 there are at least 50 programs that implement SMTP as aclient (sender of messages) or a server (receiver of messages). Some otherpopular SMTP server programs include Postfix, qmail, Novell GroupWise, Exim,Novell NetMail and Microsoft Exchange Server.

Since this protocol started out as purely ASCII text-based, it did not deal well withbinary files. Standards such as Multipurpose Internet Mail Extensions (MIME)were developed to encode binary files for transfer through SMTP. MTAsdeveloped after sendmail also tended to be implemented 8-bit-clean, so that thealternate "just send eight" strategy could be used to transmit arbitrary data viaSMTP. Non-8-bit-clean MTAs today tend to support the 8BITMIME extension,permitting binary files to be transmitted almost as easily as plain text.

Developers

Many people edited or contributed to the core SMTP specifications, among themJon Postel, Eric Allman, Dave Crocker, Ned Freed, Randall Gellens, JohnKlensin, and Keith Moore.

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Outgoing mail SMTP server

An email client requires the name or the IP address of an SMTP server as part ofits configuration. The server will take care of delivering messages on behalf ofthe user. This setting allows for various policies and network designs. Clients

from behind a firewall are able to send mail to any Internet address withoutdirectly connecting to the Internet. End-users directly connected to the Internetcan use the services of an e-mail provider that is not necessarily the same astheir connection provider.

Before SMTP-AUTH was widely implemented, the only practical setting for anend user or small office directly connected to the Internet was to use theconnection provider's SMTP server. Nowadays, decent SMTP servers supportauthentication and encrypted SMTP sessions. Even then, some still believe thatusing their connection providers SMTP server for outgoing mail would result in abetter overall resource usage, because that allows to optimize the delivery paths

of outgoing messages. After recognizing the amount of resources burned fordelivering spam, that statement is not true.

Another choice is whether to use port 25 or port 587, as established by RFC2476. Many servers support both. Some servers still support port 465 for legacysecure SMTP, it is preferable to use encryption on standard ports after RFC2487.

Security and spamming

One of the limitations of the original SMTP is that it has no facility for

authentication of senders. Therefore the SMTP-AUTH extension was defined.However, the impracticalities of widespread SMTP-AUTH implementation andmanagement means that E-mail spamming is not and cannot be addressed by it.

Modifying SMTP extensively, or replacing it completely, is not believed to bepractical, due to the network effects of the huge installed base of SMTP. InternetMail 2000 is one such proposal for replacement.

Spam is enabled by several factors, including vendors implementing brokenMTAs (that do not adhere to standards, and therefore make it difficult for otherMTAs to enforce standards), security vulnerabilities within the operating system

(often exacerbated by always-on broadband connections) that allow spammersto remotely control end-user PCs and cause them to send spam, and aregrettable lack of "intelligence" in many MTAs still a major problem.

There are a number of proposals for sideband protocols that will assist SMTPoperation. The Anti-Spam Research Group (ASRG) of the Internet ResearchTask Force (IRTF) is working on a number of E-mail authentication and otherproposals for providing simple source authentication that is flexible, lightweight,

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and scalable. Recent Internet Engineering Task Force (IETF) activities includeMARID (2004) leading to two approved IETF experiments in 2005, andDomainKeys Identified Mail in 2006.

Remote Access Service

Remote Access Services (RAS) refers to any combination of hardware andsoftware to enable the remote access to tools or information that typically resideon a network of IT devices.

Originally coined by Microsoft when referring to their built-in NT remote accesstools, RAS was a service provided by Windows NT which allows most of theservices which would be available on a network to be accessed over a modemlink. The service includes support for dialup and logon, and then presents thesame network interface as the normal network drivers (albeit slightly slower). It isnot necessary to run Windows NT on the client - there are client versions for

other Windows operating systems.

A feature built into Windows NT that enables users to log into an NT-based LANusing a modem, X.25 connection or WAN link. RAS works with several majornetwork protocols, including TCP/IP, IPX, and NBF.

To use RAS from a remote node, you need a RAS client program, which is builtinto most versions of Windows, or any PPP client software. For example, mostremote control programs work with RAS.

Over the years, many vendors have provided both hardware and software

solutions to gain remote access to various types of networked information. Infact, most modern routers include a basic RAS capability that can be enabled forany dial-up interface.

IP address

An IP address (Internet Protocol address) is a unique address that certainelectronic devices use in order to identify and communicate with each other on acomputer network utilizing the Internet Protocol standard (IP)in simpler terms,a computer address. Any participating network deviceincluding routers,

computers, time-servers, printers, Internetfaxmachines, and some telephonescan have their own unique address.

