Broadband Technologies Part 2. Overview Broadband Overview

Broadband TechnologiesPart 2

Overview

Broadband Overview

Why broadband?

• Broadband is generally defined as any sustained speed of 128K or more.

• Broadband can allow remote office staff and small office, home office (SOHOs) to connect to the central office (CO) LAN at high speeds.

• The Internet is moving from dialup modems and slow connections to a world of high-speed broadband using a variety of technologies.

• The most common problem with broadband access is lack of coverage area.

• Broadband options include digital subscriber line (DSL), fast downstream data connections from direct broadcast satellite (DBS), fixed wireless providers, and high-speed cable modems. – Note: These are typically “residential” broadband options

What is broadband? From Whatis.com

• In general, broadband refers to telecommunication in which a wide band of frequencies is available to transmit information. Because a wide band of frequencies is available, information can be multiplexed and sent on many different frequencies or channels within the band concurrently, allowing more information to be transmitted in a given amount of time (much as more lanes on a highway allow more cars to travel on it at the same time). Related terms are wideband (a synonym), baseband (a one-channel band), and narrowband (sometimes meaning just wide enough to carry voice, or simply "not broadband," and sometimes meaning specifically between 50 cps and 64 Kpbs).

• Various definers of broadband have assigned a minimum data rate to the term. Here are a few: – Newton's Telecom Dictionary: "...greater than a voice grade line of 3 KHz...some say [it should

be at least] 20 KHz."

– Jupiter Communications: at least 256 Kbps.

– IBM Dictionary of Computing: A broadband channel is "6 MHz wide."

• It is generally agreed that Digital Subscriber Line (DSL) and cable TV are broadband services in the downstream direction.

Broadband

• Broadband can be delivered to SOHOs via the following technologies:– Satellite – Wireless– Cable– DSL

Satellites

Satellite options

• Older deployments from satellite data providers used asymmetric data paths a satellite downstream to the customer, and a dialup modem for the return path to the provider.

• Today, a transmitter and a receiver are installed for communications.

• Satellite services deliver data at downstream speeds up to 1,500 kbps, and upstream speeds as high as 125 kbps.

• Due to the asymmetric nature of satellite communication, certain applications do not perform very well over satellite such as voice.

Satellite options

• The typical satellite system requires:– small 1.2 m (3.9 ft) satellite dish– two standard coaxial cables to connect the satellite disk to a satellite modem– the satellite modem that connects to a PC through an Ethernet or USB port.

• Satellite networks include:• geostationary orbit satellites (GSOs) (22,300 miles)

– Disadvantage of ¼ second ground-satellite-ground delay

• non-geostationary orbit satellites (NGSOs). – NGSOs include low-earth-orbit satellites (LEOs).

Satellite • big repeaters in the sky• transponders = repeater units carried by the satellite • 10 to 46 transponders per satellite• each transponder can carry up to 3,000 DS-0, (64 kbps) channels• uses radio waves to transmit data• bandwidth in the Mbps

Satellite Extras

Geosynchronous / Geostationary satellites• orbits earth at an altitude of 22,300 miles above the equator• takes 24 hours to orbit earth• appears stationary to the human eye or to the satellite dish

Satellite Extras

Advantages

• large footprint broadcast, send once - receive many

• cost effective for remote areas

• 3 - 4 satellites can cover the earth

Satellite Extras

Disadvantages• promulgation delay: long delay, .12 seconds for a “single hop”

or .24 seconds (1/4 second) for “roundtrip”• limited “orbit space” or positions for these satellites• initially expensive to put in orbit• subject to noise and interference• two-way communications expensive (inexpensive: receive only)

Satellite Extras

Low Earth Orbit Satellites (LEO)• non-stationary satellites of lower orbits which has a smaller promulgation delay• home satellite dishes act more like cellular phones, jumping from satellite to

satellite as the satellites move in and out of range• Teledesic Network (A Microsoft Project): satellites at a low orbit about 435

miles

Satellite Extras

• “An orbiting global constellation of 1,000 small, advanced, semi-autonomous, inter-connected satellites.”

• prevents the signal delays inherent in the use of conventional geostationary communication satellites which operate at a higher altitude

• Hughes Network Systems DirecPC: 400 kbps and in the future up to 92 Mbps

• Iridium System - uses a central control station, 66 satellites, handheld phones and gateways to the PSTN.

