Analog to Digital(digital telephony)

• Given an analog function (voice?) we wish to represent it as a sequence of digital values

Pulse Amplitude Modulation (PAM)

Pulse Code Modulation (PCM)• Pulse Code Modulation is a variation of PAM

Measurements are changed to a set of integral values

• How often should the analog signal be sampled?

• A result of Nyquist’s Theorem says

For a signal with frequency xMaximum sampling rate = 2 * x

• For a voice channelMax frequency of voice channel = 4000 HzMaximum sampling rate needed = 2 * 4000

= 8000 samples per second

Pulse Code Modulation

• Establish a set of integral valuesPlus/minus 7 bitsTotal of 8 bits per sampleTotal of 256 distinct values

• This is how voice is encoded as a digital signal• Sample 2 * 4000 = 8000 samples per second• One sample every 125 microseconds

Pulse Code Modulation

These samples are pooled into groups of 24

CodecVOICE 64 Kbps

Codec

1

2

24

Frame

8 bits 8 bits 8 bits 8 bits 8 bits F

8 * 24 = 192 bits1 frame bit

8000 samples/sec

Pulse Code Modulation (PCM)• 24 channels• Each frame is 193 bits• Sampled 8000 times per second

193 bits * 8000 samples = 1,544,000 bits per second

• This is called a T1 facility• The circuit is also called a Digital Service-1 (DS-1)• This is the fundamental digital circuit from the telephone

company• A single channel within the DS-1 is often called a DS-0• There is a European standard that groups 30 channels

instead of 24 – Called an E1 circuit

Digital Service• Uses Alternate Mark Inversion or AMI

encoding

• Requires all one bits to be alternating positive and negative voltages

• Zero bits have 0 voltage

0 1 1 1 0 0 0 1

Multiplexing

• To more fully utilize channel capacity more than one transmission can ‘share’ a single channel

• The is called multiplexing• Multiplexing can take many forms

Frequency Division

The total bandwidth is divided a number into a number of frequency ranges

Each transmission utilizes one of these ranges

Cable Television• A single coax cable system can have a total capacity of

350 Mhz and to over 650 Mhz• Each TV channel occupies 6 Mhz

Channel Band Number Mhz

2 54-60 3 60-66 4 66-72…… ………. 20 156-162……. ………. 40 318-324……. ……….. 61 444-450

Wave Division Multiplexing

• Using multiple ‘colors’ over fiber optical cable is a form of frequency division

• Multiple transmission can exist concurrently over the same fiber cable

• Each uses a different wave length, typically called a ‘lamda’

Time Division Multiplexing

• Total bandwidth is divided into a series of ‘n’ time slots

• Each transmission gets one of the time slots in a round robin fashion

T1

T2

T3

T4

T1

T2

T3

T4

T1T2T3T4T1T2T3T4

Communications Channel

Statistical Multiplexing

• Like time division except when a station does not have data to send, slot is passed on to next transmission

• Could have variable length data for each transmission

• Any one link can actually get use of full link of other are idle

• Better link utilization• More overhead

Statistical Multiplexing

T1

T2

T3

T4 T4

T3

T2

T1

T1dd T3dd T1d T4ddd T3dd T1d

Communications Channel

Inverse Multiplexing

• A number of slower speed lines are grouped together to form a higher speed circuit

MUX

MUX

MUX

MUX

Multiplexing

Inverse multiplexing

Recall PCM

• Establish a set of integral valuesPlus/minus 7 bitsTotal of 8 bits per sampleTotal of 256 distinct values

• This is how voice is encoded as a digital signal• Sample 2 * 4000 = 8000 samples per second• One sample every 125 microseconds

Pulse Code Modulation

These samples are pooled into groups of 24

CodecVOICE 64 Kbps

Codec

1

2

24

Frame

8 bits 8 bits 8 bits 8 bits 8 bits F

8 * 24 = 192 bits1 frame bit

8000 samples/sec

Multiplexing Voice Conversations

Digital Hierarchy

Phone Company pools T1s into larger circuits

24 DS-0

64 kbps

4 DS-1

1.544 mbps

DS-2

6.312 Mbps DS – 3

44.736 Mbps DS-4

7 DS-2

6 DS-3

274.176

Synchronous Optical NetworkSONET

• Standard for digital transmission over fiber optics• Also called Synchronous Digital Hierarchy (SDH)

by the ITU• Provides for interoperability between carriers• Standardizes US and European hierarchy• Extends Digital Hierarchy beyond existing DS-3

and DS-4• Makes provision for Operations, Administration,

and Maintenance

SONET

• Synchronous• Basic frame

810 bytes

8,000 frames per second

6480 bits per frame or 51.84 Mbps

• This forms a basic SONET channel

6480 6480 6480 6480

Synchronous Transport Signal -1

STS-1

SONET Signal Hierarchy

Integrated Services Digital NetworkISDN

• Designed to provide digital services to end users (total telephone redesign)

• Approved in 1984 (ISO, ITU)• All digital services all the way to the home• Uses same twisted pair cabling• Initially slow implementation• Usually priced as a measured service, even for

local calling• Very limited user acceptance

ISDN Services

• Basic Rate Interface (BRI)

2 B (basic rate) channelseach 64 Kbpsused for voice, concurrent data

1 D channel16 Kbps, used for signaling

• Primary Rate Interface (PRI)

23 B channels1 D channel (at 64 Kbps)

Digital Subscriber Line (DSL)

• Effort by telephone company to bring lower cost, high speed service to home

• Provides concurrent voice/data

• Uses regular phone twisted pair

• Intended to compete with cable service

• Multitude of variations

• Speeds from several hundred Kbps to several Mbps

DSL Variants

• Asymmetric DSL Frequency division multiplexing 0 – 25 Khz Voice 25 – 200 Khz Upstream data 250 – 1000 Khz Downstream data

• Rates up to 1 Mbps upstream

8 Mbps Downstream

are theoretically possible

Asymmetric DSL

Digital Subscriber Lines

Operation of ADSL using discrete multitone modulation (DMT).

Upstream: Usually 512 Kbps or less

Downstream: Up to 8 Mbps

Digital Subscriber Lines

Bandwidth versus distanced over category 3 UTP for DSL.

Other DSL Variants

• Symmetric DSL (SDSL) Upstream and downstream speeds the same Single copper pair

• High Data Rate DSL (HDSL) Symmetric services Requires two copper pairs Cost effective way to deliver T1 equivalent service

• Very High Data Rate (VDSL) Asymmetric Up to 50 Mbps downstream possible

DSL Modem

Digital Subscriber Line Access Multiplexor

Cable Modems

• Why cable? Infrastructure exists Coax cable better characteristics than twisted pairs

• TV channels are 6 Mhz Allows for high speed data What is maximum possible data rate?

• Shared channel – many users on a segment• Now a standard

Data Over Cable System Interface Standard

DOCSIS

Cable Spectrum Allocation

Frequency allocation in a typical cable TV system used for data

Cable modems

• Upstream: 5-42 Mhz Uses an encoding scheme of 2 bits per baud 1 baud per Hz 12 Mbps per channel possible per 6 Mhz

channel

• Downstream: 550-750 Mhz Uses 64-QAM 6 bits/baud, 1 bit is used for error correction Theoretical 30 Mbps per 6 Mhz channel

Wireless Broadband Alternatives

• Current efforts are to provide 802.11 wireless in large areas through meshed access points

• Wireless broadband to the home standard being developed

• IEEE 802.16 - WiMAX Potential of upto155 Mbps Over wide areas (miles) Considered a replacement for last mile

http://grouper.ieee.org/groups/802/16/index.html