An IP address can also be thought of as the equivalent of a street address or aphone number (compare: VoIP (voice over (the) internet protocol)) for a computeror other network device on the Internet. Just as each street address and phonenumber uniquely identifies a building or telephone, an IP address can uniquelyidentify a specific computer or other network device on a network. An IP address

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differs from other contact information, however, because the linkage of a user'sIP address to his/her name is not publicly available information.

IP addresses can appear to be shared by multiple client devices either becausethey are part of a shared hosting web server environment or because a network

address translator (NAT) or proxy server acts as an intermediary agent on behalfof its customers, in which case the real originating IP addresses might be hiddenfrom the server receiving a request. A common practice is to have a NAT hide alarge number of IP addresses, in the private address space defined by RFC1918, an address block that cannot be routed on the public Internet. Only the"outside" interface(s) of the NAT need to have Internet-routable addresses.

Most commonly, the NAT device maps TCP or UDP port numbers on the outsideto individual private addresses on the inside. Just as there may be site-specificextensions on a telephone number, the port numbers are site-specific extensionsto an IP address.

IP addresses are managed and created by the Internet Assigned NumbersAuthority (IANA). The IANA generally allocates super-blocks to Regional InternetRegistries, who in turn allocate smaller blocks to Internet service providers andenterprises.

IP versions

The Internet Protocol has two versions currently in use (see IP version history fordetails). Each version has its own definition of an IP address. Because of itsprevalence, "IP address" typically refers to those defined by IPv4.

IP version 4

IPv4 only uses 32-bit (4 byte) addresses, which limits the address space to4,294,967,296 (232) possible unique addresses. However, many are reserved forspecial purposes, such as private networks (~18 million addresses) or multicastaddresses (~270 million addresses). This reduces the number of addresses thatcan be allocated as public Internet addresses, and as the number of addressesavailable is consumed, an IPv4 address shortage appears to be inevitable in thelong run. This limitation has helped stimulate the push towards IPv6, which iscurrently in the early stages of deployment and is currently the only contender to

replace IPv4.

IP version 6

IPv6 is the new standard protocol for the Internet. Windows Vista, AppleComputer's Mac OS X, and an increasing range of Linux distributions includenative support for the protocol, but it is not yet widely deployed elsewhere.

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Addresses are 128 bits (16 bytes) wide, which, even with a generous assignmentof netblocks, will more than suffice for the foreseeable future. In theory, therewould be exactly 2128, or about 3.403 1038 unique host interface addresses.Further, this large address space will be sparsely populated, which makes itpossible to again encode more routing information into the addresses

themselves.

Example: 2001:0db8:85a3:08d3:1319:8a2e:0370:7334

One source notes that there will exist "roughly 5,000 addresses for every squaremicrometer of the Earth's surface". This enormous magnitude of available IPaddresses will be sufficiently large for the indefinite future, even though mobilephones, cars and all types of personal devices are coming to rely on the Internetfor everyday purposes.

The above source, however, involves a common misperception about the IPv6

architecture. Its large address space is not intended to provide unique addressesfor every possible point. Rather, the addressing architecture is such that it allowslarge blocks to be assigned for specific purposes and, where appropriate,aggregated for provider routing. With a large address space, there is not theneed to have complex address conservation methods as used in classless inter-domain routing (CIDR).

IP version 6 private addresses

Just as there are addresses for private, or internal networks in IPv4 (oneexample being the 192.168.0.1 - 192.168.0.254 range), there are blocks of

addresses set aside in IPv6 for private addresses. Addresses starting with FE80:are called link-local addresses and are routable only on your local link area. Thismeans that if several hosts connect to each other through a hub or switch thenthey would communicate through their link-local IPv6 address.

Early designs specified an address range used for "private" addressing, withprefix FEC0. These are called site-local addresses (SLA) and are routable withina particular site, analogously to IPv4 private addresses. Site-local addresses,however, have been deprecated by the IETF, since they create the same problemthat does the existing IPv4 private address space (RFC 1918). With that privateaddress space, when two sites need to communicate, they may have duplicate

addresses that "combine". In the IPv6 architecture, the preferred method is tohave unique addresses, in a range not routable on the Internet, issued toorganizations (e.g., enterprises).

The preferred alternative to site-local addresses are centrally assigned uniquelocal unicast addresses (ULA). In current proposals, they will start with the prefixFC00.Neither ULA nor SLA nor link-local address ranges are routable over theinternet.

