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Mobile and Wireless Computing

Networking basics

Courtesey of Abdus Salam International Centre for Theoretical Physics, Trieste




Institute for Computer Science, University of Freiburg
Western Australian Interactive Virtual Environments Centre (IVEC)
Mobile and Wireless Computing

Physical Layer

     This layer is concerned with the transmission of
      bits.

     Some of the issues handled in this layer are :
      how many volts for 0, how many for 1; number of
      bits per second to be transmitted; whether it is a
      one-way or two-way communication etc.

     Many standards have been developed, e.g., RS-
      232 for serial communication lines.
Institute for Computer Science, University of Freiburg
Western Australian Interactive Virtual Environments Centre (IVEC)
Mobile and Wireless Computing

Data Link Layer

     This layer groups bits into frames and ensures
      their correct delivery.

     It adds some bits at the beginning and end of
      each frame plus the checksum. Receiver of the
      frame verifies the checksum and requests
      retransmission if the checksum is not correct.

     This layer consists of two sublayers.

Institute for Computer Science, University of Freiburg
Western Australian Interactive Virtual Environments Centre (IVEC)
Mobile and Wireless Computing

Logical Link Control and Medium Access Control



     The LLC defines how data is transferred over
      the network medium (e.g., cables) and provides
      data link service to the higher layers.
     The Medium Access Control (MAC) layer defines
      who can use the network medium when multiple
      computers are trying to access it simultaneously.
     For example, token passing or CSMA/CD for
      Ethernet.

Institute for Computer Science, University of Freiburg
Western Australian Interactive Virtual Environments Centre (IVEC)
Mobile and Wireless Computing

Network Layer

     This layer is concerned with the transmission of
      packets. It takes routing decisions, e.g., the best
      path to send a packet.

     The network layer may be quite complex in a
      large network, e.g., the Internet.

     Most protocols are connection oriented (packets
      are sent when the destination accepts a
      connection) or connection-less, like IP.
Institute for Computer Science, University of Freiburg
Western Australian Interactive Virtual Environments Centre (IVEC)
Mobile and Wireless Computing

Transport Layer

     This layer ensures reliable service and deals
      with lost messages.

     It breaks the messages into smaller packets,
      assigns sequence number and sends them.
      Reliable transport connections are built on top of
      X.25 or IP.

     Examples are TCP (Transfer Control Protocol),
      TCP/IP, UDP etc.
Institute for Computer Science, University of Freiburg
Western Australian Interactive Virtual Environments Centre (IVEC)
Mobile and Wireless Computing

Sessions Layer

     Very few applications use this layer. It is mostly
      an enhanced version of the transport layer.

     It facilitates dialog control and synchronization.

     Many network implementations do not support it.
      For example, Internet suite does not support this
      layer.


Institute for Computer Science, University of Freiburg
Western Australian Interactive Virtual Environments Centre (IVEC)
Mobile and Wireless Computing

Presentation Layer

     Very few applications use this layer. This layer is
      concerned with the semantics of the bits.

     It defines records and fields in them. The sender
      can tell the receiver about the format.

     It makes machines with different internal
      representations to communicate. It is the best
      layer for implementing cryptographic services.

Institute for Computer Science, University of Freiburg
Western Australian Interactive Virtual Environments Centre (IVEC)
Mobile and Wireless Computing

Application Layer

     This is a collection of miscellaneous protocols
      for high level applications.

     Typical services are electronic mail, file transfer,
      connecting remote terminals etc.

     FTP, telnet, HTTP, NFS etc.



Institute for Computer Science, University of Freiburg
Western Australian Interactive Virtual Environments Centre (IVEC)
Mobile and Wireless Computing

Multiplexing

     In a mobile and wireless network, the wireless
      medium is shared by many nodes.

     Hence, multiple use of a shared medium is a
      major challenge in wireless networking.

     Most decisions for accessing the wireless
      medium is made in the MAC layer.


Institute for Computer Science, University of Freiburg
Western Australian Interactive Virtual Environments Centre (IVEC)
Mobile and Wireless Computing

Multiplexing

     The wireless channels can be multiplexed (used
      by multiple machines) in four dimensions :
        –    Space (s)
        –    Time (t)
        –    Frequency (f)
        –    Code (c)




Institute for Computer Science, University of Freiburg
Western Australian Interactive Virtual Environments Centre (IVEC)
Mobile and Wireless Computing

Space multiplexing : Cellular Networks

     Same frequency can be
      reused when the base
      stations are separated in
      space.

     The reuse of frequencies
      depend on signal
      propagation range.

