Chapter 11 Approaches to Networking Business Data Communications

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

LANs, WANs, and MANs 8 Ownership 8 WANs can be either public or private 8 LANs are usually privately owned 8 Capacity 8 LANs are usually higher capacity, to carry greater internal communications load 8 Coverage 8 LANs are typically limited to a single location 8 WANs interconnect locations 8 MANs occupy a middle ground

Comparison of Networking Options

Types of WANs 8 Circuit-switched 8 Packet-switched

Circuit-Switching 8 Definition: Communication in which a dedicated communications path is established between two devices through one or more intermediate switching nodes 8 Dominant in both voice and data communications today 8 e. g. PSTN is a circuit-switched network 8 Relatively inefficient (100% dedication even without 100% utilization)

Circuit-Switching Stages 8 Circuit establishment 8 Transfer of information 8 point-to-point from endpoints to node 8 internal switching/multiplexing among nodes 8 Circuit disconnect

Circuit Establishment 8 Station requests connection from node 8 Node determines best route, sends message to next link 8 Each subsequent node continues the establishment of a path 8 Once nodes have established connection, test message is sent to determine if receiver is ready/able to accept message

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

Circuit Disconnect 8 When transfer is complete, one station initiates termination 8 Signals must be propagated to all nodes used in transit in order to free up resources

Public Switched Telephone Network (PSTN) 8 Subscribers 8 Local loop 8 Connects subscriber to local telco exchange 8 Exchanges 8 Telco switching centers 8 Also known as end office 8>19, 000 in US 8 Trunks 8 Connections between exchanges 8 Carry multiple voice circuits using FDM or synchronous TDM 8 Managed by IXCs (inter -exchange carriers)

Digital Circuit-Switching Node

Circuit Switching Node: Digital Switch 8 Provides transparent signal path between any pair of attached devices 8 Typically full-duplex

Circuit-Switching Node: Network Interface 8 Provides hardware and functions to connect digital devices to switch 8 Analog devices can be connected if interface includes CODEC functions 8 Typically full-duplex

Circuit-Switching Node: Control Unit 8 Establishes on-demand connections 8 Maintains connection while needed 8 Breaks down connection on completion

Blocking/Nonblocking Networks 8 Blocking: network is unable to connect two stations because all possible paths are already in use 8 Nonblocking: permits all possible connection requests because any two stations can be connected

Switching Techniques 8 Space-Division Switching 8 Developed for analog environment, but has been carried over into digital communication 8 Requires separate physical paths for each signal connection 8 Uses metallic or semiconductor “gates” 8 Time-Division Switching 8 Used in digital transmission 8 Utilizes multiplexing to place all signals onto a common transmission path 8 Bus must have higher data rate than individual I/O lines

Routing in Circuit-Switched Networks 8 Requires balancing efficiency and resiliency 8 Traditional circuit-switched model is hierarchical, sometimes supplemented with peer-to-peer trunks 8 Newer circuit-switched networks are dynamically routed: all nodes are peer-topeer, making routing more complex

Alternate Routing 8 Possible routes between two end offices are predefined 8 Originating switch selects the best route for each call 8 Routing paths can be fixed (1 route) or dynamic (multiple routes, selected based on current and historical traffic)

Control Signaling 8 Manage the establishment, maintenance, and termination of signal paths 8 Includes signaling from subscriber to network, and signals within network 8 In-channel signaling uses the same channel for control signals and calls 8 Common-channel signaling uses independent channels for controls (SS 7)

ISDN 81 st generation: narrowband ISDN 8 Basic Rate Interface (BRI) 8 two 64 Kbps bearer channels + 16 Kbps data channel (2 B+D) = 144 Kbps 8 circuit-switched 82 nd generation: broadband ISDN (B-ISDN) 8 Primary Rate Interface (PRI) 8 twenty-three 64 Kbps bearer channels + 64 data channel (23 B+D) = 1. 536 Mbps 8 packet-switched network 8 development effort led to ATM/cell relay

Past Criticism of ISDN 8“Innovations Subscribers Don’t Need” , “It Still Doesn’t Network” , “It Still Does Nothing” 8 Why so much criticism? 8 overhyping of services before delivery 8 high price of equipment 8 delay in implementing infrastructure 8 incompatibility between providers' equipment. 8 Didn’t live up to early promises

