1. Data Communications TDC 362 / TDC 460
Circuit Switching and
Packet Switching

2. 8.1 Circuit Switching
Space-Division Switch
Time-Division Switch
TDM Bus
Combinations

3. Figure 8.1 Circuit-switched network

4. Figure 8.2 A circuit switch

5. 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

6. Figure 8.4 Crossbar switch

7. Figure 8.5 Multistage switch

8. Figure Switching path

9. Three Stage Switch

10. Figure 8. 7 Time-division multiplexing, without and with a
Figure Time-division multiplexing, without and with a Time-slot interchange

11. Figure 8.8 Time-slot interchange

12. Figure TDM bus

13. Figure 8.10 TST (Time-space-time) switch

14. Circuit-Switched Routing
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

15. Alternate Routing Possible routes between end offices predefined
Originating switch selects appropriate route
Routes listed in preference order
Different sets of routes may be used at different times

16. Alternate Routing Diagram

17. 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

18. Control Signals

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

20. 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)

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

22. 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

23. Common vs. In Channel Signaling

24. Signaling Modes

25. Signaling System Number 7
SS7
Most widely used common channel signaling scheme
Internationally standardized and general purpose

26. SS7 SS7 network and protocol used for:
Basic call setup, management, tear down
Wireless services such as PCS, roaming, authentication
Toll free and toll (900) wireline services
Enhanced features such as call forwarding, caller ID, 3-way calling
Efficient and secure worldwide telecommunications

27. SS7
SS7 messages are exchanged between central offices and specialized databases via signal transfer points (packet switches).
Control plane
Responsible for establishing and managing connections
Information plane
Once a connection is set up, info is transferred in the information plane

28. SS7 Signaling Network Elements
Service switching point (SSP)
SSPs enable central offices to communicate with SS7 databases (the user entry point into SS7)
Signal transfer point (STP)
A signaling point (packet switch) capable of routing control messages
Service control point (SCP)
SCPs contain databases with call routing instructions

29. SS7 SCP SCP STP SSP Central Office STP SSP Central Office SSP Central

30. SS7 Characteristics
SSPs are telephone switches that send signaling messages to other SSPs to setup, manage, and release voice circuits
An SSP may also send a query message to a centralized database (an SCP) to determine how to route a call (e.g. a toll-free number)
Because the SS7 network is critical to call processing, SCPs and STPs are deployed in mated pair configurations in separate physical locations
Links between signaling points are also in pairs

31. Packet Switching Principles
Circuit switching designed for voice
Resources dedicated to a particular call
Much of the time a data connection is idle
Data rate is fixed
Both ends must operate at the same rate
What if we don’t want a dedicated call, or the data rate is bursty? You want packet switching!

32. Basic Operation Data transmitted in small packets
Typically 1000 bytes
Longer messages split into series of packets
Each packet contains a portion of user data plus some control info (such as addressing info or packet type)
Packets are received, stored briefly (buffered) and passed on to the next node
Store and forward (only ATM does not do this)

33. 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

34. Two Basic Forms of Packet Switching
Packets handled in two ways
Datagram
Virtual circuit

35. Datagram Each packet treated independently
Packets can take any practical route
Packets may arrive out of order
Packets may get lost or delayed
Up to receiver to re-order packets and recover from missing packets

36. 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

37. Figure 18.2 Virtual Circuit Identifier (VCI)
VCI is known only between two switches. (It is not a global
address.)

38. Figure 18.4 Switch and table

39. Figure 18.5 Source-to-destination data transfer

40. S(witched)VC vs. P(ermanent)VC setup
A virtual circuit can be either switched or permanent.
If permanent, an outgoing VCI is given to the source,
and an incoming VCI is given to the destination.
The source always uses this VCI to send frames to
this particular destination.
The destination knows that the frame is coming from
that particular source if the frame carries the
corresponding incoming VCI.
If a duplex connection is needed, two virtual circuits
are established.

41. S(witched)VC vs. P(ermanent)VC setup
A PVC has several drawbacks:
1. Always connected, so always paying
2. Connection is between two parties only. If
you need a connection to another point, you
need another PVC.
Don’t like these disadvantages? Use an SVC.

42. Figure 18.6 SVC setup request
1 - Setup frame sent from A to Switch I.
Note how the Outgoing VCI is not yet known.

43. Figure 18.7 SVC setup acknowledgment
As the acknowledgment frame goes back, the VCI number
is placed into the Outgoing VCI entry in each table.

