CSCI 4550/8556Computer Networks

Comer, Chapter 12:Long Distance Digital Connection Technologies

IntroductionPrevious technologies cover “short” distances (e.g. campus, building, a few city blocks) They can be extended over short distances.We also need to cover longer distances - e.g., San Francisco to Boston The name for long distance technology is WAN - Wide Area Network There are two categories:

Long distance between other networks “Local loop”

Digital Telephony

The telephone system spans long distances. The use of digital telephony improves long distance service:

Better quality than analog technologyMore logical connections in the same wire than if analog technology is used.

Digitizing Voice

Problem: how to encode an analog audio signal as digital data Solution:

Sample the audio signal at periodic intervals Convert the samples to digital using an A-to-D converter Send the digital data samples over wire (or fiber, or …)Regenerate the audio using a D-to-A converter

ExampleThe continuous line shows one possible analog signal (e.g. voice).The vertical lines show possible digital equivalents resulting from periodic sampling of the analog signal.

Sampling Parameters

We want to carry signals up to 4000Hz (recall the telephone system bandwidth) To do this, we select a sampling rate of 8000Hz (twice the maximum frequency in the source)Each sample is in the range 0-255 (so we can use 8 bits per sample).The standard for this is called Pulse Code Modulation (PCM).

Synchronous Communication

Converting these samples back to audio requires that the data be available “on time.”Digital telephony systems use clocking for synchronous data delivery.The samples must not be delayed as network traffic increases.

Using Digital Telephony for Data Delivery

So, digital telephony can handle synchronous data delivery. Can we use that for arbitrary data delivery? An Ethernet frame is not 8-bit PCM synchronous data.Thus, to send Ethernet frames we need to convert data formats...

Conversion for Digital Circuits

To use digital telephony for data delivery: Lease a point-to-point digital circuit between sites; and Convert between local and PCM formats at each end.

The conversion uses a Data Service Unit / Channel Service Unit (DSU/CSU) at each end of the line.

CSU - manages control functions DSU - converts data

Using a DSU / CSU

Telephone Standards

Several standards exist for data transmission rates in telephone systems. These are called the T-series standards, and are similar throughout the world.

Name Bit Rate

Voice Circuits

- 0.064 Mbps 1

T1 1.544 Mbps 24

T2 6.312 Mbps 96

T3 44.736 Mbps 672

Intermediate CapacityThe price for a leased line does not go up linearly with speed. For example, the cost for a T3 line is less than the cost for 28 T1 lines.However, if all you need is 9 Mbps, the cost for a T3 line is greater than the cost for 6 T1 lines.Solution: combine multiple T1 lines with an inverse multiplexor.

Higher Capacity Circuits

Standard name Optical name Bit rate Voice circuits

STS-1 OC-1 51.840 Mbps 810

STS-3 OC-3 155.520 Mbps 2,430

STS-12 OC-12 622.080 Mbps 9,720

STS-24 OC-24 1,244.160 Mbps 19,440

STS-48 OC-48 2,488.320 Mbps 38,880

About the Terminology

T-standards define the underlying bit rate; Digital Signal Level standards (DS standards) define:

how to multiplex calls The effective bit rates A T1 line transmits data at DS-1 rate

Synchronous Transport Signal (STS) standards define high speed connections over copper, Optical Carrier (OC) standards are for fiber The C suffix indicates concatenated:

OC-3 == three OC-1 circuits at 51.84 Mbps OC-3C == one 155.52 Mbps circuit

SONETSynchronous Optical Network (SONET) defines how to use high-speed connections

Framing: STS-1 uses 810 bytes per frame Encoding: Each sample travels as one octet in payload

Payload changes with data rate STS-1 transmits 6,480 bits in 125 microseconds (== 810 octets) STS-3 transmits 19,440 bits in 125 microseconds (==2,430 octets)

Getting To Your Home

The term local loop describes the connection from a telephone end office to your home.This is sometimes called POTS (Plain Old Telephone Service). The legacy infrastructure is copper wires.Other available connections to your home include cable TV, wireless, and electric power.

ISDN (Integrated Services Digital Network)

Provides digital service (like T-series) on existing local loop copper wiring. Three separate circuits, or channels, in a typical home connection:

Two B channels, 64 Kbps each; equivalent to two voice circuits One D channel, 16 Kbps; used for control functions

Often written as 2B+D; called Basic Rate Interface (BRI) ISDN has been slow to catch on, because…

It is/was expensive. It is charged by time used. It is now (almost) equaled by analog modems.

DSL (Digital Subscriber Line)DSL is a family of technologies.

It is sometimes called xDSL. It provides high-speed digital service over the existing local loop.

One common form is ADSL (Asymmetric DSL).It has a higher speed into a home than out of it.More bits flow in (“downstream”) than out (“upstream”).

ADSL maximum speeds: 6.144 Mbps downstream 640 Kbps upstream

ADSL TechnologyUses existing local loop copper

Takes advantage of higher frequencies on most local loops

Can be used simultaneously for POTS

Adaptive TransmissionIndividual local loops have different transmission characteristics

Different maximum frequencies Different interference frequencies

ADSL uses FDM 286 frequencies

• 255 downstream • 31 upstream • 2 control

Each frequency carries data independently All frequencies are outside the audio range The bit rate adapts separately to the quality in each frequency

Other DSL Technologies

SDSL (Symmetric DSL) provides divides frequencies evenly. HDSL (High-rate DSL) provides DS1 bit rate in both directions

Useful over short distances Requires a four wire circuit (POTS is two wires)

VDSL (Very high bit rate DSL) provides up to 52 Mbps

Very short distance Requires an Optical Network Unit (ONU) as a relay

Cable Modem Technologies

Cable TV already brings high bandwidth coaxial cable into your home.Cable modems encode and decode data from cable TV coaxial cable:

One modem in the cable TV center connects to the networkOne modem in your home connects to your computer

Features of Cable Modems

The available bandwidth is multiplexed among all users.It is a multiple access medium (like Ethernet): your neighbor can see your data!Not all cable TV coaxial cable plants are bidirectional; thus not cable TV systems can support networking as well as television.

Upstream Communication

Cable TV requires sending in one direction only.The signal is broadcast from a central location.Amplifiers boost the signal as it travels through the network

Amplifiers are unidirectional, so how can data travel successfully “opposite” to television signals? Solutions:

Retrofit the system with bi-directional amplifiers.Use an alternate upstream path - e.g., dialup.

Alternatives

In addition to POTS, ISDN, xDSL and cable TV coaxial cable, there are other alternatives for connecting residences to networks.Satellite systems, either per subscriber or per “neighborhood”“Fiber to the curb”

Fiber to each subscribers home is much too expensiveFiber to the neighborhood is a potential, then use short-distance copper local loops to provide connectivity to fiber cable.

Summary

WAN links between sites use digital telephony

Based on digitized voice service Several standard rates Requires conversion via DSU/CSU

Local loop technologies ISDN xDSL Cable modem Satellite Fiber to the curb