1. Network Layer and Circuit Switching
Networks provide for communication between devices without direct connections. Circuit Switching is the Oldest Networking Technology

2. Why Networks? Why we need networks?
When point to point link or multipoint link is not available between devices
Why we can’t connect devices directly? This is obvious for two reasons:
The direct connection of every device with every other device would require N(N-1) full duplex links
This would be prohibitively expensive
Each communication link would require a port on the device.
The cost of the ports would be a limiting factor and also their physical size.

3. What is a Network?

4. Kinds of Networks By Communication Technique By Geographical Coverage
Switched Networks
Broadcast Networks e.g. LANs
By Geographical Coverage
Wide Area Network
Cover large geographical areas, often crossing public right-of-ways
Usually consist of several interconnected switching points
Local Area Network
Small scope, usually a building or cluster
Typically owned by the same organization that owns the equipment

5. Switched Network Types
Long distance transmission is typically done over a network of switched nodes
Nodes not concerned with content of data
End devices are stations
Computer, terminal, phone, etc.
A collection of nodes and connections is a communications network
Data routed by being switched from node to node
Nodes may connect to other nodes only, or to stations and other nodes

6. Switched Network Types
Node to node links usually multiplexed
Two types of switched networks
Circuit Switched Networks
Packet Switched Networks

7. Simple Switched Network

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

9. 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

10. Circuit Switching - Disadvantages
Inefficient
Channel capacity dedicated for duration of connection
If no data, capacity wasted
Set up (connection) takes time
Once connected, transfer is transparent
Developed for voice traffic (phone)

11. Circuit-Switching Stages
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
Transfer of information
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

12. Circuit-Switching Stages
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

13. Circuit Switching Application
Circuit switching is well suited for analog voice communications as in the telephone network.
In-efficient for data networks due to its resource allocation nature
Ill-suited to data communication because data traffic is BAD

14. Examples of Circuit Switching
Public Switched Telephone Network - PSTN
Private Automatic Branch Exchange - PABX
Integrated Services Digital Network - ISDN

15. Public Switched Telephone Network (PSTN)
PSTN (Public Switched Telephone Network), refers to the international telephone system based on copper wires carrying analog voice
Telephone service carried by the PSTN is often called plain old telephone service (POTS).

16. Public Switched Telephone Network (PSTN) Elements
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)

17. Telephone Network Structure

18. Telephone Network

19. Typical Circuit Route for Medium Distance Calls-

20. Circuit Switching Connection

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

22. 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

23. Circuit-Switching Node:Control Unit
Establish connections
Generally on demand
Handle and acknowledge requests
Determine if destination is free
construct path
Maintain connection -while needed
Disconnect - Breaks down connection on completion

24. Blocking or Non-blocking
A network is unable to connect stations because all paths are in use
A blocking network allows this
Used on voice systems
Short duration calls
Non-blocking
Permits all stations to connect (in pairs) at once
Used for some data connections

25. 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

26. Circuit Switch Design - Cross Bar 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

27. Cross Bar Switch
A cross bar switch connects n inputs to m outputs in a grid, using electronic micro switches (transistors) at each crosspoint.
The major limitation of this design is the number of crosspoints required.
To connect n inputs to m outputs using a crossbar switch requires nxm crosspoints.
E.g to connect 1000 inputs to 1000 outputs requires a switch with 1,000,000 crosspoints
A crossbar with this number of crosspoints is impractical.
Such a switch is also inefficient because only fewer than 25% of the crosspoints are in use at any given time.

28. Multistage Switch Reduced number of crosspoints
More than one path through network
Increased reliability
More complex control
May be blocking

29. Multistage Switch
To design a three-stage switch, we follow these steps:
We divide the N input lines into groups, each of n lines. For each group, we use one crossbar of size nxk, where k is the number of crossbars in the middle stage. In other words, the first stage has N/n crossbars of nxk crosspoints
We use k crossbars, each of size (N/n) x (N/n) in the middle stage
We use N/n crossbars, each of size kxn at the third stage

30. N/n(nxk) + k(N/n x N/n) + N/n(k x n) =
In a three-stage switch, the total number of crosspoints is 2kN + k(N/n)2
which is much smaller than the number of crosspoints in a single-stage switch (N2).

32. Example 8.3 Design a three-stage, 200 × 200 switch (N = 200) with k = 4 and n = 20.
Solution
In the first stage we have N/n or 10 crossbars, each of size
20 × 4. In the second stage, we have 4 crossbars, each of
size 10 × 10. In the third stage, we have 10 crossbars,
each of size 4 × 20. The total number of crosspoints is
2kN + k(N/n)2, or 2000 crosspoints. This is 5 percent of
the number of crosspoints in a single-stage switch (200 ×
200 = 40,000).

