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Data and Computer Communications Part 3 - Wide area Networks - Concepts. Chapter 9 Circuit Switching. Switching Networks. Long distance transmission is typically done over a network of switched nodes Nodes are not concerned with content of data End devices are referred to as stations

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switching networks
Switching Networks
  • Long distance transmission is typically done over a network of switched nodes
  • Nodes are not concerned with content of data
  • End devices are referred to as stations
    • Computer, terminal, phone, etc.
  • A collection of nodes and connections is a communications network
  • Data is routed by being switched from node to node
switching networks3
Switching Networks
  • Communication Network
    • Switching Network (used in WAN)
    • Broadcast Network (used in LAN)
  • Switching Networks
    • Circuit-switching networks
    • Packet-switching networks
nodes
Nodes
  • Nodes may connect to other nodes only, or to stations and other nodes
  • Node to node links are usually multiplexed
  • A network is usually partially connected
    • But some redundant connections are desirable for reliability
  • Two different switching technologies exist:
    • Circuit switching
    • Packet switching
simple switching network
Simple switching network

CommunicationNetworkNode

  • providesswitchingfacility (routing)

NetworkStation

  • endnode(source & destination)

Communication is achieved by transmitting

data from source to destination through a

network of switching nodes

Communication Network

switching technology
Switching technology
  • Circuit switching
    • need a connection established between end nodes
    • connection is maintained until one of end nodes terminates
    • Connection is dedicated to the communication between two nodes
    • Example : Public Switch Telephone Network (PSTN)
  • Packet switching
    • data are transmitted in short messages called packets
    • a connection between the two end-nodes is not maintained
    • a node-to-node link can be dynamically shared by many packets
    • Example : Public Data Network (PDN) like X.25, Frame Relay
key differences
Key differences

Keys Circuit switching Packet switching

data dedicated single route different routes

node status both must be ready sender ready to send is enough

connection dedicated shared

utilization poor good

data rate fixedvaries

Prioritization not supported supported

public switching telephone network
Public Switching Telephone Network

endoffice

subscriberloop

Long-distanceoffice

subscriber loop (local loop)

linkbetweensubscriberandnetwork

Intercitytrunk

connecting trunk (exchanges)

switchingcenter; localized support for

subscriber

Intercity trunk (trunks)

branches between exchanges; carry

multiple voice circuits

connectingtrunk

slide10
PSDN
  • PSDN is designed specifically for the transmission of data rather than voice
  • Communication is shared

LAN

PSDN= Public Switched Data Network

LAN

PSDN

LAN

packets
Packets

data

header data

header data

header data

packet

packet

packet

  • Messages are broken up into a series of packets
  • Header is used to route the packet through the network
psdn switching approach
PSDN switching approach
  • Virtual Circuit
    • network protocols establish a logical route called virtual circuit
    • packets use the same route; data arrive in order
    • similar to circuit switching ! but the route is not dedicated
    • need call setup
  • Datagram
    • each packet is transmitted independently
    • network protocols route each packet as though it were a separate message
    • packet may not arrive in order; need protocol to ensure ordering
    • call setup is not required
virtual circuit

1.1

1.2

1.3

1.1

1.2

1.3

2.1

2.2

2.3

2.3

2.2

2.1

4

2

1

3

5

Virtual circuit

B

  • A logical connection known as
  • virtual circuit (VC) is set up
  • between two stations.
  • Packets are labeled with
  • a virtual circuit number and
  • a sequence number

A

C

virtual circuit #1

B

A

C

virtualcircuit #2

datagram

B.1

B.2

B.3

B.1

B.2

B.3

C.1

C.2

C.3

C.3

C.2

C.1

B.2

B.3

4

2

C.1

1

B.1

3

5

C.2

C.3

Datagram

B

  • Each packet is transmitted
  • independently.
  • Packets are labeled with
  • a destination address and
  • may arrive out of sequence

