Data and Computer Communications

1
Data and Computer Communications

• Chapter 10
• Packet Switching

2
Basic Operation

• Data transmitted in small packets
• 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 past on to the next node
• Store and forward operation

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Use of Packets
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Advantages

• 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
• But delivery may slow down
• Priorities can be used

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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

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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
  missing packets

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Virtual Circuit

• Preplanned route established before any packets
  sent
• Call request and call accept packets establish
  connection (handshaking) Similar to circuit
  switching except that it is done with packets
  rather than signals
• Each packet contains a virtual circuit identifier
  instead of destination address
• No routing decisions required for individual
  packets
• Clear request packet is used to drop circuit
• Not a dedicated path (unlike circuit switching,
  the path may be shared)

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Virtual Circuits versus Datagram

• 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

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Effect of Packet Size on Transmission Time
Small packet size decreases total transmission
time but, there is a limit to this approach
because of the header. Here, d is larger than c.
So, transmission time starts to increase again
as a function of header to data ratio.
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Circuit v Packet Switching

• Performance 3 types of delay may affect
  performance
• Propagation delay Time for a signal to propagate
  from one node to another (mostly negligible)
• Transmission time Time for a transmitter to
  transmit a block of data
• Node delay Node's data processing time (delay)

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Event Timing
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External and Internal Operation

• One of the most important characteristics of a
  packet switching network is whether it uses
  datagrams or virtual circuits. There are 2
  dimensions of this characteristic one for the
  interface between station and it corresponding
  network node (external), and the other for the
  network itself (internal).
• Interface between station and network node
• Connection oriented External Virtual Circuit
  Service
• Station requests logical connection (virtual
  circuit)
• All packets identified as belonging to that
  connection sequentially numbered
• Network delivers packets in sequence
• External virtual circuit service
• e.g. X.25
• Different from internal virtual circuit operation
• Connectionless External Datagram Service
• Packets handled independently
• External datagram service
• Different from internal datagram operation

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Possible Combinations (1)

• External virtual circuit, internal virtual
  circuit
• Dedicated route through network
• External virtual circuit, internal datagram
• Network handles each packet separately
• Different packets for the same external virtual
  circuit may take different internal routes
• Network buffers at destination node for
  re-ordering

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Combinations (2)

• External datagram, internal datagram
• Packets treated independently by both network and
  user
• External datagram, internal virtual circuit
• External user does not see any connections
• External user sends one packet at a time
• Network sets up logical connections

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External Virtual Circuit andDatagram Operation
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InternalVirtualCircuit andDatagram Operation
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Routing

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

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Performance Criteria

• Used for selection of route and based on
• Minimum hop or
• Least cost
• See Stallings appendix 10A for routing algorithms

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Decision Time and Place

• 2 key characteristics concerned with routing
  decisions are
• Decision Time
• Packet or virtual circuit basis meaning decision
  is made when packet is sent (datagram approach),
  or when virtual circuit is set up
• Decision Place
• Distributed
• Decision made by each node
• Centralized
• Decision made by a central network node
• Source
• Decision made by source

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Network Information Source and Update Timing

• Routing decisions are usually (not not always)
  based on knowledge of network
• 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
• Update timing
• When is network info held by nodes, updated?
• Fixed - never updated
• Adaptive - regular updates

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Routing Strategies

• Fixed
• Flooding
• Random
• Adaptive

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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 happens

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Fixed RoutingTables
For each source-destination pair, the routing
table shows the next node on the route.
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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
• Hop count is set to a maximum value. When packet
  passes a node, it decrements the count. Packet
  is discarded if the count reaches zero before
  reaching its destination

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Flooding Example
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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)

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Random Routing

• 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 neither least cost nor minimum
  hop

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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 are more complex
• The tradeoff here is between quality of network
  info and overhead associated with the time
  involved in gathering the information, etc.

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Adaptive Routing - Advantages

• Improved performance
• Aids in congestion control (covered in chapter
  12)

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X.25

• Originally approved in 1976
• Specifies interface between host and packet
  switched network
• Almost universal on packet switched networks and
  packet switching in ISDN
• Defines three layers
• Physical
• Link
• Packet

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X.25 - Physical Layer

• Specifies interface between attached station and
  link to node
• Data terminal equipment DTE (user equipment)
• Data circuit terminating equipment DCE (node)
• Uses physical layer specification X.21, but often
  other standards such as RS232 are used instead
• Provides for a reliable transfer across physical
  link by transmitting the data as a sequence of
  frames

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X.25 - Link Layer

• Uses Link Access Protocol Balanced (LAPB)
• Subset of HDLC
• see LAPB frame structure (chapter 7 page 222) and
  User Data and X.25 Control Information (chapter
  10 page 331)

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X.25 - Packet

• Provides external virtual circuit service which
  enables logical connections (virtual circuits)
  between subscribers

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X.25 Use of Virtual Circuits
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Virtual Circuit Service

• Provides for 2 types of virtual circuits
• Virtual Call
• Dynamically established virtual circuits using a
  call setup and call clearing procedure
• Permanent virtual circuit
• Fixed network assigned virtual circuit data
  transfer happens same way as virtual calls, but
  no call setup and clearing is required.

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Virtual Call
See figure 10.16 and explanations on pages
331-333
Left side shows the packets exchanged between
user machine A and the packet switching node to
which it is attached
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Packet Format (see Page 333)
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Multiplexing

• Most important service provided by X.25
• Packets contain a 12 bit virtual circuit number
• DTE is allowed to establish up to 4095 (212-1)
  simultaneous virtual circuits with other DTEs
  over a single DTC-DCE link
• Number zero is reserved for diagnostic packets
  common to all virtual circuits
• The rest of the numbers are assigned by the DCE
  or DTE depending on which one is initiating the
  virtual circuit call

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Virtual Circuit Numbering
Used when address overflow from top or bottom
happens
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Flow and Error Control

• Like HDLC (Chapter 7) using sliding window
  protocol 3 bit, 7 bit, or 15 bit sequence
  numbers with
• P(S)Send sequence number and
• P(R)Receive sequence number number of next
  packet expected from the other side
• Acknowledgement has either local or end-to-end
  significance.
• When D bit0, acknowledgement is exercises
  between DTE and the network.
• When D bit1, acknowledgement is exercises
  between DTE and the remote DTE
• The error control scheme is Go-Back-N ARQ

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Packet Sequences

• X.25 provides the capacity to identify an
  adjacent sequence of data packets, which is
  called a complete packet sequence
• This allows the network to form longer blocks of
  data sent across network with smaller packet size
  without loss of block integrity
• To specify this mechanism, X.25 defines 2 types
  of packets
• A packets
• M bit 1 (means there are additional complete
  packets to follow), D bit 0
• B packets
• The rest (all other packets)
• In a complete packet sequence, there are zero or
  more A packets followed by a B packet. The
  network may combine or break down this sequence
  to make larger or smaller packets for
  transmission.
• See figure 10.19

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Reset and Restart

• Two methods that X.25 uses to recover from errors
  are
• Reset
• Reinitialize virtual circuit
• Sequence numbers set to zero
• Packets in transit lost
• Up to higher level protocol to recover lost
  packets
• Triggered by loss of packet, sequence number
  error, congestion, loss of network internal
  virtual circuit
• Restart
• Equivalent to a clear request on all virtual
  circuits
• E.g. temporary loss of network access