Satellite Extras

Wireless Introduction

802.11 Frames – This isn’t Ethernet!

• 802.11 has some similarities with Ethernet but it is a different protocol.• Access Points are translation bridges.• From 802.11 to Ethernet, and from Ethernet to 802.11• The “data/frame body” is re-encapsulated with the proper layer 2 frame.• Certain addresses are copied between the two types of frames.

Distribution System (DS)

General 802.11 Frame

IP Packet

IP PacketLLC

Station Connectivity

Authentication Process

• On a wired network, authentication is implicitly provided by the physical cable from the PC to the switch.

• Authentication is the process to ensure that stations attempting to associate with the network (AP) are allowed to do so.

• 802.11 specifies two types of authentication:– Open-system

– Shared-key (makes use of WEP)

Authentication Process – Open-System

• Open-system authentication really “no authentication”.• Open-system authentication is the only method required by 802.11

– You could buy an AP that doesn’t support Shared-key

• The client and the station exchange authentication frames.

Authentication Process – Shared-Key

• Shared-key authentication uses WEP (Wired Equivalent Privacy) and can only be used on products that support WEP.

• WEP is a Layer 2 encryption algorithm based on the RC4 algorithm.• 802.11 requires any stations that support WEP to also support shared-key

authentication.• Both the client and the AP must have a shared-key, password.


• We’ll look at the configuration of the client and AP later!• Example of open-system authentication.• Note: On “some” systems you can configure authentication (WEP) and

WEP encryption separately. On the ACU you can have open-system authentication and also have WEP encryption. However, if you have Shared-key (WEP) authentication, you must use WEP encryption.


• Authentication– Open-System– Shared-Key

(WEP)

• Encryption– None– WEP

oronly

If using Shared Key (WEP) authentication you are also using WEP encryption.

Open System authentication can take place with or without WEP encryption.

Station Connectivity

• If not configured specifically to look for a network, some client utilities will automatically join the network that meets their vendor’s criteria (not specified in 802.11) such as signal strength and open-system authentication.

• How a station chooses an AP is not specified in 802.11.• Or just find the open-system network and join.

Authentication Request

Beacon SSID = tsunami

Hey, I REALLY didn’t do

anything and I am on the Internet!

Authentication Response (Open-system)

Wireless Bridging

Monthly Leased Line OpEx

2 DS1: $600

1 DS3: $5000

TOTAL: $5600

RBOC provides guaranteed level of service via a Service Level Agreement (SLA)

RBOC

DS3DS1

DS1

New remote office

- No DS1 connection available

Traditional WAN Connectivity

Monthly Line Cost

2 DS1: $600

1 Fractional DS3: $3000

TOTAL: $3600

Wireless Installation Cost

7 350 Series Bridges Installed: $12,500 USD

Pay Back Period: 3 months

New building connected

Self managed

RBOC

22 Mbps

2 Mbps

2 Mbps

5 Mbps

802.11b Connectivity

Monthly Line Cost

2 DS1: $600

1 DS3: $5000

TOTAL: $5600

Wireless Installation Cost

7 1400 Series Bridges Installed: $40,000 USD

Pay Back Period: 8 months

New building connected

Self managed

RBOC

50 Mbps

14 Mbps

14 Mbps

27 Mbps

802.11a/g Connectivity

Optional 2.4GHz Antennas for Long Range

• 13.5 dBi YagiDistances over

7.3 miles @ 2 Mbps11.7 Km @ 2 Mbps3.6 miles @ 11 Mbps5.8 Km @ 11 Mbps

• 21 dBi Solid DishFor distances up to

25+ miles @ 2 Mbps40+ Km @ 2 Mbps 20.5 miles @ 11 Mbps33 Km @ 11 Mbps

802.11b Bridge Application: School District

LincolnElementaryYagi

BodeElementaryYagi

RichardsonElementaryYagi

PriceElementaryYagi

Dewitt ElementaryYagi

BolichMiddle SchoolYagi

RobertsMiddle SchoolDish

Weaver-Special EducationDish

High School 2 BridgesOne 12 dBi omniOne Dish Administration

2 BridgesOne 12 dBi omniOne Yagi

U N I V E R S I T YU N I V E R S I T Y

Channel #11

Channel #6

Channel #1

Cable Technology

Cable options - Benefits

• Cable users access the Internet through a cable modem that connects to the service provider through a cable TV connection.

