1 Chapter 9 Communications Circuits

2 Types of Circuits Point-to-point Connects only two nodes Multipoint Connects several nodes 2-wire circuits Signal + return Half-duplex data, except can use FDM to get full duplex Phone line 4-wire circuits Full duplex data

3 Digital Circuits Signal distortions easier to correct than on analog lines No A/D or D/A conversions

4 ISDN Basic Rate Interface (BRI) Two B (bearer) channels + One D (delta) channel B channel has 64 kbps capacity D channel has 16 kbps capacity B channels can be combined or separate, data or digitized voice Primary Rate Interface (PRI) 23 B (bearer) channels + One D (delta) channel B channel has 64 kbps capacity D channel has 64 kbps capacity Total capacity is T1

5 ISDN BRI and PRI

6 ISDN Example PRI can be set up with 772 kbps (13 B channels) for video 384 kbps (6 B) for 6 voice channels 256 kbps (4 B) for data Can reconfigure at later time

7 Broadband ISDN (BISDN) Services Full duplex at Mbps Full duplex at Mbps Asymmetrical circuit with 2 simplex channels One at Mbps One at Mbps Asynchronous Transfer Mode is used to transport ISDN Being deployed slowly

8 ISDN Benefits Efficient multiplexed access to public network Capability to support integrated voice and data Signaling channel for network management Internationally defined open system interface Popular in Europe and Japan Slow rollout in U.S.

9 T-Carrier Systems High-speed digital transmission system Time-division multiplexing T-# identifies the carrier type DS# identifies the signal

10 T-Carrier System

11 T-1 Uses two wire pair for full duplex Carries kbps channels plus 8 kbps signaling One way Channels multiplexed on T-1 line at transmitter and demultiplexed at receiver May be cheaper to lease a T-1 line and use less than full capacity than to lease multiple low-capacity lines Leased point-to-point

12 Fractional T-1 IXCs are offering capacities less than Mbps at 128 kbps minimum plus 64 kbps intervals CO runs full T-1 to customer, usually reserves all 24 channels for customer future expansion

13 Switched Megabit Data Service (SMDS) Connectionless No dedicated line between locations Based on Asynchronous Transfer Mode (ATM) technology Speeds: Mbps Mbps Not point-to-point like T-1 Lease circuit to nearest carrier’s office at each end of connection Carrier handles circuit in between end offices

14 Digital Subscriber Line (DSL) Asymmetric DSL (ADSL) Developed in 1987 Designed to deliver digitized video to homes over twisted pair New application: Internet access Full rate speed is 1.5 – 8 Mbps downstream, up to 1 Mbps upstream Limit is 13,000 ft Service available in Atlanta area

15 ADSL Bandwidth Allocation

16 DSL Other DSLs G.Lite 1.5 Mbps downstream, 384 kbps upstream Symmetric DSL (SDSL) 784 kbps max each way Mainly business customers Very-high-rate DSL (VDSL) Asymmetric Downstream  51 – 55 Mbps for 1000 feet or less  13 Mbps for 4000 feet or less Upstream  1.6 – 2.3 Mbps

17 Circuit Media – Copper Wire Twisted pair STP - shielded twisted pair UTP - unshielded twisted pair STP is better, but harder to terminate and more expensive EIA standards for UTP Category 1 (Cat 1) - basic, not for data Cat 2 - up to 4 Mbps Cat to 16 Mbps, at least 3 twists per inch Cat Mbps Cat Mbps, 3 to 4 twists per inch Current standard-use cable Cat 5E – up to 1 Gbps Look for Cat 6 and Cat 7 in the future

18 Coaxial Cable Bandwidth about 400 to 600 MHz Used to carry 10,800 telco FDM voice calls Repeaters every mile Used by cable TV companies for video, data Shielded Improved immunity to electromagnetic interference More difficult to work with than UTP Bulky, more expensive

