Chapter 11: Approaches to Networking

1. Chapter 11:Approaches to Networking Business Data Communications, 4e

2. LANs, WANs, and MANs • Ownership • WANs can be either public or private • LANs are usually privately owned • Capacity • LANs are usually higher capacity, to carry greater internal communications load • Coverage • LANs are typically limited to a single location • WANs interconnect locations • MANs occupy a middle ground

3. Comparison of Networking Options

4. Types of WANs • Circuit-switched • Packet-switched

5. Circuit-Switching • Definition: Communication in which a dedicated communications path is established between two devices through one or more intermediate switching nodes • Dominant in both voice and data communications today • e.g. PSTN is a circuit-switched network • Relatively inefficient (100% dedication even without 100% utilization)

6. Circuit-Switching Stages • Circuit establishment • Transfer of information • point-to-point from endpoints to node • internal switching/multiplexing among nodes • Circuit disconnect

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

8. Information Transfer • Point-to-point transfer from source to node • Internal switching and multiplexed transfer from node to node • Point-to-point transfer from node to receiver • Usually a full-duplex connection throughout

9. Circuit Disconnect • When transfer is complete, one station initiates termination • Signals must be propagated to all nodes used in transit in order to free up resources

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

11. Digital Circuit-Switching Node

12. Circuit Switching Node:Digital Switch • Provides transparent signal path between any pair of attached devices • Typically full-duplex

13. Circuit-Switching Node:Network Interface • Provides hardware and functions to connect digital devices to switch • Analog devices can be connected if interface includes CODEC functions • Typically full-duplex

14. Circuit-Switching Node:Control Unit • Establishes on-demand connections • Maintains connection while needed • Breaks down connection on completion

15. Blocking/Nonblocking Networks • Blocking: network is unable to connect two stations because all possible paths are already in use • Nonblocking: permits all possible connection requests because any two stations can be connected

16. Space-Division Switching Developed for analog environment, but has been carried over into digital communication Requires separate physical paths for each signal connection Uses metallic or semiconductor “gates” Time-Division Switching Used in digital transmission Utilizes multiplexing to place all signals onto a common transmission path Bus must have higher data rate than individual I/O lines Switching Techniques

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

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

19. Control Signaling • Manage the establishment, maintenance, and termination of signal paths • Includes signaling from subscriber to network, and signals within network • In-channel signaling uses the same channel for control signals and calls • Common-channel signaling uses independent channels for controls (SS7)

20. ISDN • 1st generation: narrowband ISDN • Basic Rate Interface (BRI) • two 64Kbps bearer channels + 16Kbps data channel (2B+D) = 144 Kbps • circuit-switched • 2nd generation: broadband ISDN (B-ISDN) • Primary Rate Interface (PRI) • twenty-three 64Kbps bearer channels + 64 data channel (23B+D) = 1.536 Mbps • packet-switched network • development effort led to ATM/cell relay

21. Past Criticism of ISDN • “Innovations Subscribers Don’t Need” , “It Still Doesn’t Network” , “It Still Does Nothing” • Why so much criticism? • overhyping of services before delivery • high price of equipment • delay in implementing infrastructure • incompatibility between providers' equipment. • Didn’t live up to early promises

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

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

24. ISDN Network Architecture • Physical path from user to office • subscriber loop, aka local loop • full-duplex • primarily twisted pair, but fiber use growing • Central office connecting subscriber loops • B channels: 64kbps • D channels: 16 or 64kbps • H channels: 384, 1536, or 1920 kbps

25. ISDN B Channel • Basic user channel (aka “bearer channel”) • Can carry digital voice, data, or mixture • Mixed data must have same destination • Four kinds of connections possible • Circuit-switched • Packet-switched • Frame mode • Semipermanent

26. ISDN D Channel • Carries signaling information using common-channel signaling • call management • billing data • Allows B channels to be used more efficiently • Can be used for packet switching

27. ISDN H Channel • Only available over primary interface • High speed rates • Used in ATM

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

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

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

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

32. Packet-Switched Routing • Adaptive routing changes based on network conditions • Factors influencing routing are failure and congestion • Nodes must exchange information on network status • Tradeoff between quality and amount of overhead

33. Packet-Switched Congestion Control • When line utilization is >80%, queue length grows too quickly • Congestion control limits queue length to avoid througput problems • Status information exchanged among nodes • Control signals regulate data flow using interface protocols (usually X.25)

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

35. Virtual-Circuit Service • External virtual circuit: logical connection between two stations on the network • Internal virtual circuit: specific preplanned route through the network • X.25 usually has a 1:1 relationship between external and internal circuits • In some cases, X.25 can be implemented as a packet-switched network

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

37. WANs for Data • Public packet-switched networks (X.25) • Private packet-switched networks • Leased lines between sites (non-switched) • Public circuit-switched networks • Private circuit-switched networks (interconnected digital PBXs) • ISDN (integrated X.25 and traditional circuit-switching)

38. WAN Considerations • Nature of traffic • stream generally works best with dedicated circuits • bursty better suited to packet-switching • Strategic and growth control--limited with public networks • Reliability--greater with packet-switching • Security--greater with private networks