Computer Networks

1
Computer Networks

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2
Switching

• How to connect multiple devices to make
  one-one-one communication?
• By point-to-point connection? ? impractical
• ? by switching with switches
• Switches create temporary connections between two
  or more devices

3
Introduction

• Taxonomy of switched networks

4
Contents

• Circuit-Switched Networks
• Introduction
• Three Phases
• Efficiency
• Delay
• Circuit-Switched Technology in Telephone Networks
• Datagram Networks
• Virtual-Circuit Networks
• Structure of a Switch
• Assignment 7

5
Circuit-Switched Networks

• Features
• consists of a set of switches connected by
  physical links
• Each connection uses only one dedicated channel
  on each link
• Each link is normally divided into n channels by
  using FDM or TDM

6
Circuit-Switched Networks

• Properties
• Circuit switching take places at the physical
  layer
• Circuit switching has three phases
• setup phase reserve the needed resources
• end-to-end addressing
• resources channels, switch buffers, switch i/o
  ports, ...
• data transfer phase
• data are not packetized ? continuous flow
  transmission
• no addressing involved
• teardown phase
• the reserved resources are released

7
Circuit-Switched Networks

• Ex.1 A circuit-switched network to connect 8
  telephones in a small area.
• Communication is through 4-kHz voice channels
• Each link uses FDM to connect a maximum of 2
  voice channels
• The BW of each link is 8kHz

temporary connections made
8
Circuit-Switched Networks

• Ex.2 A circuit-switched network that connects
  computers in two remote offices of a privacy
  company.
• Offices are connected using a T-1 line leased
  from a communication SP
• uses two 48 switches
• For each switch, four output ports are folded
  into the input ports to allow communication
  between computers in the same offices
• Four other output ports allow communication
  between two offices

9
Three Phases

• Setup phase (connection phase)
• establishes a dedicated circuit for the
  communication
• connection establish and acknowledgement
• end-to-end addressing, e.g., telephone number
• Data transfer phase
• Teardown phase (disconnection phase)

10
Efficiency and Delay

• Efficiency
• may not be efficient because resources are
  allocated during the entire duration of
  connection ? unavailable to other connections
• not useful for computer networks
• Delay
• minimal delay during data transfer the data are
  not delayed at each switch
• connection time propagation delay for request
  and acknowledgement
• data transfer time propagation delay
  transmission delay
• disconnection time propagation delay

11
Circuit-Switched Technology in Telephone Networks

• Data Transfer and Signaling Networks for
  Telephone Services
• The task of data transfer and signaling are
  separated in modern telephone networks
• The protocol used in the signaling network
  Signaling System Seven (SS7)

12
Contents

• Circuit-Switched Networks
• Datagram Networks
• Introduction
• Routing Table
• Efficiency
• Delay
• Datagram Networks in the Internet
• Virtual-Circuit Networks
• Structure of a Switch
• Assignment 7

13
Datagram Networks

• Packet switching is used in data communication
• No resource allocation for a packet
• No reserved BW on the links
• No scheduled processing time for each packet
• Resources are allocated on demand (FCFS basis)
• Properties of datagram networks
• Each packet is treated independently of all
  others
• Packets are referred to as datagrams
• Datagram switching is normally done at the
  network layer
• Sometimes referred to as connectionless networks

14
Routing Table

• How are the packets routed to their destinations
  in a datagram network?
• ? Each switch has a routing table which is based
  on the destination address
• Routing Table
• dynamic and updated periodically
• Destination address
• Every packet in a datagram network
• carries a header that contains
• the destination address of the packet
• ? remains the same during the entire
• journey of the packet

15
Efficiency and Delay

• Efficiency
• better than that of a circuit-switched network
• Resources are allocated only when there are
  packets to be transferred
• Delay
• greater delay than in a virtual-circuit network
• Although no setup and teardown phases, each
  packet may experience a wait at a switch before
  it is forwarded
• the delay is not uniform for the packets of a
  message, because not all packets in a message
  necessarily travel through the same switch

