Your friendly reminders

1
Your friendly reminders

• No Quiz this week
• Tutorial continues today at 200pm
• Tutorial questions will be printed out for you
• Labs - will continue as usual this week.
• I will be present.
• I will provide worked solutions for Lab 1 by the
  end of this week.
• You should be finishing Lab 1 this week.
• Labs next week Lab Test 1 will be next week in
  the lab session.
• Password will be provided in the venue
• LDC My LDC time is Tuesday from 400 to 500.
  (Or call to make an appointment)

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Todays lecture Data Link
L 11 Security issues
L 12

• Never lived up to early promises

• Most widely used

L 10
L 6, L9
L 5
L 7, L 8
L 4
L 2, L 3
3
Todays lecture Data Link
L 11 Security issues
L 12

• Never lived up to early promises

• Most widely used

L 10
L 6, L9
L 5
L 7, L 8
L 4
L 2, L 3
4
48740 Lecture 5 Switching

• Chapter 10
• Circuit Switching and Packet Switching

5
Recap - Big Picture
Lecture 5
Functions of Network Layer
Lecture 4
Flow control, ARQ schemes, HDLC, Multiplexing
(TDM, FDM, WDM)
Lecture 3
Data vs symbols, encoding, modulation,
synchronization, error control, interfacing
Lecture 2
Frequency and time, Analog and digital
transmission, impairments, channel capacity,
transmission media
6
Key concepts to be covered

• How a telephone call is made leading to ideas
  of Circuit Switched networks
• 0011 1 416 794 2321
• How an 8 bit sample in a 125uS frame is passed
  through a system
• How a packet is routed from a client to a server
  leading to ideas of Packet Switched networks.
• www.yahoo.com
• VoIP
• Synchronized Circuit Switching
• A synchronized Virtual Circuit Switching
• Packet Switching
• Routing

7
Type of Switching
8
What is circuit switching? What are the phases
of 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
• Generally already predetermined in the network
  architecture

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(No Transcript)
10
What are the pros and cons of circuit switching?
What are some of the 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)

11
Public Circuit Switched Network
RQ Explain briefly how a call may be switched
when someone in Sydney makes a call to someone in
London, UK
12
Telecomms Components

• Subscriber
• Devices attached to network
• Subscriber line
• Local Loop
• Subscriber loop
• Connection to network
• Few km up to few tens of km
• Exchange
• Switching centers
• End office - supports subscribers
• Trunks
• Branches between exchanges
• Multiplexed

13
What is the difference in circuit establishment
between when two subscribers are connected to (a)
the same end office and (b) different end offices
14
Circuit Switch Elements

• Digital Switch
• Provide transparent signal path between devices
• Network Interface
• Functions/hardware connects devices to network
• Control Unit
• Establish connection
• Generally on demand
• Handle and acknowledge requests
• Determine if destination is free
• construct path
• Maintain connection
• Disconnect

15
What is Space Division Switching? Where is it
used?

• Developed for analog environment
• Separate physical paths
• 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

16
Explain the operation of a Space Division Switch?
What are its pros and cons?
17
Explain the operation of a Three Stage Space
Division Switch? Why is there a need for 3 stages?
18
What is 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
Explain the sequence of events that take place
when a subscriber makes a telephone call?

• 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

20
What is Signaling System Number 7?

• Aka SS7 or CCS7.
• You dont need to know the details, but it is a
  system whereby telephone exchanges are able to
  pass the control messages between each other so
  as to establish and maintain calls

21
Explain the operation of traditional circuit
switching shown below?
22
What are the key principles associated with
packet switching?

• Recap 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

23
Explain briefly the basic operation of a packet
switching network?

• Data transmitted in small packets
• Typically 1000 octets
• Longer messages split into series of packets
• Each packet contains a portion of user data plus
  some control info
• What Control info has to be inserted in a packet?
• Routing (addressing) info
• Why is packet switching referred to as store and
  forward?
• Packets are received, stored briefly (buffered)
  and past on to the next node

24
Use of Packets where are packets fragmented and
re-assembled?
25
What are the advantages and disadvantages of
packet and circuit switching?

• Line efficiency
• 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

26
Are there different types of packet switching
techniques?

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

27
What is Datagram Packet Switching?

• 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

28
DatagramDiagram
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Datagram networks the Internet model

• no call setup at network layer
• routers no state about end-to-end connections
• no network-level concept of connection
• packets typically routed using destination host
  ID
• packets between same source-dest pair may take
  different paths

1. Send data
2. Receive data
30
What is a Virtual Circuit Packet Switching?

• 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

31
VirtualCircuitDiagram
32
Virtual circuits signaling protocols

• used to setup, maintain teardown VC
• used in ATM, frame-relay, X.25
• not used in todays Internet

6. Receive data
5. Data flow begins
4. Call connected
3. Accept call
1. Initiate call
2. incoming call
33
Compare and contrast datagram and virtual circuit
approaches for packet switching?