ISDN Principles 8 Support of voice and nonvoice using limited set of standard facilities 8 Support for switched and nonswitched applications 8 Reliance on 64 kbps connections 8 Intelligence in the networks 8 Layered protocol architecture (can be mapped onto OSI model) 8 Variety of configurations

ISDN User Interface 8“Pipe” to user’s premises has fixed capacity 8 Standard physical interface can be used for voice, data, etc 8 Use of the pipe can be a variable mix of voice and data, up to the capacity 8 User can be charged based on use rather than time

ISDN Network Architecture 8 Physical path from user to office 8 subscriber loop, aka local loop 8 full-duplex 8 primarily twisted pair, but fiber use growing 8 Central office connecting subscriber loops 8 B channels: 64 kbps 8 D channels: 16 or 64 kbps 8 H channels: 384, 1536, or 1920 kbps

ISDN B Channel 8 Basic user channel (aka “bearer channel”) 8 Can carry digital voice, data, or mixture 8 Mixed data must have same destination 8 Four kinds of connections possible 8 Circuit-switched 8 Packet-switched 8 Frame mode 8 Semipermanent

ISDN D Channel 8 Carries signaling information using commonchannel signaling 8 call management 8 billing data 8 Allows B channels to be used more efficiently 8 Can be used for packet switching

ISDN H Channel 8 Only available over primary interface 8 High speed rates 8 Used in ATM

ISDN Basic Access 8 Basic Rate Interface (BRI) 8 Two full-duplex 64 kbps B channels 8 One full-duplex 16 kbps D channel 8 Framing, synchronization, and overhead bring total data rate to 192 kbps 8 Can be supported by existing twisted pair local loops 82 B+D most common, but 1 B+D available

ISDN Primary Access 8 Primary Rate Interface (PRI) 8 Used when greater capacity required 8 No international agreement on rates 8 US, Canada, Japan: 1. 544 mbps (= to T 1) 8 Europe: 2. 048 mbps 8 Typically 23 64 kbps B + 1 64 kbps D 8 Fractional use of n. B+D possible 8 Can be used to support H channels

Packet-Switching Networks 8 Includes X. 25, ISDN, ATM and frame-relay technologies 8 Data is broken into packets, each of which can be routed separately 8 Advantages: better line efficiency, signals can always be routed, prioritization option 8 Disadvantages: transmission delay in nodes, variable delays can cause jitter, extra overhead for packet addresses

Packet-Switching Techniques 8 Datagram 8 each packet treated independently and referred to as a datagram 8 packets may take different routes, arrive out of sequence 8 Virtual Circuit 8 preplanned route established for all packets 8 similar to circuit switching, but the circuit is not dedicated

Packet-Switched Routing 8 Adaptive routing changes based on network conditions 8 Factors influencing routing are failure and congestion 8 Nodes must exchange information on network status 8 Tradeoff between quality and amount of overhead

Packet-Switched Congestion Control 8 When line utilization is >80%, queue length grows too quickly 8 Congestion control limits queue length to avoid througput problems 8 Status information exchanged among nodes 8 Control signals regulate data flow using interface protocols (usually X. 25)

X. 25 Interface Standard 8 ITU-T standard for interface between host and packet-switched network 8 Physical level handles physical connection between host and link to the node 8 Technically X. 21, but other standards can be substituted, including RS-232 8 Link level provides for reliable data transfer 8 Uses LAPB, which is a subset of HDLC 8 Packet level provides virtual circuits between subscribers

Virtual-Circuit Service 8 External virtual circuit: logical connection between two stations on the network 8 Internal virtual circuit: specific preplanned route through the network 8 X. 25 usually has a 1: 1 relationship between external and internal circuits 8 In some cases, X. 25 can be implemented as a packetswitched network

WANs for Voice 8 Requires very small and nonvariable delays for natural conversation--difficult to provide this with packet-switching 8 As a result, the preferred method for voice transmission is circuit-switching 8 Most businesses use public telephone networks, but a few organizations have implemented private voice networks

WANs for Data 8 Public packet-switched networks (X. 25) 8 Private packet-switched networks 8 Leased lines between sites (non-switched) 8 Public circuit-switched networks 8 Private circuit-switched networks (interconnected digital PBXs) 8 ISDN (integrated X. 25 and traditional circuitswitching)

WAN Considerations 8 Nature of traffic 8 stream generally works best with dedicated circuits 8 bursty better suited to packet-switching 8 Strategic and growth control--limited with public networks 8 Reliability--greater with packet-switching 8 Security--greater with private networks