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

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

Comparison of Networking Options

Types of WANs 8Circuit-switched (today’s lecture) 8Packet-switched (Thursday’s lecture)

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

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

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

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

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

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

Digital Circuit-Switching Node

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

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

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

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

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

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

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

Control Signaling 8Manage the establishment, maintenance, and termination of signal paths 8Includes signaling from subscriber to network, and signals within network 8In-channel signaling uses the same channel for control signals and calls 8Common-channel signaling uses independent channels for controls (SS7)

ISDN 81st generation: narrowband ISDN 8Basic Rate Interface (BRI) 8two 64Kbps bearer channels + 16Kbps data channel (2B+D) = 144 Kbps 8circuit-switched 82nd generation: broadband ISDN (B-ISDN) 8Primary Rate Interface (PRI) 8twenty-three 64Kbps bearer channels + 64 data channel (23B+D) = Mbps 8packet-switched network 8development 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” 8Why so much criticism? 8overhyping of services before delivery 8high price of equipment 8delay in implementing infrastructure 8incompatibility between providers' equipment. 8Didn’t live up to early promises

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

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

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

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

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

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

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

ISDN Primary Access 8Primary Rate Interface (PRI) 8Used when greater capacity required 8No international agreement on rates 8US, Canada, Japan: 1.544mbps (= to T1) 8Europe: 2.048mbps 8Typically 23 64kbps B kbps D 8Fractional use of nB+D possible 8Can be used to support H channels

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

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

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

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

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

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

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

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

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