1. CIS 325: Data Communications1 CIS-325 Data Communications Dr. L. G. Williams, Instructor

2. CIS 325: Data Communications2 Chapter Seven Traditional Wide-Area Networks

3. CIS 325: Data Communications3 Network Types n Wide Area Network – Cover large geographical areas, often crossing public right-of-ways – Usually consist of several interconnected switching points n Local Area Network – Small scope, usually a building or cluster – Typically owned by the same organization that owns the equipment

4. CIS 325: Data Communications4 Circuit-Switching n Definition: Communication in which a dedicated communications path is established between two devices through one or more intermediate switching nodes n Dominant in both voice and data communications today n Relatively inefficient (100% dedication even without 100% utilization)

5. CIS 325: Data Communications5 Circuit-Switching Stages n Circuit establishment n Transfer of information – point-to-point from endpoints to node – internal switching/multiplexing among nodes n Circuit disconnect

6. CIS 325: Data Communications6 Circuit Establishment n Station requests connection from node n Node determines best route, sends message to next link n Each subsequent node continues the establishment of a path n Once nodes have established connection, test message is sent to determine if receiver is ready/able to accept message

7. CIS 325: Data Communications7 Information Transfer n Point-to-point transfer from source to node n Internal switching and multiplexed transfer from node to node n Point-to-point transfer from node to receiver n Usually a full-duplex connection throughout

8. CIS 325: Data Communications8 Circuit Disconnect n When transfer is complete, one station initiates termination n Signals must be propagated to all nodes used in transit in order to free up resources

9. CIS 325: Data Communications9 Public Switched Telephone Network (PSTN) n Subscribers n Local loop – Connects subscriber to local telco exchange n Exchanges – Telco switching centers – Also known as end office – >19,000 in US n Trunks – Connections between exchanges – Carry multiple voice circuits using FDM or synchronous TDM – Managed by IXCs (inter-exchange carriers)

10. CIS 325: Data Communications10 Circuit Switching Node: Digital Switch n Provides transparent signal path between any pair of attached devices n Typically full-duplex

11. CIS 325: Data Communications11 Circuit-Switching Node

12. CIS 325: Data Communications12 Blocking/Nonblocking Networks n Blocking: network is unable to connect two stations because all possible paths are already in use n Nonblocking: permits all possible connection requests because any two stations can be connected

13. CIS 325: Data Communications13 Routing in Circuit-Switched Networks n Requires balancing efficiency and resiliency n Traditional circuit-switched model is hierarchical, sometimes supplemented with peer-to-peer trunks n Newer circuit-switched networks are dynamically routed: all nodes are peer-to- peer, making routing more complex

14. CIS 325: Data Communications14 Alternate Routing n Possible routes between two end offices are predefined n Originating switch selects the best route for each call n Routing paths can be fixed (1 route) or dynamic (multiple routes, selected based on current and historical traffic)

15. CIS 325: Data Communications15 Control Signaling n Manage the establishment, maintenance, and termination of signal paths n Includes signaling from subscriber to network, and signals within network n In-channel signaling uses the same channel for control signals and calls n Common-channel signaling uses independent channels for controls

16. CIS 325: Data Communications16 Packet-Switching Networks n Includes X.25, ISDN, ATM and frame-relay technologies n Data is broken into packets, each of which can be routed separately n Advantages: better line efficiency, signals can always be routed, prioritization option n Disadvantages: transmission delay in nodes, variable delays can cause jitter, extra overhead for packet addresses

17. CIS 325: Data Communications17 Packet-Switching Techniques n Datagram – each packet treated independently and referred to as a datagram – packets may take different routes, arrive out of sequence n Virtual Circuit – preplanned route established for all packets – similar to circuit switching, but the circuit is not dedicated

18. CIS 325: Data Communications18 Packet-Switched Routing n Adaptive routing changes based on network conditions n Factors influencing routing are failure and congestion n Nodes must exchange information on network status n Tradeoff between quality and amount of overhead

19. CIS 325: Data Communications19 Packet-Switched Congestion Control n When line utilization is >80%, queue length grows too quickly n Congestion control limits queue length to avoid throughput problems n Status information exchanged among nodes n Control signals regulate data flow using interface protocols (usually X.25)

20. CIS 325: Data Communications20 X.25 Interface Standard n ITU-T standard for interface between host and packet-switched network n Physical level handles physical connection between host and link to the node – Technically X.21, but other standards can be substituted, including RS-232 n Link level provides for reliable data transfer – Uses LAPB, which is a subset of HDLC n Packet level provides virtual circuits between subscribers

21. CIS 325: Data Communications21 ISDN n First generation is narrowband ISDN – uses 64kbps channels – circuit-switched n Second generation is broadband ISDN (B- ISDN – uses higher data rates (hundreds of mbps) – packet-switched network

22. CIS 325: Data Communications22 ISDN Principles n Support of voice and nonvoice using limited set of standard facilities n Support for switched and nonswitched applications n Reliance on 64kbps connections n Intelligence in the networks n Layered protocol architecture (can be mapped onto OSI model) n Variety of configurations

23. CIS 325: Data Communications23 ISDN User Interface n “Pipe” to user’s premises has fixed capacity n Standard physical interface can be used for voice, data, etc n Use of the pipe can be a variable mix of voice and data, up to the capacity n User can be charged based on use rather than time

24. CIS 325: Data Communications24 ISDN Network Architecture n Physical path from user to office – subscriber loop, aka local loop – full-duplex – primarily twisted pair, but fiber’s coming n Central office connecting subscriber loops – B channels: 64kbps – D channels: 16 or 64kbps – H channels: 384, 1536, or 1920 kbps

25. CIS 325: Data Communications25 ISDN B Channel n Basic user channel n Can carry digital voice, data, or mixture – Mixed data must have same destination n Four kinds of connections possible – Circuit-switched – Packet-switched – Frame mode – Semipermanent

26. CIS 325: Data Communications26 ISDN D Channel n Carries signaling information using common-channel signaling – call management – billing data n Allows B channels to be used more efficiently n Can be used for packet switching

27. CIS 325: Data Communications27 ISDN H Channel n High speed rates n Only available over primary interface

28. CIS 325: Data Communications28 ISDN Basic Access n Basic Rate Interface (BRI) n Two full-duplex 64kbps B channels n One full-duplex 16kbps D channel n Framing, synchronization, and overhead bring total data rate to 192kbps n Can be supported by existing twisted pair local loops n 2B+D most common, but 1B+D available

29. CIS 325: Data Communications29 ISDN Primary Access n Primary Rate Interface (PRI) n Used when greater capacity required n No international agreement on rates – US, Canada, Japan: 1.544mbps (= to T1) – Europe: 2.048mbps n Typically 23 64kbps B + 1 64kbps D n Fractional use of nB+D possible n Can be used to support H channels

30. CIS 325: Data Communications30 WANs for Voice n Requires very small and nonvariable delays for natural conversation--difficult to provide this with packet-switching n As a result, the preferred method for voice transmission is circuit-switching n Most businesses use public telephone networks, but a few organizations have implemented private voice networks

31. CIS 325: Data Communications31 WANs for Data n Public packet-switched networks (X.25) n Private packet-switched networks n Leased lines between sites (non-switched) n Public circuit-switched networks n Private circuit-switched networks (interconnected digital PBXs) n ISDN (integrated X.25 and traditional circuit-switching)

32. CIS 325: Data Communications32 WAN Considerations n Nature of traffic – stream generally works best with dedicated circuits – bursty better suited to packet-switching n Strategic and growth control--limited with public networks n Reliability--greater with packet-switching n Security--greater with private networks