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Static and dynamic IP addresses

A Static IP address is where a computer uses the same address every time auser logs on to a network, such as the Internet. With a static IP address, acomputer's identity can be easily identified by others, and users can easily

connect with it. That way, for example, a website, email server, or other type ofserver connection can be hosted.

This contrasts with a Dynamic IP address, wherein an IP address is assigned toa computer, usually by a remote server which is acting as a Dynamic HostConfiguration Protocol server. IP addresses assigned using DHCP may changedepending on the addresses available in the set scope. Dynamic IP Addressesassigned by Dynamic Host Configuration Protocol servers are used because itcreates effiency within a network. When there is no need to assign everybody aspecific IP Address, users can simply log in and out and use the network withoutthe hassle of having to get an IP assigned to them.

IP address legality in Europe

It is important to note that unlike the U.S., under European Union law IPAddresses are considered to be personal data as defined by article 2(a) ofDirective 95/46/EC " 'personal data' shall mean any information relating to anidentified or identifiable natural person ('data subject'); an identifiable person isone who can be identified, directly or indirectly, in particular by reference to anidentification number or to one or more factors specific to his physical,physiological, mental, economic, cultural or social identity; " Also see Directive2006/24/EC.

In association with time codes, IP Addressing information will always identifyunique ISP account holders unless there is translation of that information.

It is important that this significant difference in legal status be understood,because Websites that provide for third-party interception of IP addressinginformation and traffic data, without Website visitor consent, are committing acriminal offence in the UK by virtue of the Regulation of Investigatory Powers Act2000, where through the requirements of European Council Decision2005/222/JHA such Website owners face serious sanctions, including thewinding up of their businesses, being debarred from running a business, and

more than 2 years imprisonment.

Directive 95/46/EC

Directive 2006/24/EC

European Council Framework Decision 2005/222/JHA

the Regulation of Investigatory Powers Act 2000

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Subnetting

Subnetting an IP Network can be done for a variety of reasons, includingorganization, use of different physical media (such as Ethernet, FDDI, WAN,etc.), preservation of address space, and security. The most common reason is

to control network traffic. In an Ethernet network, all nodes on a segment see allthe packets transmitted by all the other nodes on that segment. Performance canbe adversely affected under heavy traffic loads, due to collisions and the resultingretransmissions. A router is used to connect IP networks to minimize the amountof traffic each segment must receive.

Subnet Masking

Applying a subnet mask to an IP address allows you to identify the network andnode parts of the address. The network bits are represented by the 1s in themask, and the node bits are represented by the 0s. Performing a bitwise logical

AND operation between the IP address and the subnet mask results in theNetwork Address or Number.

Integrated Services Digital Network

Integrated Services Digital Network (ISDN) is a circuit-switched telephonenetwork system, designed to allow digital transmission of voice and data overordinary telephone copper wires, resulting in better quality and higher speedsthan that which is available with the PSTN system. More broadly, ISDN is a set ofprotocols for establishing and breaking circuit switched connections, and for

advanced call features for the user.

In a videoconference, ISDN provides simultaneous voice, video, and texttransmission between individual desktop videoconferencing systems and group(room) videoconferencing systems.

ISDN elements

The English term is a backronym that was thought to be better for English-language advertisements than the original, "Integriertes Sprach- und Datennetz"(German for "Integrated Speech and Data Net").

Integrated Services refers to ISDN's ability to deliver at minimum two

simultaneous connections, in any combination of data, voice, video, andfax, over a single line. Multiple devices can be attached to the line, andused as needed. That means an ISDN line can take care of most people'scomplete communications needs at a much higher transmission rate,without forcing the purchase of multiple analog phone lines.

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Digitalrefers to its purely digital transmission, as opposed to the analog

transmission of plain old telephone service (POTS). Use of an analogtelephone modem for Internet access requires that the Internet serviceprovider's (ISP) modem converts the digital content to analog signalsbefore sending it and the user's modem then converts those signals back

to digital when receiving. When connecting with ISDN there is no analogconversion. ISDN transmits data digitally, resulting in a very cleartransmission quality. There is none of the static and noise of analogtransmissions that can cause slow transmission speed.

Networkrefers to the fact that ISDN is not simply a point-to-point solution

like a leased line. ISDN networks extend from the local telephoneexchange to the remote user and includes all of the telecommunicationsand switching equipment in between.

The purpose of the ISDN is to provide fully integrated digital services to theusers. These services fall under three categories: bearer services,

supplementary services and tele services.

