Business data communications 4e chapter 11

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 commun

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

✘ 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

✘ 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)

Digital Circuit-Switching Node

Circuit Switching Node:

Digital Switch

✘ Provides transparent signal path between any pair of attached devices

✘ Typically full-duplex

Circuit-Switching Node:

Control Unit

✘ Establishes on-demand connections

✘ Maintains connection while needed

✘ Breaks down connection on completion

Switching Techniques

✘ 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

Routing in Circuit-Switched

Networks

✘ Requires balancing efficiency and resiliency

✘ Traditional circuit-switched model is

hierarchical, sometimes supplemented with to-peer trunks

peer-✘ Newer circuit-switched networks are

dynamically routed: all nodes are peer-to-peer, making routing more complex

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)

✘ 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

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

✘ Intelligence in the networks

✘ Layered protocol architecture (can be mapped onto OSI

model)

✘ Variety of configurations

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

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

✘ 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 + 1 64kbps D

✘ Fractional use of nB+D possible

✘ Can be used to support H channels

✘ 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

✘ preplanned route established for all packets

✘ similar to circuit switching, but the circuit is not dedicated

✘ 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)

✘ 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

digital PBXs)

circuit-switching)

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