T1
t1
Total delay T1T2T3 t1t2t3
w1w2 3T3t w1w2
w1
t2
T2
w2
t3
T3
16
Datagram Networks in the Internet

• Switching in the Internet is done by using the
  datagram approach to packet switching at the
  network layer

17
Contents

• Circuit-Switched Networks
• Datagram Networks
• Virtual-Circuit Networks
• Introduction
• Addressing
• Three Phases
• Efficiency
• Delay
• Circuit-Switched Technology in WANs
• Structure of a Switch
• Assignment 7

18
Virtual-Circuit Networks

• Approach
• A cross between a circuit-switched (CS) network
  and a datagram (DG) network
• Setup and teardown phases ? CS network
• Resources can be allocated during the setup phase
  ? CS network
• Data are packetized and each packet carries an
  address in the header ? DG network
• The address in the header has local jurisdiction,
  not end-to-end
• All packets follow the same path established
  during a connection ? CS network
• Normally implemented in the data link layer
• a CS network ? physical layer
• a DG network ? network layer

19
Virtual-Circuit Networks

• An example of a virtual-circuit network

20
Addressing

• Two types of addressing
• Global vs. Local addressing
• Global addressing
• A source or a destination needs to have a global
  address
• a global address in virtual-circuit networks is
  used only to create a virtual-circuit identifier
• Virtual-Circuit Identifier (VCI) local
  addressing
• The identifier actually used for data transfer
• small number that has only switch scope ? local
  address
• used by a frame between two switches ? local
  address

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Three Phases

• The same 3 phases as in a CS network
• Setup phase switches creates an entry for a VC
  with global addresses
• Data transfer phase
• teardown phase
• Data transfer phase
• all switches need to have a table entry for a
  virtual-circuit

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Three Phases

• Data transfer phase (cont.)

23
Three Phases

• Setup phase
• Two Steps Setup Request and the Acknowledgment
• Setup request

24
Three Phases

• Setup phase
• Acknowledgment
• Teardown phase
• Source sends a special frame called a teardown
  request and destination responds with a teardown
  confirmation frame
• All switches delete the corresponding entries
  from their tables

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Efficiency and Delay

• Efficiency
• all packets belonging to the same source and
  destination travels the same path
• Resource reservation in a VC network can be made
  
• during their setup delay for each packet is the
  same (normal cases)
• on demand during the data transfer phase each
  packet may encounter different delays
• The source can check the availability of the
  resource without actually reserving it ? like a
  restaurant using on-demand basis (no reservation)
• Delay

Total delay T1T2T3 t1t2t3 setup delay
teardown delay 3T3t setup delay teardown
delay
t1
T1
t2
T2
t3
T3
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Circuit-Switched Technology in WANs

• Virtual-Circuit networks are used in Switched
  WAN Frame Relay, ATM
• Data link layer of Switched WAN is well-suited to
  the virtual-circuit technology
• Frame Relay
• ATM

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Contents

• Circuit-Switched Networks
• Datagram Networks
• Virtual-Circuit Networks
• Structure of a Switch
• Structure of Circuit Switches
• Structure of Packet Switches
• Assignment 7

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Structure of Circuit Switches

• Space-Division Switch
• The path in the circuit are separated from one
  another spatially
• Crossbar Switch

• connects n inputs to m outputs in a grid with
  electronic microswitches (transistors) at each
  crosspoint
• Problem huge number of crosspoints and
  inefficiency
• connect n inputs by m output -- require n m
  crosspoints.
• ex 1000 input, 1000 output ? 1,000,000
  crosspoints
• fewer than 25 of the crosspoints are in use at a
  given time
• ? the rest are idle

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Structure of Circuit Switches

• Space-Division Switch
• Multistage Switch
• combines crossbar switches in several stages
  (normally three)

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Structure of Circuit Switches

• Space-Division Switch
• Multistage Switch
• combines crossbar switches in several stages
  (normally three)
• many optional paths exist

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Structure of Circuit Switches

