1. William Stallings Data and Computer Communications 7th Edition
Chapter 10
Circuit Switching and Packet Switching

2. Switching Networks
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

3. Nodes
Nodes may connect to other nodes only, or to stations and other nodes
Node to node links usually multiplexed
Network is usually partially connected
Some redundant connections are desirable for reliability
Two different switching technologies
Circuit switching
Packet switching

4. Simple Switched Network

5. Switching Activities
Some nodes connect only to other nodes (intermediary nodes). Sole purpose is to switch data
Some nodes have one or more stations attached. They accept from and deliver data to the attached station.
Node-to-node links are usually multiplexed
Multiple paths enhance reliability

6. Circuit Switching Originated in public telephone networks
Well suited to analog transmission of voice signal
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

7. Circuit Switching - Applications
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)

8. Public Circuit Switched Network

9. Telecom Components Subscriber Subscriber line Exchange Trunks
Devices attached to network
Subscriber line
Link between subscriber and network
Also called Local Loop or Subscriber Loop
Almost all Local Loops are TPW
Range from Few km up to tens of km
Exchange
Switching center in the network
End office specific switching center that supports subscribers
Trunks
Branches between exchanges
Multiplexed

10. Circuit Establishment

11. Circuit Switching Concepts
Digital Switch
Provide transparent signal path between devices
Typically allows full duplex transmission
Network Interface
Functions and h/w needed to connect digital devices to the network
Control Unit
Establish connections - Generally on demand, Handle and acknowledge requests, Determine if destination is free,construct path
Maintain connection
Disconnect

12. Blocking or Non-blocking Circuit Switching
A network may not be able to connect stations because all paths are in use (more stations than path)
Used on voice systems
Short duration calls
Non-blocking
Permits all stations to connect (in pairs) at once (at least as many paths as stations)
Used for some data connections

13. Space Division Switching
Developed for analog environment, but carried over into digital
Signal paths are physically separate (slide 15)
Each connection requires dedicated path (crossbar switch)

14. Crossbar switch
Number of crosspoints grows as square of number of stations
Loss of crosspoint prevents connection
Inefficient use of crosspoints
If all stations connected, only a few crosspoints in use
Non-blocking

15. Space Division Switch

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

17. Three Stage Space Division Switch

18. Time Division Switching
Modern digital systems rely on intelligent control of space and time division elements
Use digital time division techniques to set up and maintain virtual circuits
Partition low speed bit stream into pieces that share higher speed stream

19. 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

20. 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

21. 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

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

23. In Channel Signaling Use same channel for signaling control and call
Requires no additional transmission facilities
Inband
Control signals have same electromagnetic properties (frequency) 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)

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

25. 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

26. Common Channel Signaling Modes

27. Signaling System Number 7
SS7
Common channel signaling scheme
ISDN
Overall purpose to provide international standardized common channel signaling system
Performs call management (setup, maintenance, termination) and network management functions
Network is circuit switched, but control is packet switched

28. Softswitch Architecture
Latest trend in circuit-switching technology
General purpose computer running software to make it a smart phone switch
Lower cost, greater functionality
Can packetize digitized voice data, allowing voice over IP
Performs call routing
Separates call processing from hardware function of switch

29. Traditional Circuit Switching

30. Softswitch

31. Circuit Switching Shortcomings
Inefficient for data because of idle time
Provides for transmission at constant rate – must transmit and receive at same data rate. Limits versatility

32. Packet Switching Basic Operation
Data transmitted in small packets
Typically 1000 octets (8 bit byte)
Longer messages split into series of packets
Each packet contains a portion of user data plus some control info
Control info
Routing (addressing) info
Packets are received, stored briefly (buffered) and passed on to the next node
Store and forward

33. Use of Packets

34. Advantages Line efficiency Data rate conversion
Single node to node link can be shared by many packets over time
Packets queued and transmitted as fast as possible
Data rate conversion
Each station connects to the local node at its own speed
Nodes buffer data if required to equalize rates
Packets are accepted even when network is busy
Delivery may slow down
Priorities can be used

35. Switching Technique Station breaks long message into packets
Packets sent one at a time to the network
Packets handled in two ways
Datagram
Virtual circuit

36. Datagram Each packet treated independently
Packets can take any practical route
Packets may arrive out of order
Packets may go missing
Up to receiver to re-order packets and recover from missing packets

37. Datagram Diagram

38. Virtual Circuit Preplanned route established before any packets sent
Call request and call accept packets establish connection (handshake)
Each packet contains a virtual circuit identifier instead of destination address
No routing decisions required for each packet
Clear request to drop circuit
Not a dedicated path

39. Virtual Circuit Diagram

40. Virtual Circuits v Datagram
Network can provide sequencing and error control
Packets are forwarded more quickly
No routing decisions to make
Less reliable
Loss of a node loses all circuits through that node
Datagram
No call setup phase
Better if few packets
More flexible
Routing can be used to avoid congested parts of the network