Consumer and industry perspectives

There are two points of view into the ISDN world. The most common viewpoint isthat of the end user, who wants to get a digital connection into the telephone/datanetwork from home, whose performance would be better than an ordinary analogmodem connection. The typical end-user's connection to the Internet is related tothis point of view, and talk about the merits of various ISDN modems, carriers'offerings and tarriffing (features, pricing) are from this perspective. Much of thefollowing discussion is from this point of view, but it should be noted that as a

data connection service, ISDN has been mostly superseded by DSL.

There is, however, a second viewpoint: that of the telephone industry, whereISDN is not a dead issue. A telephone network can be thought of as a collectionof wires strung between switching systems. The common electrical specificationfor the signals on these wires is T1 or E1. On a normal T1, the signalling is donewith A&B bits to indicate on-hook or off-hook conditions and MF and DTMF tonesto encode the destination number. ISDN is much better because messages canbe sent much more quickly than by trying to encode numbers as long (100 msper digit) tone sequences. This translated to much faster call setup times, whichis greatly desired by carriers who have to pay for line time and also by callers

who become impatient while their call hops from switch to switch.

It is also used as a smart-network technology intended to add new services tothe public switched telephone network (PSTN) by giving users direct access toend-to-end circuit-switched digital services.

ISDN BRI (Basic Rate Interface) has never gained popularity as a telephoneaccess technology in North America and today remains a niche product.

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However, most modern non-VoIP PBXs use PRI (Primary Rate Interface) T1 linesto communicate with a Telco Class 5 central office switch, replacing older analogtwo-way and Direct Inward Dialing (DID) trunks. PRI is capable of deliveringcaller ID in both directions so that the telephone number of an extension, ratherthan a company's main number, can be sent. It is still commonly used in

recording studios, when a voice-over actor is in one studio, but the director andproducer are in a studio at another location. ISDN is used because of its"guaranteed" real-time, not-over-the-Internet service, and its superior audiofidelity as compared to POTS service. A few companies make video conferencecall equipment that combine three BRI lines and six 64K channels to create agood quality picture.

In Japan, it became popular to some extent from around 1999 to 2001, but nowthat ADSL has been introduced, the number of subscribers is in decline. NTT, adominant Japanese telephone company, provides an ISDN service with thenames INS64 and INS1500, which are much less recognized than ISDN.

In the UK, British Telecom (BT) provides ISDN2e (BRI) as well as ISDN30 (PRI).Until April 2006, they also offered Home Highway and Business Highway, whichare BRI ISDN-based services that offer integrated analogue connectivity as wellas ISDN. Later versions of the Highway products also included built-in USBsockets for direct computer access. Home Highway has been bought by manyhome users, usually for Internet connection, although not as fast as ADSL,because it was available before ADSL and in places where ADSL does not reach.

France Tlcom offers ISDN services under their product name Numeris (2B+D), of which a professional Duo and home Itoo version is available. ISDN is

generally known as RNIS in France and has widespread availability. Theintroduction of ADSL is reducing ISDN use for data transfer and Internet access,although it is still common in more rural and outlying areas, and for applicationssuch as business voice and point-of-sale terminals.

In Germany, ISDN is very popular with an installed base of 25 million channels(29% of all subscriber lines in Germany as of 2003 and 20% of all ISDN channelsworldwide). Due to the success of ISDN, the number of installed analog lines isdecreasing. Deutsche Telekom (DTAG) offers both BRI and PRI. Competingphone companies often offer ISDN only and no analog lines. Because of thewidespread availability of ADSL services, ISDN is today primarily used for voicetraffic, but is still very popular thanks to the pricing policy of German telcos.Today ISDN (BRI) and ADSL/VDSL are often bundled on the same line.

In India, ISDN was very popular until the introduction of ADSL. Bharat SancharNigam Limited (A Govt. Of India Ent.), the largest communication serviceprovider in India, is offering both ISDN BRI and PRI services across the countryover its ISDN network. After the introduction of ADSL broadband technology withstatic IPs, the data transfer load is taken up by ADSL. But ISDN still plays a very

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big role as a backup network for point-to-point leased line customers and lowcost reliable data network for organisations located all over India, such asBanks,E-seva centres, LIC, and so on.

Configurations

In ISDN, there are two types of channels, B (for "Bearer") and D (for "Delta"). Bchannels are used for data (which may include voice), and D channels areintended for signaling and control (but can also be used for data).