• In this case, the Internet Service Provider (ISP) is the cable company. • Minimum of 27 Mbps downstream to customers and as much as 9.4

Mbps in the return path. • Another key benefit of constant connectivity

Cable options - DOCSIS

• Cable specifications are defined by Data Over Cable Service Interface Specification (DOCSIS).

• Current specification: DOCSIS 2.0• Defines the use of data over cable and other

functional details.– Defines technical specifications for subscriber

locations and cable operators’ headends (coming).– Allows for interoperability for multi-vendor solutions.

• DOCSIS managed by non-profit CableLabs.

The original cable plant

• Community Antenna Television (CATV), commonly called cable TV, was invented to solve a dire consumer problem, which was poor TV reception.

• Cable service providers (CSPs) offer IP-based data and voice services to the business market as an opportunity to substantially expand their revenue potential and enhance their profit margins.

Cisco UBR 7223 Cable Modem Router as used for high speed internet access over cable systems

Data over cable

• Fiber is used to replace cable amplifiers throughout the plant. • Amplifiers are placed approximately every 610 m (2000 ft) to ensure all RF

signals will be delivered to the home of the end-users with enough power and clarity to receive all channels within the spectrum, which is 50 to 860 MHz.

• In a 20-mile plant, approximately 52 amplifiers would be used to reach the last house 27.3 km (20 miles) away.

• Fiber allows the cable operator to run longer distances with a cleaner signal.

• Fiber also allows the cable operator to remove amplifiers from the link.

• The downstream traffic emanates from the headend and is injected into a trunk cable.

• A cable system consists of the headend and its connected coaxial cables and subscribers.

• The operator of a cable system is referred to as a cable operator. • The headend is where the cable operator puts the different channels on the

frequencies that are compatible with the cable network.• Larger cable systems are much more complex, and they may serve several

communities in a geographical area. • Big companies that operate multiple systems are called multiple system operators

(MSOs).

Data over cable

Data over cable

• Headend at Dascom’s Minnesota Facility - a 45 channel system

Data over cable

• The distribution network, which is made up of fiber and coaxial cabling, delivers television signals to the subscriber.

• The last part and also one of the best known parts of the cable network is what is called the subscriber drop.

• The subscriber drop includes the following:– All cable splitters, couplers, and amplifiers running from the nearest utility pole

or pedestal to devices such as TV sets and cable modems. – Set-top box (STP) – Grounding and attachment hardware – Cable

Data over cable• Broadcast analog signal

strength attenuates or weakens as it moves through conducting material (coax).

• Outside noise, weather, and temperature all affect the impact signal strength through coaxial cable.

• To combat these problems, cable operators came up with the idea to use fiber-optic cable in place of coaxial cable trunks.

• The total system would have both fiber and coaxial cables, which created the term hybrid fiber-coaxial (HFC) networks. www.knology.com

Hybrid fiber-coaxial (HFC) architecture

• To deliver data services over a cable network: – one 6 MHz television channel that is in the 50 MHz to 750 MHz range

is typically allocated for downstream traffic to homes – one 6 MHz channel in the 5 MHz to 42 MHz band is used to carry

upstream signals

• A headend cable modem termination system (CMTS) communicates through these channels with cable modems located in subscriber homes to create a virtual LAN connection

Scientific-Atlanta CMTS


• The cable modem network only operates at Layers 1 and 2

www.twcarolina.com


• An individual cable modem subscriber may experience access speeds from 500 kbps to 2.5 Mbps, depending on the network architecture and traffic load.

• If congestion does begin to occur due to high usage, cable operators have the flexibility to add more bandwidth for data services.

• A cable operator can simply allocate an additional 6 MHz video channel for high-speed data, which would double the downstream bandwidth available to users.

Scientific-Atlanta CMTS

HFC

• Another option for adding bandwidth is to subdivide the physical cable network by running fiber-optic lines deeper into neighborhoods.

