Lecture 9

Chapter 12: 

Chapter 12 Long distance and local loop technologies

Motivation : 

Motivation Connect computers across – Large geographic distance – Public right-of-way * Streets * Buildings * Railroads

Long-Distance Transmission Technologies : 

Long-Distance Transmission Technologies General solution: lease transmission facilities from telephone company Point-to-point topology NOT part of conventional telephone system Copper, fiber, microwave, or satellite channels available Customer chooses analog or digital

Equipment For Leased Connections : 

Equipment For Leased Connections Analog circuit – Modem required at each end Digital circuit – DSU/CSU required at each end

Digital Circuit Technology : 

Digital Circuit Technology Developed by telephone companies Designed for use in voice system Analog audio from user’s telephone converted to digital format Digital format sent across network Digital format converted back to analog audio

Illustration Of Digitized Signal : 

Illustration Of Digitized Signal Pick nearest digital value for each sample Telephone standard known as Pulse Code Modulation (PCM)

PCM: 

PCM Telephone uses 4000 Hz to represent the human voice Nyquist sampling theorem states we must sample at least twice the highest frequency component Sampled at 8000 samples per second Each sample is encoded as an 8 bit value 8000 sample/sec * 8bits/sample = 64 kbits/sec This is one audio channel data rate

Synchronous Communications: 

Synchronous Communications Voice audio bits must arrive on time and with minimal delay Bursting data can not be tolerated Synchronous transmission assures that bits exit the system at the very same rate that they enter the system Additional information is sent to ensure the data stays synchronous

Illustration Of DSU/CSU : 

Illustration Of DSU/CSU Cost of digital circuit depends on – Distance – Capacity

DSU/CSU : 

DSU/CSU Performs two functions; usually a single ‘‘box’’ Needed because telephone industry digital encoding differs from computer industry digital encoding DSU portion Translates between two encodings CSU portion Terminates line Allows for maintenance

Telephone Standards For Digital Circuits : 

Telephone Standards For Digital Circuits Specified by the telephone industry in each country Differ around the world Are known by two-character standard name In the US these are the “T” standards Note: engineers refer to circuit capacity as ‘‘speed’’

Example Circuit Capacities : 

Example Circuit Capacities Note: T2 not popular

Common Digital Circuit Terminology : 

Common Digital Circuit Terminology Most common in North America – T1 circuit (24 voice circuits) – T3 circuit (28 times T1) d Also available – Fractional T1 (e.g., 64 Kbps circuit) The phone company uses TDM to multiplex multiple voice calls onto a higher speed digital circuit

Inverse Multiplexing : 

Inverse Multiplexing What if we want more than T1 rates, but don’t want to go to T3? Combines two or more circuits into one logical circuit Produces intermediate capacity circuit Special hardware required Needed at each end Called inverse multiplexor Also known as Bonding

Example Of Inverse Multiplexing : 

Example Of Inverse Multiplexing Can alternate between circuits for – Every other bit – Every other byte There are also off the shelf analog modems which will do this with multiple phone lines as well

High-Capacity Digital Circuits : 

High-Capacity Digital Circuits Also available from phone company Use optical fiber Electrical standards called Synchronous Transport Signal (STS) Optical standards called Optical Carrier (OC) These are sometimes referred to as “trunk” lines

High-Capacity Circuits : 

High-Capacity Circuits STS-is standard for electrical signals OC-is standard for optical signals Engineers usually use OC-terminology for everything OC-3 is a popular rate

SONET: 

SONET The phone companies have defined a broad set of standards for digital transmission Called Synchronous Optical Network (SONET) Defines How data is frames How data is multiplexed onto high speed circuits How synchronous clock information is sent SONET describes the logical bit structure, not the line carrier technology So, SONET may run over T1, T3, OC-3 etc

SONET cont: 

SONET cont Example SONET frame Each frame is 810 octets long Octets are divided into 9 “rows” with 90 “columns” per row

The Local Subscriber Loop : 

The Local Subscriber Loop Telephone terminology Refers to connection between residence/business and central office Crosses public right-of-way Originally for analog POTS (Plain Old Telephone Service) Remember: This refers simply as the telephone line that goes from your house to the central office (CO)

Digital Local Loop Technologies : 

Digital Local Loop Technologies Integrated Services Digital Network (ISDN) Digital Subscriber Line (DSL) Cable modems Hybrid Fiber Coax Fiber to the curb