A

C

B

A

C

routing methods in packet switching
Routing methods in packet switching
  • Fixed routing
    • each packet is transmitted independently
  • Flooding
    • network protocol establishes a logical route called virtual circuit
    • packets use the same route; data arrive in order
  • Random routing
    • the route taken is random
  • Adaptive routing
    • the route reacts to changing conditions within the network
circuit switching networks
Circuit-Switching Networks
  • During communication, a dedicated communication path exists between sender and receiver; e.g., telephone system.
  • Communication involves 3 phases:
    • Circuit establishment: “Call request” propagates, hop by hop through the network, to establish a dedicated link (channels in each component link in some path, from sender to receiver)
    • Data transfer: continuous transfer; either analog or digital signal
    • Circuit disconnect: “circuit disconnect” signal to deallocate the links
circuit switching
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 switching18
Circuit Switching
  • Dedicated communication path between two stations
  • Three phases
    • Establish
    • Transfer
    • Disconnect
  • Must have switching capacity and channel capacity to establish connection
  • Must have intelligence to work out routing
circuit switching stages
Circuit-Switching Stages
  • Circuit establishment
  • Transfer of information
    • point-to-point from endpoints to node
    • internal switching/multiplexing among nodes
  • Circuit disconnect
circuit switching networks20
Circuit-Switching Networks
  • Disadvantages:
    • Both stations need to be simultaneously ready
    • Inefficient use of link capacities, especially with bursty intermittent traffic
  • Advantages
    • Low delay once circuit is established
circuit switching networks21
Circuit-Switching Networks
  • Public telephone network
    • Best-known example of a circuit-switching network
    • Four generic architectural components
      • Subscribers - Devices that attach to the network
      • Local loop - The link between the subscriber and the network (end office). Also called subscriber loop
      • Exchanges - Switching centers in the network. End offices, long-distance offices, …
      • Trunks - Links between exchanges. Carry multiple voice channels by using FDM or synchronous TDM
circuit switching applications
Circuit Switching - Applications
  • Inefficient
    • Channel capacity dedicated for duration of connection
    • If there is no data, capacity wasted
  • Set up (connection) takes time
  • Once connected, transfer is transparent
  • Developed for voice traffic (phone)
switching concepts
Switching Concepts
  • Single circuit-switched node network
    • A collection of stations attached to a central switching unit
    • The central switch establishes a dedicated path between any two devices that wish to communicate
switching concepts27
Switching Concepts
  • Digital switch
    • Establishes a dedicated path between any two devices
  • Control unit
    • Establishes, maintains, and tears down the connection
  • Network interface
  • (circuit) switch
    • Blocking
    • Nonblocking
telecommunication components
Telecommunication Components
  • Subscriber
    • Devices attached to network
  • Local Loop
    • Subscriber loop
    • Connection to network
  • Exchange
    • Switching centers
    • End office - supports subscribers
  • Trunks
    • Branches between exchanges
    • Multiplexed
circuit switching node s elements30
Circuit Switching Node's Elements
  • Digital Switch
    • Provides transparent signal path between devices
  • Network Interface (functions & hardware needed to connect devices to the network)
  • Control Unit; its function is to:
    • Establish connections
      • Generally on demand
      • Handle and acknowledge requests
      • Determine if destination is free
      • construct path
    • Maintain connection
    • Disconnect
circuit switching node digital switch
Circuit Switching Node:Digital Switch
  • Provides transparent signal path between any pair of attached devices
  • Typically full-duplex
circuit switching node network interface
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
Circuit-Switching Node:Control Unit
  • Establishes on-demand connections
  • Maintains connection while needed
  • Breaks down connection on completion
blocking or non blocking
Blocking or Non-blocking
  • An important characteristic of a circuit-switching device is whether it is blocking or non-blocking.
  • Blocking
    • A network is unable to connect stations because all paths are in use
    • Used on voice systems
      • example: Short duration calls
  • Non-blocking
    • Permits all stations to connect (in pairs) at once
    • Used for some data connections
blocking nonblocking networks
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
slide36

There are 2 Types of Switching Techniques Internal to a Single Circuit-Switching Node:

