# Analog to Digital (digital telephony) Given an analog function (voice?) we wish to represent it as a sequence of digital values Pulse Amplitude Modulation.

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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 x Maximum sampling rate = 2 * x For a voice channel Max frequency of voice channel = 4000 Hz Maximum sampling rate needed = 2 * 4000 = 8000 samples per second

Pulse Code Modulation Establish a set of integral values Plus/minus 7 bits Total of 8 bits per sample Total 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 Codec VOICE64 Kbps Codec 1 2 24 Frame 8 bits 8 bits 8 bits 8 bits 8 bitsF 8 * 24 = 192 bits 1 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 ChannelBand Number Mhz 2 54-60 3 60-66 4 66-72 ……………. 20 156-162 …….………. 40318-324 …….……….. 61444-450

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 T1T2T3T4 T1T2 T3 T4 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 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 MUXMUX MUXMUX MUXMUX MUXMUX Multiplexing Inverse multiplexing

Recall PCM Establish a set of integral values Plus/minus 7 bits Total of 8 bits per sample Total 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 Codec VOICE64 Kbps Codec 1 2 24 Frame 8 bits 8 bits 8 bits 8 bits 8 bitsF 8 * 24 = 192 bits 1 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 Network SONET 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 Synchronous Transport Signal -1 STS-1

SONET Signal Hierarchy

Integrated Services Digital Network ISDN 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) channels each 64 Kbps used for voice, concurrent data 1 D channel 16 Kbps, used for signaling Primary Rate Interface (PRI) 23 B channels 1 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 KhzVoice  25 – 200 KhzUpstream data  250 – 1000 KhzDownstream 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

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