• This reduces the number of homes served by each network segment, and it increases the amount of bandwidth available to customers.

www.synchronous.net

Digital signals over RF channels

• When an FM radio is tuned to different radio stations across the spectrum, that radio is being tuned to different electromagnetic frequencies across the spectrum.

• Cable works the same way. Cable carries TV channels or data carriers at different frequencies.

• The equipment in the subscriber home can be tuned to those different frequencies.

• This allows the customer to view the channel on the TV or through a cable modem and route that information to a computer.

• The CATV industry uses the portion of the electromagnetic spectrum between approximately 5 MHz and 1 GHz

Identifying cable technology terms

• Broadband refers to the ability to frequency-division multiplex (FDM) many signals in a wide RF bandwidth over an HFC network. – It also refers to the ability to handle vast amounts of information.

• CATV is originally an acronym for community antenna television. – Today the term is generally accepted to mean cable TV.

• Coaxial Cable is the principal physical media with which CATV systems are built. – Coaxial cable is used to transport RF signals. – Coaxial cable signal loss or attenuation is a function of the diameter of the cable, dielectric

construction, ambient temperature, and operating frequency (f).

• Headend is the location where the cable company aggregates, combines, mixes, and modulates all signals in order to send them downstream. – Upstream signals usually are received in the headend.

• Downstream (DS) is the RF signal flow from headend toward subscribers. – It is also called forward path.

• Upstream (US) is the RF signal flow from the subscribers to the headend. – It is also called the return or reverse path.

DSL Technology

What is DSL?

• While considered an end-to-end solution, DSL only operates on the local loop between the customer premises equipment (CPE) and the DSL access multiplexer (DSLAM).

• A DSLAM is a device in the central office (CO) (sometimes) used to terminate many Layer 1 DSL connections, such as dialup, cable, wireless, and T1.

What is DSL?

• DSL uses the high frequency range of up to about 1 MHz. • For example, asymmetric digital subscriber line (ADSL) uses the frequency range

of about 20 kHz to 1MHz. – ADSL does not overlap the plain old telephone service (POTS) voice frequency range. (300

– 3,400 Hz)– POTS and ADSL service can coexist over the same wire.

• Other DSL variants like single-line digital subscriber line (SDSL) use a frequency range that overlaps the POTS voice frequency range. – POTS and SDSL service cannot coexist over the same wire.

DSL Implementations

• Asymmetric– Faster downstream than upstream transfer rate– ADSL– G.lite ADSL– RADSL– VDSL

• Symmetric– Same downstream and upstream transfer rates– SDSL– SHDSL– HDSL– HDSL2– IDSL

DSL limitations

• The distance from CO to the DSL CPE must be considered. – The longer the distance, the lower the speed.

• The gauge of wire used in the local loop is important. • Thicker wire gauge supports higher speed.

ADSL

• An installer must check with the service provider to determine which modulation technique is being used.

• The modulation method used must be matched between the ADSL CPE (DSL Modem) and the ADSL modems on the DSLAM.

My Alcatel ADSL Modem uses DMT

ADSL and POTS coexistence

• There is a POTS splitter at the central office (CO) (or at home) to split up the POTS called voice and ADSL called data traffic.

• The POTS traffic goes to the voice switch in the CO, and the ADSL traffic goes to the DSLAM in the CO.

• ADSL offloads the data or modem traffic from the voice switch and keeps analog POTS separate from data.

www.consultronics.com/ psts450.htm

ADSL channels and encoding

• There are two competing and incompatible standards (modulation methods) for ADSL. – The official ANSI and ITU standard for ADSL is a system called discrete

multitone (DMT). • Most of the ADSL equipment installed today uses DMT.

– An earlier and more easily implemented modulation method was the carrierless amplitude/phase (CAP) system, which was used on many of the early installations of ADSL.

• Unlike DMT, CAP is proprietary.


• CAP divides the signals on the telephone line into three distinct bands on a single channel. – Voice: 0-kilohertz to 4-kilohertz (kHz) band, same as POTS circuits. – The upstream channel: between 25 and 160 kHz. – The downstream channel: begins at 240 kHz and goes up to a point

that varies depending on a number of conditions, which are line length, line noise, and number of users in a particular telephone company switch.

• The downstream channel has a maximum of about 1.5 MHz.