• Space-Division Switch
• Multistage Switch
• Design steps of a three-stage switch
• 1. Divide the N input lines into groups, each of
  n lines
• 2. Use k crossbar in the middle stage, each of
  size (N/n)(N/n)
• 3. Use N/n crossbar at the third stage, each of
  size kn
• The total number of crosspoints
• N/n (nk) k ((N/n)(N/n)) N/n (kn) 2kN
  k(N/n)2 ltlt N2
• Ex.1 Design a three-stage, 200200 switch (N200)
  with k4 and n20
• 1st stage N/n 10 crossbars, each of size
  204
• 2nd stage 4 crossbars, each of size 1010
• 3rd stage N/n 10 crossbars, each of size
  420
• Total crosspoints 2kN k(N/n)2 2000 ltlt 40000
  (just 5)

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Structure of Circuit Switches

• Space-Division Switch
• Example of Blocking in a Multi State Switch

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Structure of Circuit Switches

• Space-Division Switch
• Multistage Switch
• Problems blocking (in case of heavy traffic)
• more stages, more blockings
• Clos criterion the condition of nonblocking in
  multistage switches
• In a nonblocking switch, no. of middle-stage
  switches (k) 2n-1
• Worst Case All other inputs have seized top n-1
  middle switches AND all other outputs have seized
  next n-1 middle switches
• If k2n-1, there is another path left to connect
  desired input to desired output

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Structure of Circuit Switches

• Space-Division Switch
• Multistage Switch
• Clos criterion the condition of nonblocking in
  multistage switches
• Let's minimize the no. of crosspoints with a
  fixed N by using the Clos criteria
• C(n) number of crosspoints in Clos switch
• 2Nk k(N/n)2 2N(2n 1)(2n
  1)(N/n)2
• Differentiate with respect to n
• 0 dC/dn 4N 2N2/n2 2N2/n3 4N 2N2/n2 ?
  n (N/2)1/2
• The minimized number of crosspoints is then
• C (2N N2/(N/2))(2(N/2)1/2 1) 4N
  ((2N)1/2-1)
• This is lower than N2 for large N
• Ex.2 Redesign the previous three-stage, 200200
  switch using the Clos criteria with a minimum no.
  of crosspoints
• let n (200/2)1/2 10, k 2n-1 19
• total number of crosspoints 9500 (24 of a
  single-stage switch)

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Structure of Circuit Switches

• Time-Division Switch
• uses TDM inside a switch
• Time-Slot Interchange (TSI)
• consists of RAM with memory locations
• size of each location size of a single time
  slot
• RAM fills up with incoming data from time slots
• Time delays exist in RAM filling up

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Structure of Circuit Switches

• Time- and Space-Division Switch Combinations
• take advantage of the best of both
• less crosspoints and less delay (due to TSI)
• TST, TSST, STTS (where T-time, S-space)
• a TST example

37
Structure of Packet Switches

• Packet switch components
• Input ports

38
Structure of Packet Switches

• Packet switch components
• Output ports
• Routing processor
• Table lookup for routing table ? takes some time
• To facilitate and expedite the processor, the
  function of routing processor is being moved to
  the input ports, in the newer packet switches

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Structure of Packet Switches

• Switching Fabrics
• Crossbar switch
• Banyan switch
• multistage switch with microswitches at each
  stage that route the packets based on the output
  port represented as a binary string
• For n inputs and n outputs, log2n stages with n/2
  microswitches at each state

40
Structure of Packet Switches

• Switching Fabrics
• Banyan switch
• example of routing in a banyan switch
• ? possibility of internal collision due to
  multistages

41
Structure of Packet Switches

• Switching Fabrics
• Batcher Banyan switch
• To solve the problem of Banyan switches, sort the
  arrived packets based on their destination port
• Refer to http//www.cs.bham.ac.uk/rzm/teaching/n
  etworks/Queenie/program/BatcherBanyanApplet.html

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Structure of Packet Switches

• Switching Fabrics
• Batcher Banyan switch

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Assignment 7

• Exercises
• 12, 13, 15, 18, 19, 21, 22, 24, 26
• Due Date
• The next class