33. Crosspoints ≥ 4N [(2N)1/2 – 1]
Clos criterion
Clos investigated the condition of nonblocking in multistage switches and came up with the following formula
In a nonblocking switch, the number of middle-stage switches must be at least 2n-1. In other words, we need to have k > 2n-1
According to the Clos criterion:
n = (N/2)1/2
k > 2n – 1
Crosspoints ≥ 4N [(2N)1/2 – 1]

34. Example 8.4
Redesign the previous three-stage, 200 × 200 switch, using the Clos criteria with a minimum number of crosspoints.
Solution
We let n = (200/2)1/2, or n = 10. We calculate k = 2n − 1 = 19. In the first stage, we have 200/10, or 20, crossbars, each with 10 × 19 crosspoints. In the second stage, we have 19 crossbars, each with 10 × 10 crosspoints. In the third stage, we have 20 crossbars each with 19 × 10 crosspoints. The total number of crosspoints is 20(10 × 19) + 19(10 × 10) + 20(19 ×10) = 9500.

35. Circuit Switch Design - TDM Bus Switch
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

36. Issues in Circuit Switched Networks
Routing
Control Signalling

37. Routing
Routing in the network tries to determine the path from a given source to a given destination
Many connections will need paths through more than one switch
Need to find a route
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
Uses a peer structure for nodes

38. Routing in Circuit-Switched Networks
Requires balancing, efficiency and resiliency
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 - almost like packet switching

39. Static Hierarchical Routing
Tracing up the tree to the first common node, and then tracing down the tree to the called subscriber
Resiliency to the network, additional high usage trunks were added that cut across the tree structure to connect exchanges.
Minimum switching costs, flexibility only via alternate trunks
Not able to adapt to changing conditions
Typically in such designs the result of a failure is a major local congestion near the site of the failure

40. Alternate Routing
A dynamic routing approach is one in which routing decisions are influenced by current traffic conditions
In Alternate routing schemes the possible routes between two end offices are predefined
Originating switch selects the best route for each call
Routes listed in preference order
Different sets of routes may be used at different times
Routing paths can be fixed (1 route) or dynamic (multiple routes, selected based on current and historical traffic)

41. Alternate Routing
A form of the dynamic alternate routing technique is employed by the Bell Operating Companies for providing local and regional telephone service, referred as multialternate routing (MAR)
This approach also used in AT&T in its long distance network which is referred as dynamic non-hierarchal routing (DNHR)

42. Adaptive Routing
Traffic reporting and analysis with new paths computed periodically, adapts to net load, events
Need to use algorithms to determine paths dynamically, based on load/congestion vectors

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

44. Control Signaling Functions
Audible communication with subscriber
Transmission of dialed number
Call can not be completed indication
Call ended indication
Signal to ring phone
Billing info
Equipment and trunk status info
Diagnostic info
Control of specialist equipment

45. 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 not busy, send ringer signal to target subscriber
Feedback to caller
Ringing tone, engaged tone, unobtainable
Target accepts call by lifting receiver
Switch terminates ringing signal and ringing tone
Switch establishes connection
Connection release when Source subscriber hangs up

46. Switch to Switch Signaling
Subscribers connected to different switches
Originating switch seizes interswitch trunk
Send off hook signal on trunk, requesting digit register at target switch (for address)
Terminating switch sends off hook followed by on hook (wink) to show register ready
Originating switch sends address

47. Control Signals

48. Location of Signaling Subscriber to network Within network
Depends on subscriber device and switch
Within network
Management of subscriber calls and network
More complex
SS7

49. 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
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
Need extra electronics
Slower signal rate (narrow bandwidth)

50. Drawbacks of In Channel Signaling
Limited transfer rate
Delay between entering address (dialing) and connection
Overcome by use of common channel signaling

51. Common Channel Signaling
Control signals carried over paths independent of voice channel
One control signal channel can carry signals for a number of subscriber channels
Common control channel for these subscriber lines
Associated Mode
Common channel closely tracks interswitch trunks
Disassociated Mode
Additional nodes (signal transfer points)
Effectively two separate networks

52. Common v. In Channel Signaling

53. Common Channel Signaling Modes

54. Signaling System Number 7
SS7
Common channel signaling scheme
Used in ISDN and inside PSTN
Optimized for 64k digital channel network
Call control, remote control management and maintenance
Reliable means of transfer of info in sequence without loss or duplication
Will operate over analog and below 64k
Point to point terrestrial and satellite links
Every element of the SS7 is replicated for resilience

55. SS7 Benefits SS7 adds intelligence to a network
Basis of new end user services e.g.
800 and 900 services
Mobile Telephone Service
Mobile subscriber authentication
Caller identification
Charging calls to a credit card
Charging calls to a calling card

56. SS7 Signaling Network Elements
Signaling point (SP)
Any point in the network capable of handling SS7 control message
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

57. Transfer Points

58. 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

59. Circuit Switching - Summary
A type of communications in which a dedicated channel (or circuit) is established for the duration of a transmission.
The most ubiquitous circuit-switching network is the telephone system, which links together wire segments to create a single unbroken line for each telephone call.

60. Circuit Switching - Summary
Circuit-switching networks are sometimes called connection-oriented networks because the devices must first establish a connection before exchanging data
Circuit-switching systems are ideal for communications that require data to be transmitted in real-time. Packet-switching networks are more efficient if some amount of delay is acceptable.