1. Space-Division Switching

2. Time-Division Switching

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

Switching Techniques
space division switch
Space Division Switch
  • Developed for the analog environment (but is now used for both analog & digital signals)
  • Separate physical paths for every channel (signal path)
  • Its basic device is the Crossbar switch
    • Number of crosspoints grows as square of number of stations
    • Loss of crosspoint prevents connection
    • Inefficient use of crosspoints
      • All stations connected, only a few crosspoints in use
    • Non-blocking
multistage switch
Multistage Switch
  • Reduced number of crosspoints
  • More than one path through network
    • Results in increased reliability
  • More complex control
  • May be blocking
time division switching
Time Division Switching
  • Partition low speed bit stream into pieces that share higher speed stream
  • e.g. TDM bus switching
    • based on synchronous time division multiplexing
    • Each station connects through controlled gates to high speed bus
    • Time slot allows small amount of data onto bus
    • Another line’s gate is enabled for output at the same time
routing in circuit switched networks
Routing in Circuit-Switched Networks
  • Traditional circuit-switched model is hierarchical, sometimes supplemented with peer-to-peer trunks
  • Newer circuit-switched networks are dynamically routed: all nodes are peer-to-peer, making routing more complex
routing in circuit switching networks
Routing in Circuit-Switching Networks
  • Many connections will need paths through more than one switch
  • Therefore, the network needs to find an appropriate route. Important factors are:
    • Efficiency
    • Resilience
  • Public telephone switches are a tree structure
    • Static routing uses the same approach all the time
    • Dynamic routing allows for changes in routing depending on traffic
alternate routing
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
Control Signaling
  • Control unit manages 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 (e.g. SS7)
control signaling functions
Control Signaling Functions
  • The means by which the network is managed, calls are established, maintained, and terminated
  • Audible communication with subscriber
  • Transmission of dialed number
  • Call cannot be completed indication
  • Call ended indication
  • Signal to ring phone
  • Billing info
  • Equipment and trunk status info
  • Diagnostic info
  • Control of special equipment
control signal sequence
Control Signal Sequence
  • Both phones on hook
  • Subscriber lifts receiver (off hook)
  • End office switch signaled
  • Switch responds with dial tone
  • Caller dials number
  • If target is not busy, ringer signal is sent to target subscriber
  • Feedback to caller
    • Ringing tone, engaged tone, unobtainable (disconnected line, etc.)
  • Target accepts call by lifting receiver
  • Switch terminates ringing signal and ringing tone
  • Switch establishes connection
  • Connection is released when Source subscriber hangs up
switch to switch signaling
Switch to Switch Signaling
  • Subscribers connected to different switches
  • Originating switch seizes inter-switch trunk
  • Off hook signal is sent on trunk, requesting a digit register at target switch (so that address may be communicated)
  • Terminating switch sends off hook followed by on hook (known as wink) to show register-ready status
  • Originating switch sends address
location of signaling
Location of Signaling
  • Subscriber to network
    • Depends on subscriber device and switch
  • Within network
    • is concerned with management of subscriber calls and network
    • More complex
  • Two types of control signaling are used in circuit switching networks:
    • In-channel signaling
    • Common channel signaling
in channel signaling
In-Channel Signaling
  • Use same channel for signaling and call
    • Requires no additional transmission facilities
  • Inband
    • Uses same frequencies as voice signal
    • Can go anywhere a voice signal can
    • Impossible to set up a call on a faulty speech path (because the control signals used to set up the path have to follow same path)
  • Out of band
    • Voice signals do not use full 4kHz bandwidth
    • Narrow signal band within 4kHz used for control
    • Can be sent whether or not voice signals are present
    • Requires extra electronics
    • Slower signal rate (narrow bandwidth)
drawbacks of in channel signaling
Drawbacks of In Channel Signaling
  • Limited transfer rate
  • Delay between entering address (dialing) and connection
  • This is overcome by use of common channel signaling
common channel signaling
Common Channel Signaling
  • Control signals carried over paths independent of voice channel
  • One control signal channel (path) can carry signals for a number of subscriber channels and is therefore the common control channel for these subscriber lines (channels)
  • Two modes of operation are used in Common channel signaling:
  • Associated Mode
    • Common channel closely tracks inter-switch trunks
  • Disassociated Mode
    • Additional nodes (signal transfer points)
    • Effectively two separate networks
signaling system number 7
Signaling System Number 7
  • SS7
  • It is the most widely used common channel signaling scheme
  • Used in ISDN
  • Optimized for 64kbps digital channel network
  • Call control, remote control, management and maintenance
  • Reliable means of transfer of info in sequence
  • Will operate over analog and below 64k
  • Point to point terrestrial and satellite links
  • Although the network being controlled is a circuit switched network, the control signaling is implemented using packet switching technology
ss7 signaling network elements
SS7 Signaling Network Elements
  • Signaling point (SP)
    • Any point in the network capable of handling SS7 control messages
  • Signal transfer point (STP)
    • A signaling point capable of routing control messages
  • Control plane
    • Responsible for establishing and managing connections
  • Information plane
    • Once a connection is set up, info is transferred in the information plane
signaling network structures
Signaling Network Structures
  • STP capacities
    • Number of signaling links that can be handled
    • Message transfer time
    • Throughput capacity
  • Network performance
    • Number of SPs
    • Signaling delays
  • Availability and reliability
    • Ability of network to provide services in the face of STP failures