There are two ISDN implementations. Basic Rate Interface (BRI), also calledBasic Rate Access (BRA) in Europe consists of two B channels, each withbandwidth of 64 kbit/s, and one D channel with a bandwidth of 16 kbit/s. Togetherthese three channels can be designated as 2B+D. Primary Rate Interface (PRI),also called Primary Rate Access (PRA) in Europe contains a greater numberof B channels and a D channel with a bandwidth of 64 kbit/s. The number of B

channels for PRI varies according to the nation: in North America and Japan it is23B+1D, with an aggregate bit rate of 1.544 Mbit/s (T1); in Europe, India and

Australia it is 30B+1D, with an aggregate bit rate of 2.048 Mbit/s (E1). BroadbandIntegrated Services Digital Network (BISDN) is another ISDN implementation andit is able to manage different types of services at the same time. It is primarilyused within network backbones and employs ATM.

Another alternative ISDN configuration can be used in which the B channels ofan ISDN basic rate interface are bonded to provide a total duplex bandwidth of128 kbit/s. This precludes use of the line for voice calls while the internetconnection is in use.

Using bipolar with eight-zero substitution encoding technique, call data istransmitted over the data (B) channels, with the signalling (D) channels used forcall setup and management. Once a call is set up, there is a simple 64 kbit/ssynchronous bidirectional data channel between the end parties, lasting until thecall is terminated. There can be as many calls as there are data channels, to thesame or different end-points. Bearer channels may also be multiplexed into whatmay be considered single, higher-bandwidth channels via a process called Bchannel bonding.

The D channel can also be used for sending and receiving X.25 data packets,

and connection to X.25 packet network, this is specified in X.31. In practice, X.31was only commercially implemented in France and Japan.

Reference points

A set of reference points are defined in the ISDN standard to refer to certainpoints between the telco and the end user ISDN equipment.

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R - defines the point between a non-ISDN device and a terminal adapter

(TA) which provides translation to and from such a device

S - defines the point between the ISDN equipment (or TA) and a Network

Termination Type 2 (NT-2) device

T - defines the point between the NT-2 and NT-1 devices1

U - defines the point between the NT-1 and the telco switch2

Most NT-1 devices can perform the functions of the NT-2 as well, and so the Sand T reference points are generally collapsed into the S/T reference point.Inside North America, the NT-1 device is considered customer premisesequipment(CPE) and must be maintained by the customer, thus, the U interfaceis provided to the customer. In other locations, the NT-1 device is maintained bythe telco, and the S/T interface is provided to the customer. In India, serviceproviders provide U interface and an NT1 may be supplied by Service provideras part of service offering

Types of communications

Among the kinds of data that can be moved over the 64 kbit/s channels arepulse-code modulated voice calls, providing access to the traditional voice PSTN.This information can be passed between the network and the user end-point atcall set-up time. In North America, ISDN is now used mostly as an alternative toanalog connections, most commonly for Internet access. Some of the servicesenvisioned as being delivered over ISDN are now delivered over the Internetinstead. In Europe, and in Germany in particular, ISDN has been successfullymarketed as a phone with features, as opposed to a POTS phone (Plain OldTelephone Service) with few or no features. Meanwhile, features that were first

available with ISDN (such as Three-Way Call, Call Forwarding, Caller ID, etc.)are now commonly available for ordinary analog phones as well, eliminating thisadvantage of ISDN. Another advantage of ISDN was the possibility of multiplesimultaneous calls (one call per B channel), e.g. for big families, but with theincreased popularity and reduced prices of mobile telephony this has becomeless interesting as well, making ISDN unappealing to the private customer.However, ISDN is typically more reliable than POTS, and has a significantlyfaster call setup time compared with POTS, and IP connections over ISDNtypically have some 3035ms round trip time, as opposed to 120180ms (bothmeasured with otherwise unused lines) over 56k or V.34 modems, making ISDNmore pleasant for telecommuters.

Where an analog connection requires a modem, an ISDN connection requires aterminal adapter (TA). The function of an ISDN terminal adapter is often deliveredin the form of a PC card with an S/T interface, and single-chip solutions seem toexist, considering the plethora of combined ISDN- and ADSL-routers.

ISDN is commonly used in radio broadcasting. Since ISDN provides a highquality connection this assists in delivering good quality audio for transmission in

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radio. Most radio studios are equipped with ISDN lines as their main form ofcommunication with other studios or standard phone lines.

Sample call

The following is an example of a Primary Rate (PRI) ISDN call showing theQ.921/LAPD and the Q.931/Network message intermixed (i.e. exactly what wasexchanged on the D-channel). The call is originating from the switch where thetrace was taken and goes out to some other switch, possibly an end-office LEC,who terminates the call.

The first line format is . If the message is an ISDN level message, then adecoding of the message is attempted showing the various Information Elementsthat make up the message. All ISDN messages are tagged with an ID numberrelative to the switch that started the call (local/remote).

