Business data communications 4e chapter 11

Chapter 11: Approaches to Networking Business Data Communications, 4e 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 Comparison of Networking Options Types of WANs ✘ Circuit-switched ✘ Packet-switched 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) Circuit-Switching Stages ✘ Circuit establishment ✘ Transfer of information ✘ point-to-point from endpoints to node ✘ internal switching/multiplexing among nodes ✘ Circuit disconnect 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 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 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 Public Switched Telephone Network (PSTN) ✘ 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) 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 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 ISDN D Channel ✘ Carries signaling information using commonchannel signaling ✘ call management ✘ billing data ✘ Allows B channels to be used more efficiently ✘ Can be used for packet switching ISDN H Channel ✘ Only available over primary interface ✘ High speed rates ✘ Used in ATM 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 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 + 64kbps D ✘ Fractional use of nB+D possible ✘ Can be used to support H channels 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 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 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 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) 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 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 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 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 circuitswitching) 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 ... 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... 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... 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