41. Circuit vs. Packet Switching
Circuit Switched
Bandwidth guaranteed
Circuit capacity not reduced by other network traffic
Circuit costs independent of amount of data transmitted, resulting in wasted bandwidth
Packet Switched
Bandwidth dynamically allocated on as-needed basis
May have concurrent transmissions over physical channel
May have delays and congestion
More cost-effective, offer better performance

42. X.25
Old ITU standard
Interface between host and packet switched network
Almost universal on packet switched networks and packet switching in ISDN
Defines three layers
Physical
Link
Packet

43. X.25 Use of Virtual Circuits

44. Virtual Circuit Service
Logical connection between two stations
External virtual circuit
Specific preplanned route through network
Internal virtual circuit
Typically one to one relationship between external and internal virtual circuits
Can employ X.25 with datagram style network
External virtual circuits require logical channel
All data considered part of stream

45. Frame Relay Designed to be more efficient than X.25
Developed before ATM
Larger installed base than ATM
ATM now of more interest on high speed networks

46. Frame Relay Public WAN packet-switching protocol
Provides LAN-LAN connectivity
Relays frames across a network from source to destination
Connection-oriented protocol – must first establish a connection before two nodes can communicate

47. LAN-LAN Connectivity Prior to Frame Relay
Conventional dial-up circuit switching
Dedicated leased line using point-to-point protocols or X.25 Packet Switching
(Both have significant problems in today’s technology)

48. Frame Relay Circuits Connection-oriented protocol
Relies on permanent virtual circuit (PVC) – provide non-dedicated connections through a shared medium (bandwidth is shared among multiple sites (simplex lines)
Can also support switched virtual circuit (SVC)

49. Frame Relay Permanent Virtual Circuit
Pre-determined link between source and destination
Bandwidth is shared among multiple sites, not dedicated
Bandwidth cost based on Committed Information Rate (CIR)
Frames encapsulate data contained in layers 3-7

50. CIR
If Leased line, a fixed amount of bandwidth is provided, regardless of use.
If CIR, frame relay provider guarantees a given throughput (bps) during normal workloads - user hedges bet and hopes for more
CIR can be symmetric or asymmetric (as in ADSL)
If CIR is too small, network becomes congested, and frames may be dropped
If CIR is too high, you are paying for excessive bandwidth (think of the Sprint commercials)
Service provider does not guarantee delivery above CIR

51. BURST A data transmission that is equal to, or exceeds the CIR
Committed Burst – Maximum amount of data the provider guarantees to deliver within a specified time period (CIR/time) – In general, CIR should not exceed 70% of port speed
Excessive Burst – Maximum amount of uncommitted data a provider will attempt to deliver within a specified time period

52. Committed plus Excessive Burst

53. Switched Virtual Circuit
Frame Relay also supports switched virtual circuits (SVC)
SVCs also support CIRs
Circuits between source and destination are established when needed (logical dynamic, rather than logical permanent)
Analogous to PSTN - paths change between connections

54. PVC Advantages and Disadvantages
Widespread availability
Less complex network design
Less expensive equipment
Permanent connections - always paying for a certain amount of bandwidth, regardless of use
Every time a new connection is required, a new permanent circuit must be established

55. Frame Relay Basic Operation
Packet switching – every frame carries source and destination address
Data link layer protocol, but does not support flow control, error detection, frame sequencing or ACK (all performed at end nodes (routers))
Since frame integrity performed by end nodes, FR is fast and efficient
Statistical multiplexing – multiple subscribers share same backbone

56. Congestion Control and Management
If no ACK within a given time period, sending node assumes frame was discarded
Discarded frames must be retransmitted, increasing network traffic
Provider responsibilities
Design networks to provide sufficient bandwidth
Ensure links are error free
Prevent any node from monopolizing the system
Distribute resources in fair and equitable manner
Refer to Lucent White Paper

57. Advantages and Disadvantages
Lost link by link error and flow control
Increased reliability makes this less of a problem
Streamlined communications process
Lower delay
Higher throughput

58. Required Reading Stallings Chapter 10

59. Chapter 10 Review Questions
Why is it useful to have more than one possible path through a network for each pair of stations?
Describe a simple switched network; describe a circuit switched network
What is a dedicated path?
Describe the differences between blocking and non-blocking circuit switching
Describe how circuit switching is inefficient.
Describe SS7.
Describe packet switching. What are the advantages of packet switching compared to circuit switching?
Explain the difference between datagram and virtual circuit operation.
Continued on next page!

60. Chapter 10 Review Questions (cont.)
What is the significance of packet size in a packet-switching network?
What is the difference between inchannel and common channel signaling? Which is preferred, and why?
Describe frame relay. What are the relative advantages and disadvantages of frame relay?
Compare and contrast PVC and SVC
Explain the concept of the committed information rate (CIR)
Describe frame relay’s method of congestion control.