Leased line

A leased line is a symmetric telecommunications line connecting two locations.Unlike traditional PSTN lines it does not have a telephone number, each side ofthe line being permanently connected to the other. Leased lines can be used fortelephone, data or Internet services. Some are ringdown services, and someconnect two PBXes.

In the U.K., leased lines are usually available at speeds of 64k, 128k, 256k, 512k,2M and provided to the customer on X.21 presentation. Higher speeds are

available on alternative interfaces.

In the U.S., low-speed leased lines (56 kbit/s and below) are usually providedusing analog modems. Higher-speed leased lines are usually presented usingFT1 (Fractional T1): a T1 bearer circuit with 1 to 24 56k or 64k timeslots.Customers must manage their own network termination equipmentChannelService Unit or Data Service Unit (CSU/DSU).

For many purposes, leased lines are gradually being replaced by DSL links.

NarrowbandNarrowband refers to a situation in radio communications where the bandwidth ofthe message does not significantly exceed the channel's coherence bandwidth. Itis a common misconception that narrowband refers to a channel which occupiesonly a "small" amount of space on the radio spectrum.

The opposite of narrowband is wideband.

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In the study of wireless channels, narrowband implies that the channel underconsideration is sufficiently narrow that its frequency response can be consideredflat. The message bandwidth will therefore be less than the coherence bandwidthof the channel. This is usually used as an idealizing assumption; no channel hasperfectly flat fading, but the analysis of many aspects of wireless systems is

greatly simplified if flat fading can be assumed.

Narrowband can also be used with the audio spectrum to describe sounds whichoccupy a narrow range of frequencies. In telephony narrowband is usuallyconsidered to cover frequencies 3003400 Hz.

Dial-up access

Dial-up access is a form of Internet access via telephone line. The client uses amodem connected to a computer and a telephone line to dial into an Internetservice provider's (ISP) node to establish a modem-to-modem link, which is then

routed to the Internet

Availability

Dial-up requires no additional infrastructure on top of the telephone network. Astelephone points are available throughout the world, dial-up remains useful totravelers. Dial-up is usually the only choice available for most rural or remoteareas where getting a broadband connection is impossible due to low populationand demand. Sometimes dial-up access may also be an alternative to peoplewho have limited budgets as it is offered for free by some, though broadband isnow increasingly available at lower prices in countries such as the United States,

Canada and the United Kingdom due to market competition.

Dial-up requires time to establish a telephone connection (several seconds,depending on the location) and perform handshaking before data transfers cantake place. In locales with telephone connection charges, each connection incursan incremental cost. If calls are time-charged, the duration of the connectionincurs costs.

Dial-up access is a transient connection, because either the user or the ISPterminates the connection. Internet service providers will often set a limit onconnection durations to prevent hogging of access, and will disconnect the user

requiring reconnection and the costs and delays associated with it.

Performance

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Modern dial-up modems typically have a maximum theoretical speed of 56 kbit/s(using the V.92 protocol), although in most cases only up to 53 kbit/s is possibledue to overhead and, in the United States, FCC regulation. These speeds arecurrently considered the maximum possible; in many cases transfer speeds willbe lower, averaging anywhere between 33-43 kbit/s. Factors such as phone line

noise and conditions, as well as the quality of the modem itself, play a large partin determining connection speeds.

Dial-up connections usually have high latency that can be as high as 400 ms oreven more, which can make online gaming or videoconferencing difficult, if notimpossible. Some games, such as Star Wars: Galaxies, The Sims Online,Warcraft 3, Halo: Combat Evolvedand Guild Wars are capable of running on 56K dial-up. Gamers with dial-up connections are often disconnected from gameservers due to the "lag", or high latency, of the connection. Many computergames released in 2005 (such as Battlefield 2orStar Wars: Battlefront 2) are notcompatible for online play with dial-up modems. These first person shooter style

games are the most sensitive to latency, making playing them impractical on dial-up.

High-speed dial-up

What is often advertised as "high-speed dial-up Internet" or "accelerated dial-up"by service providers such as Earthlink, People PC, NetZero in the United Statesand Gonuts4free in Spain, is a form of dial-up access that uses the newermodem standard V.92 to shorten the log-on (or handshake) process, and thenonce a connection has been established the provider will selectively compress,filter, and cache data being sent to the user with the overall effect of increasing

the speed of browsing most standard web pages (see also proxy server).