19 Trunk Cable with Multiple Coax

20 Optical Fiber Advantages High bandwidth 135+ Mbps over 40 miles 1.7+ Gbps over short distances Much lower loss than copper Immune to electrical interference No crosstalk Smaller and lighter than copper cables Secure Disadvantages More expensive than copper Harder to terminate

21 Optical Fiber Single mode  5 micron core Multimode  62.5 micron core More dispersion than single mode Lower BW Used in undersea cables First was TAT-8 40,000 simultaneous calls

22 Optical Cable SONET Synchronous optical network Primarily used by phone companies Replacing T-3 and T-4 Data rates to Gbps

23 Atlantic Ocean Cables

24 Microwave Radio Most common medium for long distance 4 GHz to 28 GHz carrier frequencies 30 MHz channels Up to 6000 voice circuits Line-of-sight repeater locations 20 to 30 miles apart Mostly analog, some digital Transmission affected by rain Sometimes used for private networks

25 Microwave Tower

26 Satellite Usually placed in geosynchronous orbits (GEO) 23,000 miles above the earth Appears stationary Positioned to cover certain areas Uplink - transmission to satellite from base station Downlink - transmission from satellite to a base station

27 Satellite - Advantages and Disadvantages Main advantage Communication between widely separated locations without telco infrastructure Disadvantages Security Anyone can receive transmissions Must encrypt Time delay High orbit - delays on the order of 0.25 seconds minimum Very annoying Telcos switched back to terrestrial links where possible Satellites still used for international calls Weather can affect link

28 Other Satellite Applications Used for PCS and paging systems Low Earth Orbit (LEO)  435 miles up  100 minute orbits About 70 satellites required for full coverage Medium Earth Orbit  6200 miles up  6 hour orbits 9 to 12 satellites required for full coverage Satellite TV Direct Broadcast Satellite (DBS) also known as Direct Satellite Service (DSS) Downlink only Competes with cable TV

29 Comparison of Media

30 Circuit Ownership Private circuit Maintained by company other than a common carrier Typical application within a company’s campus or buildings Very inexpensive once installed

31 Circuit Ownership Leased circuit Circuits owned by common carrier but leased to a customer for their exclusive use Reasons to lease a circuit A private circuit cannot be installed When leased circuit cost is less than dial up cost for time used When 4-wire service is needed (full duplex) For high-speed transmission Cost Based on speed and distance Will involve LEC and/or IXC Leased 4-wire circuit  10% more than leased 2-wire

32 Circuit Ownership Switched circuits (dial-up) Normal telephone circuit (half duplex)

33 Multiplexing and Concentrating Time-Division Multiplexing (TDM) A circuit’s capacity is divided into time slots A TDM frame is made up of a bit, character, or message (depending on method) from each source At receiver, frames disassembled and signals reconstructed Ex.: A circuit with 9600 bps can contain four multiplexed 2400 bps channels Statistical Time Division Multiplexing (STDM) Time slots not fixed STDM multiplexer samples the input sources and only sends data from active sources Addresses accompany source characters

34 FDM and TDM: Time x Bandwidth

35 TDM

36 STDM

37 Multiplexing and Concentrating Concentrator Combines several low-speed circuits into one high-speed circuit Inverse concentrator Divides up data on a high-speed line and puts it on several low-speed lines

38 Network Access Technology Tradeoffs

39 Case Study: Dow Corning’s Data Communications Large LAN at Midland headquarters PCs and terminals communicate with servers and mainframe Speed at least 16 Mbps Response time less than 1 second Facilities near Midland Using high-speed digital lines Excellent response times Lines not heavily used

40 Case Study: Dow Corning’s Data Communications Outside Midland Use 9.6 kbps connecting sales offices and warehouses on multipoint leased lines Connections to large plants use digital circuits of 128 kbps or higher Fiber optics Used on campus Carrying video and data Several point-to-point T1s between facitilites Multiplex voice, data, and video

41 Case Study: Dow Corning’s Data Communications Broadband coax Used in Midland plant for in-plant television now Overseas Using frame relay 128 kbps to 512 kbps