Chapter 9

Communication Circuits

Agenda

• Definition

• Types

• Media

• Ownership

• Error conditions, prevention, and correction

Definitions

• Telecommunication circuit

• Link

• Data link

• Channel

• Data circuit– Forward channel (high speed for data)– Reverse channel (low speed for control)

• Node

Type of Circuits

• Point-to-point circuits

• Multipoint circuits or multidrop circuits

• Two-wire circuits and four-wire circuits

• Analog

• Digital circuits

• ISDN

• T-carrier systems

Point-to-Point Circuits

• Only two points or nodes

• Between two telephones or terminal to computer

• More expense

• Necessary & high volume of traffic

Multipoint circuits

• Multidrop circuit or a network

• Low volume of traffic

• Less expensive

Two-wire and Four-wire circuits

• Two-wire circuits– Half-duplex circuits– Frequency division multiplexing for full-duplex data

flow– Standard dial-up telephone– Less expensive

• Four-wire circuits– Full-duplex circuits– Leased lines– More expensive

Analog Circuits

• Voice grade circuits– Low speed but up to 56,00bps

– Noise

– Error

• Low-speed circuits or subvoice-grade circuits– Subdivide voice grade circuit to 12 or 24 circuits

– Speed ranges between 45 to 200 bps

– Burglar alarm, fire alarm, telegraph, & Teletypewriter usage

• Use Amplifier for long distance

Digital Circuits

• Low error rate, noise, & high speed

• Data service unit/channel service unit (DSU/CSU) – Shape and time the pulses– Less complicated– Less expensive

• Need repeater for long distance

Integrated Services Digital Network (ISDN) - I

• Types of channels– B (bearer) channel: 64kbps data– D (delta) channel: 16kbps signal– Broadband ISDN (BISDN): a full-duplex

circuit or two asymmetrical simplex channel for high speed

• Access methods– Basic rate interface: 2B+D– Primary rate interface: 23B+D– Broadband ISDN (B-ISDN)

Integrated Services Digital Network (ISDN) - II

• Benefits– Efficient – Integrated voice and data– Programmable to meet user’s need– Robust signaling channel for network management– International defined open system interface

• Problems in US– Difficulty in agreeing on precise standards– High cost– Not active marketed service

T-carrier Systems• T1, T2, T3, T4• T2 and T4 for carriers• T1 and T3 for carriers and customers• T1 Line

– Repeater for every mile– 24 circuits of 64,000 bps– Pulse Code Modulation (PCM) or Adaptive Differential

Pulse Code Modulation (ADPCM)– Multiplexing equipment needed– Flexible to reconfiguration, low cost for high quality

and volume

• Fractional T1 (any multiple of 64kbps)

Switched Multimegabit Data Service (SMDS)

• High-speed switched digital service from carrier• Two speeds

– T1 speed (1.544 Mbps)– T3 speed (44.736 Mbps)

• Configuration– Lease line from organization to central office– Normal, shared communication facilities between

central offices

• Benefit: less expensive

Digital Subscriber Line (DSL) - I

• Deliver digitized signals over telephone lines• Higher data communication speed for Internet

usage• xDSL for dedicated, point-to-point, public

network access over twisted pair copper wire– More bandwidth downstream than upstream

– More for individual customer than business organization

Digital Subscriber Line (DSL) - II

• Asymmetric digital subscriber line (ADSL)– Standard voice band– Medium speed upstream band– High speed downstream band

• New offerings– G.Lite ADSL: lower bandwidths for less cost– SDSL (symmetric digital subscriber line): equal speed

in both direction– VDSL (very-high-rate digital subscriber line): high

speed over short twisted pair telephone lines up to 1,000 feet

Circuit Media

• Conducted or guided media– Twisted pair or unshielded twisted pair (UTP)– Shield twisted pair– Coaxial cable– Optical fiber

• Radiated or unguided media (wireless)– Microwave radio– Satellite– Infrared

Twisted Pair

• 22, 24, 26 gauge• Twisted pair or unshielded twisted pair

(UTP)• Electrical noise & other interference• Electrical Industries Association (EIA):

Category 1, 2, 3, 4, 5,5E (extended), 6, and 7 (data grade starting Cat 3)