The term high speed is misleading as these processes do not increase theoverall throughput of the line, only making more efficient use of the bandwidththat is already there. Certain applications cannot be accelerated, such as SHTTP,streaming media, or file transfers. The compression of certain files such aspictures can have a negative effect on the browsing experience of the user, dueto the lower quality that it imposes. However, the user can manually choose toview the images in true quality whenever he or she chooses to do so.

Broadband

Broadband in telecommunications is a term which refers to a signaling methodwhich includes or handles a relatively wide range of frequencies which may bedivided into channels orfrequency bins. Broadbandis always a relative term,understood according to its context. The wider the bandwidth, greater is theinformation carrying capacity. In radio, for example, a very narrow-band signalwill carry Morse code; a broader band will carry speech; a still broader band isrequired to carry music without losing the high audio frequencies required for

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realistic sound reproduction. A television antenna described as "normal" may becapable of receiving a certain range of channels; one described as "broadband"will receive more channels. In data communications a modem will transmit abandwidth of 64 kilobits per seconds (kbit/s) over a telephone line; over the sametelephone line a bandwidth of several megabits per second can be handled by

ADSL, which is described as broadband(relative to a modem over a telephoneline, although much less than can be achieved over a fibre optic circuit, forexample).

Introduction

Broadband in data communications may have the same meaning as above, sothat data transmission over a fiber optic cable would be referred to as broadbandas compared to a telephone modem operating at 600 bits per second.

However, broadband in data communications is frequently used in a more

technical sense to refer to data transmission where multiple pieces of data aresent simultaneously to increase the effective rate of transmission, regardless ofactual data rate. In network engineering this term is used for methods where twoor more signals share a medium.

Various forms of Digital Subscriber Line(DSL) services are broadband in thesense that digital information is sent over one channel and voice over anotherchannel sharing a single pair of wires. Analog modems operating at speedsgreater than 600 bit/s are technically broadband. They obtain higher effectivetransmission rates by using multiple channels with the rate on each channellimited to 600 baud. For example, a 2400 bit/s modem uses four 600 baud

channels (see baud). This is in contrast to a baseband transmission where onetype of signal uses a medium's full bandwidth such as 100BASE-T Ethernet.

Ethernet, however, is the common user interface even to DSL data links.Ethernet provisioned over cable modem often is a competitive alternative to DSL,especially in the small office/home office market.

Users who need more than DSL or cable modem speeds will often use metroethernet, when available, rather than older and often more expensive (permegabit) than T-carrier (or E-carrier in appropriate parts of the world, or

Asynchronous Transfer Mode. Metro ethernet is usually implemented over a

metropolitan all-optical network.

Multiplexing

Communications may utilize a number of distinct physical channelssimultaneously; this is multiplexing for multiple access. Such channels may be

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distinguished by being separated from each other in time (time divisionmultiplexing or TDMA), in carrier frequency (frequency division multiplexing(FDMA) or wavelength division multiplexing (WDM)), or in access method (codedivision multiple access or CDMA). Each channel that takes part in such amultiplexing exercise is by definition narrowband (because it is not utilising the

whole bandwidth of the medium), whereas the whole set of channels takentogether and utilized for the same communication could be described asbroadband.

BSNL has commissioned a world class, multi-gigabit, multi-protocol, convergentIP infrastructure through National Internet Backbone-II (NIB-II), that providestriple play services through the same backbone and broadband access network.The Broadband service is available on DSL technology (on the same coppercable that is used for connecting telephone) in a large number of cities/towns oncountry-wide basis.

In terms of infrastructure for broadband services NIB-II has put India at par withmore advanced nations. The services that are supported includes: always-onbroadband access to the Internet for residential and business customers,Content based services, Video multicasting, Video-on-demand and Interactivegaming, Audio and Video conferencing, IP Telephony, Distance learning,Messaging: plain and feature rich, Multi-site MPLS VPNs with Quality of Service(QoS) guarantees. The subscribe will be able to access the above servicesthrough Subscriber Service Selection System (SSSS) portal.

Key Objectives

To provide high speed Internet connectivity (upto 8 Mbps)

To provide Virtual Private Network (VPN) service to the broadband customers To provide dial VPN service to MPLS VPN customers.

To provide multicast video services, video-on-demand, etc. through the

Broadband Remote Access Server (BRAS).

To provide a means to bill for the aforesaid services by either time-based or

volume-based billing. It shall provide the customer with the option to select

the services through web server To provide both pre-paid and post paid broadband services

Technical Capability of the Backbone

The Broadband Service is given through the state of the art Multi Protocol Label

Switching (MPLS) based IP Infrastructure, which is designed to provide reliable

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routes to cover all possible destinations within and outside the country. Layer-1 ofthe network consists of a high speed Backbone comprising of 24 powerful Core

Routers connected with high speed 2.5 Gbps(STM-16) links. The routers are locatedon the national DWDM network interfacing at STM-16 optical level to provide for high

transmission speeds. Layer-2 of the network consists of approx. 100 routers throughwhich the customers access services like MPLS/VPN, content etc.

