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)


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:
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


Circuit-Switching Node:
Control Unit
✘ Establishes on-demand connections
✘ Maintains connection while needed

✘ Breaks down connection on completion


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


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 peerto-peer trunks
✘ Newer circuit-switched networks are
dynamically routed: all nodes are peer-to-peer,
making routing more complex


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)


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)


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


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

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


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