ISDN: 

ISDN Integrated Services Digital Network (ISDN) – Handles voice and data – Relatively high cost for low bandwidth Contains two B channels plus one D channel (2B+D) B = 64 kbs D = 16 kbs Effective data rate is 128 kbs Now is expensive and not used extensively Because analog modems became better Newer, less expensive technologies emerged rapidly to replace ISDN

Asymmetric Digital Subscriber Line (ADSL) : 

Asymmetric Digital Subscriber Line (ADSL) Popular DSL variant Runs over conventional POTS wiring Higher capacity downstream than upstream Most users tend to download more data than they upload Some businesses may want more symmetric versions of DSL Uses frequencies above POTS

Illustration Of ADSL Wiring : 

Illustration Of ADSL Wiring Downstream can reach 6.4 Mbps Upstream can reach 640 Kbps

How ADSL Works: 

How ADSL Works Runs over existing phone wires It turns out that many local loops will support much more than 4000 Hz No two local loops have the same characteristics DLS uses frequency division multiplexing DSL is adaptive. The two ends determine which frequencies the line can handle and which modulation techniques to use

How ADSL Works: 

How ADSL Works FDM in DLS divides the bandwidth into 286 frequencies or subchannels 255 used for downstream 31 used for upstream Adapts modulation on each channel for best performance All these frequencies lie above the 4kHz phone service Inverse multiplexing is used to “bond” these frequencies to a single channel

How ADSL Works: 

How ADSL Works This gives a robust technology that can adapt to changing line conditions Note: ADSL can not guarantee a specific bandwidth It can only do as well as the line will allow Some lines can not use DSL Too far from the CO Some lines contain amplifiers and loads that kill DSL

Other DSL Technologies: 

Other DSL Technologies Symmetric DSL Operates on some local loops where ADSL can’t Symmetric upload and download High rate DSL 1.544 Mbs in both directions Two twisted pairs Tolerates graceful failure Very high Speed DSL Up to 52 Mbs Requires intermediate concentration points

Cable Modems : 

Cable Modems Runs over the cable coax system Send/receive over CATV wiring Use FDM Very high data rate, BUT: Group of subscribers in neighborhood share bandwidth

Cable Modems cont: 

Cable Modems cont Original cable system only had downstream capabilities Amplifiers and such were only one direction System had to be retrofitted to handle two way data Uses FDM. One frequency for a local neighborhood (usually over fiber) Within neighborhood uses TDM

Hybrid Fiber Coax : 

Hybrid Fiber Coax Wiring scheme for cable to allow digital access Optical fiber – Highest bandwidth – Extends from central office to neighborhood concentration points Coaxial cable – Less bandwidth – Extends from neighborhood concentration point to individual subscribers (e.g., residence) Downstream – 6Mbs (but remember TDM) Upstream – 1.5Mbs (same)

Fiber to the Curb: 

Fiber to the Curb Run Optical fiber close to the end subscribers Use copper to feed into homes Could be done either by cable companies or by the phone company

Wireless Alternatives: 

Wireless Alternatives Long distance WiFi TeamWiFi uses 802.11b with special antennas New IEEE standard WiMAX 802.16 155Mbs Intended for Metropolitan Area Networks (MAN) 3G and 4G cellular services Cingular EDGE Verizon and Sprint EVDO Current rates 500kbs to 1Mbs Next gen 5Mbs

Broadcast Satellites: 

Broadcast Satellites Can reach remote geographic locations Satellite contains multiple transmitters (FDM) Within each transmitter uses TDM Can support high data rates Original approach Downlink on the satellite (maybe 1Mbs) Uplink on a phone modem (ick) Newer technology Uplink directly from the satellite dish Maybe 60 to 200 kbs What problems might exist? Think about latency Positioning? Cost?

Summary : 

Summary Technologies exist that span long distances – Leased analog lines (require modems) – Leased digital circuits (require DSU/CSUs) Digital circuits – Available from phone company – Cost depends on distance and capacity – Popular capacities called T1 and T3 – Fractional T1 also available

Summary (continued) : 

Summary (continued) High capacity circuits available – Popular capacities known as OC-3, OC-12 Local loop refers to connection between central office and subscriber Local loop technologies include DSL (especially ADSL) Cable modems Wireless Satellite

Homework: 

Homework Problems 12.3, 12.4, 12.5 (check out Hughesnet), 12.7, 12.8, 12.9 Due Monday October 1