What advantage does MPLS have over other Technologies?

MPLS VPN is a technology that allows a service provider like BSNL to have complete

control over parameters that are critical to offering its customers service guaranteeswith regard to bandwidth throughputs, latencies and availability.

Services available through Broadband

High speed Internet Access: This is the always-on Internet access service

with speed ranging from 2 Mbps to 8 Mbps.

Bandwidth on Demand: This will facilitate customer to change bandwidth as

per his / her requirement. For example a customer with 2 Mbps can change to4 Mbps during the IP TV session.

Multicasting: This is to provide video multicast services for application in

distance education, telemedicine etc

Dial VPN Service: This service allows remote users to access their private

network securely over the NIB-II infrastructure.

VPN on Broadband: This facilitates access of VPN using existing Broadband

connection.

Video and Audio Conferencing.

Content based Services: Like Video on Demand, Interactive Gaming, Live

and time shifted TV

What the customer needs in order to be able to use Broadband?

BSNL's Bfone (Basic phone) connection

Personel Computer with 10/100 Ethernet Port / USB Port

ADSL CPE (Customer Premise Equipment). This can be taken from BSNL at

nominal rental per month or can be purchased outright.

Cities where the service has been launched

Having first launched its Broadband service in Bangalore, Chennai, Hyderabad

and Kolkata on 14th January 2005, BSNL has now extended this service to coverapproximately 708 cities through out the country (as on 01/01/2007)

Digital subscriber line access multiplexer

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A Digital Subscriber Line Access Multiplexer(DSLAM) allows telephone linesto make faster connections to the Internet. It is a network device, located nearthe customer's location, that connects multiple customer Digital Subscriber Lines(DSLs) to a high-speed Internet backbone line using multiplexing techniques. Bylocating DSLAMs at locations remote to the telephone company central office(CO), telephone companies are now providing DSL service to consumers whopreviously did not live close enough for the technology to work.

Path taken by data to DSLAM

1. Residential/commercial source: DSL modem plugged into the customer'scomputer.

2. Local loop: the telephone company wires from a customer to thetelephone company's central office, often called the "last mile".

3. Main Distribution Frame (MDF): a wiring rack that connects outsidesubscriber lines with internal lines. It is used to connect public or privatelines coming into the building to internal networks. In a telco CO, the MDFis generally in proximity to the cable vault and not far from the telephone

switch.4. DSLAM: a device for DSL service. Sending on the customer ordownstream side, it intermixes voice traffic and VDSL traffic onto thecustomer's DSL line. Receiving on that side, it accepts and separatesoutgoing phone and data signals from the customer. It directs the datasignals upstream towards the appropriate carrier's network, and the phonesignals towards the voice switch.

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5. From the DSLAM the telephone wires, now cleansed of DSL signals, gothrough the MDF again to the voice switch so the customer will have dialtone phone service. Old-fashioned voice signals pass between voiceswitch and subscriber line through DSLAM, which does not disturb thembut adds a higher frequency signal to carry data for Internet service.

Role of the DSLAM

The DSLAM at the CO collects the digital signals from its many modem ports andcombines them into one signal, via multiplexing.

Depending on the product, DSLAMs connect DSL lines with some combination ofAsynchronous Transfer Mode (ATM), frame relay or Internet Protocol networks.

In terms of the OSI 7 Layer Model, the DSLAM acts like a massive networkswitch, since its functionality is purely Layer 2.

The aggregated signal then loads onto backbone switching equipment, travelingthrough an access network (AN) also known as a Network Service Provider(NSP) at speeds of up to 10 Gbit/s and connecting to the Internet-backbone.

The DSLAM, functioning as a switch, collects the ADSL modem data (connectedto it via twisted or non-twisted pair copper wire) and multiplexes this data via thegigabit link that physically plugs into the DSLAM itself, into the Telco's backbone.

A DSLAM is not always located in the telephone company central office, but mayalso serve customers within a neighborhood Serving Area Interface (SAI),

sometimes in association with a digital loop carrier. DSLAMs are also used byhotels, lodges, golfing estates, residential neighbourhoods and othercorporations setting up their own private telephone exchange.

Besides being a data switch and multiplexer, a DSLAM is also a large number ofmodems, each mod

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