• Punchdown block

Shielded Twisted Pair

• Pros:– Less interference– Faster speed

• Cons:– More expensive– Physically larger and difficult to connect to a

terminating block

Coaxial Cable

• Pros:– Large bandwidth (400 - 600 MHz)– High capacity– Less interference– Good for noise environment

• Cons:– Tapped easy (security problem)– Bulky to install

Optical Fiber - I

• Types:– Single mode: more expensive & high capacity

– Multimode: less expensive and less capacity

– Inexpensive plastic

– Expensive glass

• Standard– Synchronous optical network (SONET) in US

Optical Fiber - II

• Pros:– High bandwidth

– No interference

– No crosstalk

– Small physical size and light weight for installation

– Very secure

• Cons:– Repeater for long distance

– Expansive

Microwave Radio - I

• Characteristics– Long distance– Straight line transmission– 20-30 miles between transmission towers– Radio license by FCC

• Private microwave links connecting office in a city

Microwave Radio - II

• Pros:– Analog or digital signals

– For voice, data and television signals

– Long distance

• Cons:– Interference by heavy rain

– Straight line transmission

– Security

• Vendors– Motorola

– Rockwell Communication Systems

Satellite - I

• Characteristics:– Geosynchronous orbit

– Microwave radio transmission

– Uplink and downlink

• Types– Geosynchronous satellites (22,300 m)

– Medium earth orbit (MEO) satellites (6,000 m)

– Low earth orbit (LEO) satellite (300 – 1,000 m)

• Usages– Direct broadcast satellite (DBS) system for TV broadcasting

– Digital satellite service (DSS) for high speed Internet receive-only access

– Global telephone systems

Satellite - II

• Pros:– Analog or digital signals– For voice, data and television signals– Long distance

• Cons:– Security and encryption– Propagation delay

Infrared

• Light waves

• Line of sight

• Wireless communication between nearby equipment

Circuit Ownership

• Private circuits• Leased circuits• Bypass• Switched (dial-up) circuits• Selection criteria

– Cost, speed, availability, reliability, maintenance, users’ requirements

– Others?

Circuit Identification

• Blue prints and document circuit and equipment

• Circuit number to each circuit for identification

Multiplexing and Concentrating

• Time division multiplexing (TDM)– Bit, character, or entire message

• Statistical time division multiplexing (STDM) using address field

• Frequency division multiplexing (FDM)• Concentrator using buffering and intelligence

– Circuit multiplexer

– Concentration

– Inverse concentration

Circuit Error Conditions

• Background noise (electrical phenomenon)• Impulse Noise (spike)• Attenuation• Attenuation distortion• Envelope delay distortion (different propagation

delay)• Phase jitter (by carrier signal)• Echo• Crosstalk• Dropouts

Error Prevention

• Line conditioning for leased lines reduce– Attenuation (class C)– Envelope delay distortion (class C)– Noise (class D)– Distortion (class D)

• Methods– Shielding – Improving connection for cables and connectors– Electronic versus mechanical equipment

• Quality• Trouble-free

Error Detection

• Echo checking every character

• Vertical redundancy checking (VRC) or parity checking

• Longitudinal redundancy checking (LRC) or block check character (BCC)

• Cyclic redundancy checking (CRC-16) of polynomial error checking

Parity Check Example

• Parity BCC• 1 1 1 1 1 1 0• 0 0 0 0 0 0 0• 1 0 1 0 1 1 0• 0 0 0 1 0 1 0• 0 0 0 0 1 0 1• 0 0 1 0 0 0 1• 0 1 0 1 0 1 1

• 0 0 1 1 1 0 1 VRC

Error Correction• Equipment and circuit requirement

– Buffer

– Reverse channel or channel turn around

• Retransmission - automatic repeat request (ARQ)• Methods

– Stop and wait ARQ - ACK and NAK: long data block , low error rate, & having a reverse channel

– Continuous ARQ: long propagation time

– Forward error correction (FEC): simplex transmission• using Hamming code, Hagelbarger code, Bose-Chaudhuri

code

• High cost (extra bits & codes)

Guidelines for Wiring

• 25% more capacity

• Internet usage

• Two Cat 5 at each desk

• Fiber optic cable for backbone

• Good wiring contractor

• Others?

Future

• High-speed digital service

• Bandwidth-on-demand

Points to Remember

• Definition

• Types

• Media

• Ownership

• Error conditions, prevention, and correction

Discussion

• Design the communications circuits needs for a bank and state you reasons to insure the security and accuracy