44. Virtual Circuits vs Datagram
Network can provide sequencing and error control
Packets are forwarded more quickly
No routing decisions to make
Less reliable
Loss of a node looses 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

45. Packet Size

46. Event Timing

47. Routing Complex, crucial aspect of packet switched networks
Characteristics required
Correctness
Simplicity
Robustness
Stability
Fairness
Optimality
Efficiency

48. Performance Criteria Used for selection of route Minimum hop
Least cost
Dijkstra’s algorithm most common
Finds the least cost path from one starting node to all other nodes
Algorithm can be repeated for each starting node

49. Dijkstra’s Least Cost Example

50. Dijkstra’s Least Cost Example

51. Decision Time and Place
Packet or virtual circuit basis
Place
Distributed
Made by each node
Centralized - dead
Source - dead

52. Basic Routing Strategies
Adaptive versus Fixed (dead?)
Distributed versus Centralized (dead?)
Flooding

53. Centralized and Distributed Routing Tables

54. Flooding No network info required
Packet sent by node to every neighbor
Incoming packets retransmitted on every link except incoming link
Eventually a number of copies will arrive at destination
Each packet is uniquely numbered so duplicates can be discarded
Nodes can remember packets already forwarded to keep network load in bounds
Can include a hop count in packets

55. Flooding Example

56. Properties of Flooding
All possible routes are tried
Very robust
At least one packet will have taken minimum hop count route
Can be used to set up virtual circuit
All nodes are visited
Useful to distribute information (e.g. routing)

57. Adaptive Routing Used by almost all packet switching networks
Routing decisions change as conditions on the network change
Failure
Congestion
Requires info about network
Decisions more complex
Tradeoff between quality of network info and overhead
Reacting too quickly can cause oscillation
Reacts too slow to be relevant

58. Adaptive Routing - Advantages
Improved performance
Aid congestion control
Complex system
May not realize theoretical benefits

59. Where does routing info come from?
Local (isolated)
Route to outgoing link with shortest queue
Can include bias for each destination
Rarely used - do not make use of easily available info
Adjacent (neighbor) nodes only
All nodes in network

60. Unicast Routing Overview:
Chapter 21
Unicast
Routing Overview:
Routing Protocols
(Details in TDC 365/463)

61. Three basic unicast routing protocols: RIP, OSPF, BGP
Figure Unicasting
In unicast routing, the router forwards the received packet through only one of its ports.
Three basic unicast routing protocols: RIP, OSPF, BGP

62. R1, R2, R3 and R4 use an interior and exterior routing
Figure Autonomous systems
R1, R2, R3 and R4 use an interior and exterior routing
protocol. The other routers use only an interior protocol.
RIP and OSPF are interior, BGP is exterior.

63. RIP (Routing Information Protocol) is an interior routing
Protocol based on distance vector routing which uses the
Bellman-Ford algorithm.
Each router shares its routing knowledge with its neighbors,
every 30 seconds.
This shared information is used to update a router’s routing
table. An entry in the routing table consists of the destination
network address, the shortest distance to reach the
destination in hop count, and the next router to which the
packet should be delivered. (see next slide)

64. Table 21.1 A distance vector routing table
Destination
Hop Count
Next Router
Other information 7 5 4 6

65. RIP Updating Algorithm
Receive: a response RIP message
1. Add one hop to the hop count for each advertised destination.
2. Repeat the following steps for each advertised destination:
1. If (destination not in the routing table)
1. Add the advertised information to the table.
2. Else
1. If (next-hop field is the same)
1. Replace entry in the table with the advertised one.
1. If (advertised hop count smaller than one in the table)
1. Replace entry in the routing table.
3. Return.

66. Figure 21.4 Example of updating a routing table

67. OSPF (Open Shortest Path First) protocol is another interior
routing protocol for autonomous systems.
Special routers called autonomous system boundary routers
are responsible for dissipating information about other
autonomous systems into the current system.
To handle routing efficiently and in a timely manner, OSPF
divides an autonomous system into areas.

68. Figure 21.7 Areas in an autonomous system

69. In OSPF, each router sends the state of its neighborhood to
every other router in the area. It does this by flooding.
The state of its neighborhood is only shared when there is
new information. This generates much less traffic than does
distance vector routing (RIP).
OSPF keeps information on its links (the connection between
two routers). There are 4 types of links: point-to-point,
transient, stub, and virtual.
To share information about their neighbors, each entity
distributes link state advertisements (LSAs).

70. There are 5 different types of LSAs: router link, network link,
OSPF
There are 5 different types of LSAs: router link, network link,
summary link to network, summary link to AS boundary
router, and external link.
Every router in an area receives the router link LSAs and
network link LSAs from every other router and forms a
link state database.
Dijkstra’s least cost algorithm is applied to this link state
database to create the routing table. The routing table shows
the cost of reaching each network in the area.

71. RIP and OSPF have shortcomings.
BGP
RIP and OSPF have shortcomings.
RIP (distance vector routing) is not always optimal because
The smallest hop count is not always the optimal route. Plus,
bad news moves slowly.
OSPF (link state routing) has the shortcoming of a possibly
huge routing table. To use link state routing for the whole
internet would require each router to have a huge database.
What about BGP (Border Gateway Protocol)? It is an inter-
autonomous system routing protocol and is based on a routing
method called path vector routing.