Wide Area Networks (WAN)Terms that are commonly used by service providers: Customer premises equipment (CPE)

Equipment that’s owned by the subscriber and located on the subscriber’s premises.

Demarcation point The spot where the service provider’s responsibility ends

and the CPE begins Generally a device in a telecommunications closet owned

and installed by the telecommunications company (telco) The customer is responsible to cable (extended demarc)

from this box to the CPE

Wide Area Networks (WAN) Local loop

It connects the demarc to the closest switching office, called a central office.

Central office (CO) This point connects the customers to the provider’s

switching network Central office (CO) is sometimes referred to as a

point of presence (POP) Toll network

It is a trunk line inside a WAN provider’s network This network is a collection of switches and facilities

owned by the ISP

Data Terminal Equipment andData Communication Equipment

Router interfaces are, by default, data terminal equipment (DTE) , and they connect into data communication equipment (DCE) —for example, a channel service unit/data service unit (CSU/DSU)

The CSU/DSU then plugs into a demarcation location (demarc) and is the service provider’s last responsibility

Most of the time, the demarc is a jack that has an RJ-45 (8-pin modular) connector located in a telecommunications closet

The idea behind a WAN is to be able to connect two DTE networks together through a DCE network

Data Terminal Equipment andData Communication Equipment

Data Terminal Equipment andData Communication Equipment

The DCE network includes the CSU/DSU, through the provider’s wiring and switches, all the way to the CSU/DSU at the other end

The network’s DCE device (CSU/DSU) provides clocking to the DTE-connected interface (the router’s serial interface)

As mentioned, the DCE network provides clocking to the router

In a non-production network that do not have a CSU/DSU, there is a need to provide clocking on the DCE end of the cable

WAN Connection Types A WAN can use

a number of different connection types

Figure shows different WAN connection types that can be used to connect your LANs together (DTE) over a DCE network

WAN Connection Types

Leased lines Typically, these are referred to as a point-to-point

connection or dedicated connection A leased line is a pre-established WAN

communications path from the CPE, through the DCE switch, to the CPE of the remote site, allowing DTE networks to communicate at any time with no setup procedures before transmitting data

When cost is no object, it’s really the best choice. It uses synchronous serial lines up to 45Mbps

HDLC and PPP encapsulations are frequently used on leased lines

WAN Connection Types Circuit switching

The big advantage is cost—you only pay for the time you actually use

No data can transfer before an end-to-end connection is established

Circuit switching uses dial-up modems or ISDN, and is used for low-bandwidth data transfers

WAN Connection Types Packet switching

This is a WAN switching method that allows you to share bandwidth with other companies to save money

Packet switching can be thought of as a network that’s designed to look like a leased line, yet charges you (and costs) more like circuit switching

There is a downside: If you need to transfer data constantly, its not convenient

Packet switching will only work well if your data transfers are bursty in nature

Frame Relay and X.25 are packet-switching technologies. Speeds can range from 56Kbps to T3 (45Mbps)

High-Level Data-Link Control(HDLC) Protocol

The High-Level Data-Link Control (HDLC) protocol is a popular ISO-standard, bit-oriented Data Link layer protocol

It specifies an encapsulation method for data on synchronous serial data links using frame characters and checksums

HDLC is a point-to-point protocol used on leased lines No authentication can be used with HDLC Bit-oriented protocols include SDLC, LLC, HDLC, TCP,

IP, and others In byte-oriented protocols, control information is

encoded using entire bytes On the other hand, bit-oriented protocols may use

single bits to represent control information

High-Level Data-Link Control(HDLC) Protocol

Every vendor has a proprietary HDLC encapsulation method

This is because each vendor has a different way for the HDLC protocol to encapsulate multiple Network layer protocols

If the vendors didn’t have a way for HDLC to communicate the different layer 3 protocols, then HDLC would only be able to carry one protocol

This proprietary header is placed in the data field of the HDLC encapsulation

High-Level Data-Link Control(HDLC) Protocol

Point-to-Point Protocol (PPP)

Point-to-Point Protocol (PPP) is a Data Link layer protocol that can be used over either asynchronous serial (dial-up) or synchronous serial (ISDN) media

It uses the LCP (Link Control Protocol) to build and maintain data-link connections

Network Control Protocol (NCP) is used to allow multiple Network layer protocols (routed protocols) to be used on a point-to-point connection

Point-to-Point Protocol (PPP)

When would you choose to use PPP? The basic purpose of PPP is to transport

layer 3 packets across a Data Link layer point-to-point link

It is non-proprietary PPP can encapsulate several layer 3

routed protocols and provide authentication, dynamic addressing, and callback

Point-to-Point Protocol (PPP)

PPP contains four main components: EIA/TIA-232-C, V.24, V.35, and ISDN A

Physical layer international standard for serial communication

HDLC A method for encapsulating datagrams over serial links

LCP A method of establishing, configuring, maintaining, and terminating the point-topoint connection

NCP A method of establishing and configuring different Network layer protocols, designed to allow the simultaneous use of multiple Network layer protocols

Point-to-Point Protocol (PPP)

PPP protocol stack is specified at the Physical and Data Link layers only

NCP is used to allow communication of multiple Network layer protocols by encapsulating the protocols across a PPP data link

Link Control Protocol (LCP) Configuration Options

Authentication This option tells the calling side of the link to send information that can identify the user The two methods are PAP and CHAP

Compression This is used to increase the throughput of PPP connections by compressing the data or payload prior to transmission PPP decompresses the data frame on the receiving

end Error detection PPP uses Quality options to

ensure a reliable, loop-free data link

Link Control Protocol (LCP) Configuration Options

PPP callback PPP can be configured to call back after successful authentication

With callback enabled, a calling router (client) will contact a remote router (server) and authenticate as described in the previous section

Both routers must be configured for the callback feature

Once authentication is completed, the remote router will terminate the connection and then re-initiate a connection to the calling router from the remote router

PPP Session Establishment When PPP connections are started, the links go

through three phases of session establishment, as shown in Figure

PPP Session Establishment

Link-establishment phase LCP packets are sent by each PPP device to configure and test the link

These packets contain a field called the Configuration Option that allows each device to see the size of the data, compression, and authentication

If no Configuration Option field is present, then the default configurations are used

PPP Session Establishment Authentication phase If required, either CHAP

or PAP can be used to authenticate a link Authentication takes place before Network layer

protocol information is read It is possible that link-quality determination may occur

at this same time Network layer protocol phase PPP uses the

Network Control Protocol (NCP) to allow multiple Network layer protocols to be encapsulated and sent over a PPP data link Each Network layer protocol (e.g., IP, IPX, AppleTalk,

which are routed protocols) establishes a service with NCP

PPP Authentication Methods

There are two methods of authentication that can be used with PPP links:

Password Authentication Protocol (PAP) It is the less secure of the two methods Passwords are sent in clear text, and PAP is only

performed upon the initial link establishment When the PPP link is first established, the

remote node sends back to the originating router the username and password until authentication is acknowledged

PPP Authentication Methods

Challenge Handshake Authentication Protocol (CHAP)

Used at the initial startup of a link and at periodic checkups on the link to make sure the router is still communicating with the same host

After PPP finishes its initial link-establishment phase, the local router sends a challenge request to the remote device

The remote device sends a value calculated using a one-way hash function called MD5

The local router checks this hash value to make sure it matches

If the values don’t match, the link is immediately terminated