• DMT also divides signals into separate channels. • DMT divides the data into 250 separate channels, each 4 kHz wide. • Each channel is monitored. • If the quality is too impaired, the signal is shifted to another channel. • This system constantly shifts signals between different channels, searching for the

best channels for transmission and reception. • Since DMT uses 250 channels, it is more complex to implement than CAP. • However, it gives it more flexibility on lines of differing quality.

Data over ADSL with bridging

• DSL is a high-speed Layer 1 transmission technology that works over copper wires.

• ATM is used as the data-link layer protocol over DSL.


• A DSLAM is basically an ATM switch with DSL interface cards in it. • The DSL Layer 1 connection from the CPE is terminated at the DSLAM. • The DSLAM terminates the ADSL connections and then switches the

traffic over an ATM network to an aggregation router (I.e. via another Layer 1 connection such as OC3).

• For example, on the Cisco 6160 DSLAM, it has an OC-3 ATM uplink and can terminate up to 256 DSL subscriber lines.


• To encapsulate an IP packet over an ATM/DSL connection, there are three major approaches:1. RFC 1483/2684 Bridged (Not covered – We need to examine BVI, Bridge Virtual

Interfaces to properly cover this.)2. PPP over Ethernet (PPPoE) 3. PPP over ATM (PPPoA)

Data over ADSL: PPPoE

• To encapsulate an IP packet over an ATM/DSL connection, there are three major approaches:1. RFC 1483/2684 Bridged

2. PPP over Ethernet (PPPoE)

3. PPP over ATM (PPPoA)


• PPPoE is a bridged solution similar to RFC 1483/2684 bridging.

• The CPE is bridging the Ethernet frames from the PC of the end-user to an aggregation router over ATM, like RFC 1483/2684 bridging.

• However, in this case, the Ethernet frame is carrying a PPP frame inside it.

• The PPP session is established between the end-user PC (PPPoE Client) and the aggregation router.


• In the PPPoE architecture, the PC of the end-user runs the PPPoE client software to connect to the ADSL service.

• The PPPoE client software first encapsulates the end-user data into a PPP frame, and then the PPP frame is further encapsulated inside an Ethernet frame.

DSL Modem acting as the client

How does PPPoE work?

• PPP normally works over a point-to-point connection only. • Additional enhancements to PPP were needed to support PPP over an

Ethernet multiaccess environment. • As specified in RFC 2516, PPPoE has two distinct stages.

1. Discovery stage

2. PPP Session stage.

How does PPPoE work?

• There are four steps to the Discovery stage. • PC or router must first identity the Ethernet MAC address of the

peering device and establish a PPPoE SESSION ID.

1. PC or CPE router sends an Initiation packet (PADI). 2. DSLAM responds with an Offer packet (PADO). 3. PC or CPE router continues with the Session phase (PADR) 4. DSLAM continues with the Session phase(PADS).

• Once the PPPoE session begins, PPP goes through the normal LCP and NCP or IPCP process.


• To encapsulate an IP packet over an ATM/DSL connection, there are three major approaches:1. RFC 1483/2684 Bridged

2. PPP over Ethernet (PPPoE)

3. PPP over ATM (PPPoA)

Data over ADSL with PPPoA

• PPPoA is a routed solution, unlike RFC 1483, which is a bridged solution where the CPE is set up as a bridge.

• The CPE is routing the packets from the PC of the end-user over ATM to an aggregation router.

• The PPP session is established between the CPE and the aggregation router. • PPP over ATM requires no host-based software like PPPoE. • The CPE device must have a PPP username and password configured for

authentication to the aggregation router that terminates the PPP session from the CPE.

Data over ADSL with PPPoA

• The aggregation router that authenticates the users can either use a local database on the aggregation router or a Radius (AAA) Server.

• The PPPoA session authentication can be based on PAP or CHAP. • Once the PPP username and password is authenticated, IPCP negotiation

takes place, and the IP address is assigned to the CPE. • Once the IP address has been assigned, there is a host route established

both on the CPE and the aggregation router. • The aggregation router only needs to assign one IP address to the CPE. • The CPE can be configured as a DHCP server and use NAT/PAT to support

multiple hosts connected via Ethernet behind the CPE.

Documents

Broadband Technologies Part 2. Overview Broadband Overview