• Virtual circuits
• 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

34
RQ What are the differences between circuit and
packet switching?

• Type of connection
• Connection Set-up
• Bandwidth
• Delay and Variation of Delay
• Charging
• Best suited for
• Buffering
• Processing at each node
• Utilisation of resources
• Example Networks

35
How are Circuit v Packet Switching different in
terms of the following performance metrics?

• Performance
• Propagation delay
• Transmission time
• Node delay

36
William StallingsData and Computer
Communications7th Edition

• Chapter 12
• Routing

37
Facts about 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

38
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

39
AlternateRoutingDiagram
40
Routing in Packet Switched Network

• Complex, crucial aspect of packet switched
  networks
• Important characteristics required of routing.
• Correctness
• Simplicity
• Robustness
• Stability
• Fairness
• Optimality
• Efficiency

41
Routing in packet switched networks

• Used for selection of route
• Minimum hop
• Least cost
• See Stallings appendix 10A for routing algorithms
• Analogy driving from one end of Sydney to
  another?

42
What is the shortest path between 1 and 3?
43
When is a routing decision made? Where is it made?

• Time
• Packet or virtual circuit basis
• Place
• Distributed
• Made by each node
• Centralized
• Source

44
Network Information Source and Update Timing

• RQ What is the source of information needed to
  make a routing decision?
• Routing decisions usually based on knowledge of
  network (not always)
• Distributed routing
• Nodes use local knowledge
• May collect info from adjacent nodes
• May collect info from all nodes on a potential
  route
• Central routing
• Collect info from all nodes
• RQ How often should this network information
  source be updated?
• Update timing
• When is network info held by nodes updated
• Fixed - never updated
• Adaptive - regular updates

45
What routing strategies are commonly used?

• Fixed
• Flooding
• Random
• Adaptive

46
Fixed Routing

• Single permanent route for each source to
  destination pair
• Determine routes using a least cost algorithm
  (appendix 10A)
• Route fixed, at least until a change in network
  topology
• RQ Routing Tables for Network in Figure 12.2.

47
How does Flooding work? Where is it used? What
are it pros and cons?

• 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

48
Flooding Example
49
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)

50
How does random routing work? What are its pros
and cons?

• Node selects one outgoing path for retransmission
  of incoming packet
• Selection can be random or round robin
• Can select outgoing path based on probability
  calculation
• No network info needed
• Route is typically not least cost nor minimum hop

51
What is 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
• Too slowly to be relevant

52
Advantages of adaptive routing

• Improved performance
• Aid congestion control (See chapter 13)
• Complex system
• May not realize theoretical benefits

53
Adaptive routing techniques classification

• Based on information sources
• 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 nodes
• All nodes

54
What routing strategies were used in the first
generation Arpanet?

• First Generation
• 1969
• Distributed adaptive
• Estimated delay as performance criterion
• Bellman-Ford algorithm
• Node exchanges delay vector with neighbors
• Update routing table based on incoming info
• Doesn't consider line speed, just queue length
• Queue length not a good measurement of delay
• Responds slowly to congestion

55
What routing strategies were used in the second
generation Arpanet?

• Second Generation
• 1979
• Uses delay as performance criterion
• Delay measured directly
• Uses Dijkstras algorithm (appendix 10a)
• Good under light and medium loads
• Under heavy loads, little correlation between
  reported delays and those experienced

56
What routing strategies were used in the third
generation Arpanet?

• Third Generation
• 1987
• Link cost calculations changed
• Measure average delay over last 10 seconds
• Normalize based on current value and previous
  results

57
What are Least Cost Algorithms?

• Basis for routing decisions
• Can minimize hop with each link cost 1
• Can have link value inversely proportional to
  capacity
• Given network of nodes connected by
  bi-directional links
• Each link has a cost in each direction
• Define cost of path between two nodes as sum of
  costs of links traversed
• For each pair of nodes, find a path with the
  least cost
• Link costs in different directions may be
  different
• E.g. length of packet queue

58
Routing Algorithm classification

• Global or decentralized information?
• Global
• all routers have complete topology, link cost
  info
• link state algorithms
• Decentralised
• router knows physically-connected neighbors, link
  costs to neighbors
• iterative process of computation, exchange of
  info with neighbors
• distance vector algorithms

• Static or dynamic?
• Static
• routes change slowly over time
• Dynamic
• routes change more quickly
• periodic update
• in response to link cost changes

59
Dijkstras Algorithm Definitions

• You must be able to do a Dijkstras algorithm
  problem.
• See pages 368 to 370 in Stallings
• Work through the example on page 369.
• Nice exam question.

60
Bellman-Ford Algorithm Definitions

• Work through pages 370 to 370 so that you know
  the difference between these two.

61
Compare Bellman-Ford and Dijkstra?

• Results from two algorithms agree
• Information gathered
• Bellman-Ford
• Calculation for node n involves knowledge of link
  cost to all neighboring nodes plus total cost to
  each neighbor from s
• Each node can maintain set of costs and paths for
  every other node
• Can exchange information with direct neighbors
• Can update costs and paths based on information
  from neighbors and knowledge of link costs
• Dijkstra
• Each node needs complete topology
• Must know link costs of all links in network
• Must exchange information with all other nodes

62
Evaluation

• Dependent on processing time of algorithms
• Dependent on amount of information required from
  other nodes
• Implementation specific
• Both converge under static topology and costs
• Converge to same solution
• If link costs change, algorithms will attempt to
  catch up
• If link costs depend on traffic, which depends on
  routes chosen, then feedback
• May result in instability

63
Further Reading

• Stalling Chapter 12
• Routing information from Comer D. Internetworking
  with TCP/IP Volume 1, Prentice Hall, Upper Saddle
  River NJ.