1. Chapter 9 Circuit Switching
Data and Computer Communications Part 3 - Wide area Networks - Concepts
Chapter 9
Circuit Switching

2. 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

3. Switching Networks Communication Network Switching Networks
Switching Network (used in WAN)
Broadcast Network (used in LAN)
Switching Networks
Circuit-switching networks
Packet-switching networks

4. 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

5. Simple Switched Network

6. Simple switching network
Communication Network Node
provides switching facility (routing)
Network Station
end node (source & destination)
Communication is achieved by transmitting
data from source to destination through a
network of switching nodes
Data communication takes place between two devices that are directly connected by some form of point-to-point transmission medium. However, it is impractical for two devices to be directly, point-to-point connected because of :
- The devices are very far apart.
- Some device may require a link to many of the others at various time.
The solution is to attach each device to a communication network. The devices are refereed to network stations. The stations may be computer, terminal, telephones, or other communicating devices. Each station attaches to a network node. The set of nodes to which stations attach is a boundary of a communication network that is capable of transferring data between pairs of stations.
Communication Network

7. Switching technology Circuit switching Packet 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
The type of data communication facility used is a function of the nature of the application, the number of computers involved, and their physical interface.

8. 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 fixed varies
Prioritization not supported supported
The type of data communication facility used is a function of the nature of the application, the number of computers involved, and their physical interface.

9. Public Switching Telephone Network
end office
subscriber loop
Long-distance office
Intercity trunk
subscriber loop (local loop)
link between subscriber and network
connecting trunk (exchanges)
switching center; localized support for
subscriber
Data communication takes place between two devices that are directly connected by some form of point-to-point transmission medium. However, it is impractical for two devices to be directly, point-to-point connected because of :
- The devices are very far apart.
- Some device may require a link to many of the others at various time.
The solution is to attach each device to a communication network. The devices are referred to network stations. The stations may be computer, terminal, telephones, or other communicating devices. Each station attaches to a network node. The set of nodes to which stations attach is a boundary of a communication network that is capable of transferring data between pairs of stations.
Intercity trunk (trunks)
branches between exchanges; carry
multiple voice circuits
connecting trunk

10. PSDN
PSDN is designed specifically for the transmission of data rather than voice
Communication is shared
PSDN= Public Switched Data Network
LAN
LAN
PSDN
Telephone companies normally provide a public data service. Such networks is known as public switch data network (PSDN), like the PSTN, are now interconnected internationally and have been designed specifically for the transmission of data rather than voice. A network is public in the sense that many company share a switching network. A cost of this service is normally lower than a leased line.
LAN

11. Packets Messages are broken up into a series of packets packet packet
data
header data
header data
header data
packet
packet
packet
To control communication operation, control information, as well as user data, must be transmitted. The sending application generates a block of data and passes this to the transport layer. The transport layer may break this block into smaller pieces to make it more manageable. To each of these pieces, the transport layer appends a transport header, containing protocol information as a header. The overall data (control + data) is then passed to the Network access layer as the same manner.
Messages are broken up into a series of packets
Header is used to route the packet through the network

12. 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
The type of data communication facility used is a function of the nature of the application, the number of computers involved, and their physical interface.

13. Virtual circuit A logical connection known asB
1.3
1.2
1.1
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
1.3
1.2
1.1 A
2.3
2.2
2.1
2.3
2.2
2.1 C
virtual circuit #1 B
The OSI Layers
1.Physical - Concerned with transmission of unstructured bit stream over physical medium; deals with the mechanical, electrical, functional, and procedural characteristic to access the physical medium.
2.Data link - Provides for the reliable transfer of information across the physical link; send block of data (frames) with the necessary synchronization, error control , and flow control.
3.Network - Provides upper layers with independence from the data transmission and switching technologies used to connect systems; responsible for establishing, maintaining, and terminating connections.
4.Transport - Provides reliable, transparent transfer of data between end points; provides end-to-end error recovery and flow control.
5.Session - Provides the control structure for communication between applications; establishes, manages, and terminates connections (session) between cooperating applications.
6.Presentation - Provides independence to the application processes from difference in data representation (syntax).
7.Application - Provide access to the OSI environment for user and also provides distributed information services. 2 4 A 1 3 5 C
virtual circuit #2

14. Datagram Each packet is transmitted independently.B
B.3
B.2
B.1
Each packet is transmitted
independently.
Packets are labeled with
a destination address and
may arrive out of sequence
B.3
B.2
B.1 A
C.3
C.2
C.1
C.3
C.2
C.1 C B
The OSI Layers
1.Physical - Concerned with transmission of unstructured bit stream over physical medium; deals with the mechanical, electrical, functional, and procedural characteristic to access the physical medium.
2.Data link - Provides for the reliable transfer of information across the physical link; send block of data (frames) with the necessary synchronization, error control , and flow control.
3.Network - Provides upper layers with independence from the data transmission and switching technologies used to connect systems; responsible for establishing, maintaining, and terminating connections.
4.Transport - Provides reliable, transparent transfer of data between end points; provides end-to-end error recovery and flow control.
5.Session - Provides the control structure for communication between applications; establishes, manages, and terminates connections (session) between cooperating applications.
6.Presentation - Provides independence to the application processes from difference in data representation (syntax).
7.Application - Provide access to the OSI environment for user and also provides distributed information services.
B.3
B.2 2 4
C.1 A 1
B.1 3 5
C.3
C.2 C

15. 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
The OSI Layers
1.Physical - Concerned with transmission of unstructured bit stream over physical medium; deals with the mechanical, electrical, functional, and procedural characteristic to access the physical medium.
2.Data link - Provides for the reliable transfer of information across the physical link; send block of data (frames) with the necessary synchronization, error control , and flow control.
3.Network - Provides upper layers with independence from the data transmission and switching technologies used to connect systems; responsible for establishing, maintaining, and terminating connections.
4.Transport - Provides reliable, transparent transfer of data between end points; provides end-to-end error recovery and flow control.
5.Session - Provides the control structure for communication between applications; establishes, manages, and terminates connections (session) between cooperating applications.
6.Presentation - Provides independence to the application processes from difference in data representation (syntax).
7.Application - Provide access to the OSI environment for user and also provides distributed information services.

16. 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

17. 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)

18. 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

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

20. 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

21. 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

22. 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)

23. Public Circuit Switched Network
Connecting Trunk

24. Circuit-Switching Networks

25. AT&T telephone hierarchy

26. 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

27. 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

28. 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

29. Circuit Switching Node's Elements

30. 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

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

32. 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

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

34. 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

35. 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

36. There are 2 Types of Switching Techniques Internal to a Single Circuit-Switching Node:
1. Space-Division Switching
2. Time-Division Switching

37. Switching Techniques Space-Division Switching Time-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

38. 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

39. Crossbar Matrix (switch)

40. Multistage Switch Reduced number of crosspoints
More than one path through network
Results in increased reliability
More complex control
May be blocking

41. Three Stage Switch

42. 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

43. 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

44. 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

45. 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)

46. Alternate Routing Diagram

47. 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)

48. 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

49. 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

50. 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

51. Control Signals through a Circuit-Switching Telephone Network

52. Location of Signaling Subscriber to network Within 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

53. In-Channel Signaling Use same channel for signaling and call Inband
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)

54. Drawbacks of In Channel Signaling
Limited transfer rate
Delay between entering address (dialing) and connection
This is overcome by use of common channel signaling

55. 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

56. Common v. In Channel Signaling

57. Common Channel Signaling Modes

58. 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

59. 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

60. Transfer Points

61. 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