     Example : fixed frequency
      assignment for reuse with
      distance 2.
Institute for Computer Science, University of Freiburg
Western Australian Interactive Virtual Environments Centre (IVEC)
Mobile and Wireless Computing

Frequency Division Multiplex (FDM)

     The whole spectrum is
      separated into smaller
      frequency bands.
     A band is allocated to a
      channel for the whole
                                                                                                   f
      time.
     This is somewhat
      inflexible if the traffic is
      non-uniform.
     An example is radio or TV
      broadcast. The bandwidth
      is wasted if a station is off
      the air.                                                      t
Institute for Computer Science, University of Freiburg
Western Australian Interactive Virtual Environments Centre (IVEC)
Mobile and Wireless Computing

Time Division Multiplex (TDM)

     A channel gets the whole
      frequency spectrum for a
      certain amount of time.
     Only one user for the                                                                       f
      medium at a time.
     Usually the throughput is
      high even with many
      users.
     However, no two users
      should use the medium at
      the same time. Precise
      synchronization is                                            t
      needed.
Institute for Computer Science, University of Freiburg
Western Australian Interactive Virtual Environments Centre (IVEC)
Mobile and Wireless Computing

An example of TDM

     Ethernet uses a protocol called CSMA/CD
       Carrier Sense Multiple Access with Collision
      Detection
     When a node wants to broadcast, it checks
      whether any other node is broadcasting (senses
      the carrier).

     A node broadcasts when no other node is
      broadcasting. Otherwise, it tries later at a
      random interval.
Institute for Computer Science, University of Freiburg
Western Australian Interactive Virtual Environments Centre (IVEC)
Mobile and Wireless Computing

CSMA Problems in Wireless Medium

     Collision detection is easy in wired networks but
      difficult in wireless medium.

     With only one antenna, nodes can only listen or
      send.

     Full duplex radios are extremely expensive.

     CSMA gives rise to hidden terminal and exposed
      terminal problems.
Institute for Computer Science, University of Freiburg
Western Australian Interactive Virtual Environments Centre (IVEC)
Mobile and Wireless Computing

Hidden Terminal Problem

     Wireless transmission is usually short range.
      Even if the medium is free near the transmitter, it
      may not be free near the intended receiver.



                                 A                             B     C

                                                         Collision
                                                          at B

Institute for Computer Science, University of Freiburg
Western Australian Interactive Virtual Environments Centre (IVEC)
Mobile and Wireless Computing

Exposed Terminal Problem

     Even if the medium is busy near the transmitter,
      it may be free near the intended receiver.




                        A                           B               C   D




                     C cannot transmit because B is transmitting.

Institute for Computer Science, University of Freiburg
Western Australian Interactive Virtual Environments Centre (IVEC)
Mobile and Wireless Computing

Message Loss due to Collision

     Using CSMA in wireless medium results in
      message loss and requires retransmission of
      lost messages.

     A node spends much more energy while
      receiving or transmitting messages. Hence,
      retransmission wastes a lot of energy.

     The other alternative is to use a reservation
      based TDM protocol.
Institute for Computer Science, University of Freiburg
Western Australian Interactive Virtual Environments Centre (IVEC)
Mobile and Wireless Computing

Demand Assignment Multiple Access (DAMA)

     In a DAMA protocol, nodes first reserve slots
      which they intend to use for broadcasting.

     Each round of broadcast is preceded by a
      reservation round.

     DAMA protocols are widely used in satellite
      communication and increasingly being used in
      wireless networking.

Institute for Computer Science, University of Freiburg
Western Australian Interactive Virtual Environments Centre (IVEC)
Mobile and Wireless Computing

Code Division Multiplex (CDM)

    Each channel uses a
     unique code for                                                c
     transmitting.

    All channels use the
     same frequency
     spectrum at the same
     time.                                                                                    t
                                                                    f
      However, signal
       regeneration is very
       complex and requires
       complex HW/SW support.
Institute for Computer Science, University of Freiburg
Western Australian Interactive Virtual Environments Centre (IVEC)
Mobile and Wireless Computing

Code Division Multiplexing

     CDMA has ben adopted for the 3G mobile phone
      technology.

     CDMA is not very suitable for ad hoc networking
      as we cannot expect specialized
      hardware/software support at the nodes.

     TDMA and its variations are most suitable for ad
      hoc networking.

Institute for Computer Science, University of Freiburg
Western Australian Interactive Virtual Environments Centre (IVEC)
Mobile and Wireless Computing

The Organization of the Course

     We will mainly study routing protocols in this
      course.

     Routing messages is the most important issue in
      any network. Conventional protocols for wired
      networks do not work well for ad hoc networks.

     We will study routing protocols both for ad hoc
      mobile networks and mobile IP.

Institute for Computer Science, University of Freiburg
Western Australian Interactive Virtual Environments Centre (IVEC)

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wireless trends

  • 1. Mobile and Wireless Computing Networking basics Courtesey of Abdus Salam International Centre for Theoretical Physics, Trieste Institute for Computer Science, University of Freiburg Western Australian Interactive Virtual Environments Centre (IVEC)
  • 2. Mobile and Wireless Computing Physical Layer  This layer is concerned with the transmission of bits.  Some of the issues handled in this layer are : how many volts for 0, how many for 1; number of bits per second to be transmitted; whether it is a one-way or two-way communication etc.  Many standards have been developed, e.g., RS- 232 for serial communication lines. Institute for Computer Science, University of Freiburg Western Australian Interactive Virtual Environments Centre (IVEC)
  • 3. Mobile and Wireless Computing Data Link Layer  This layer groups bits into frames and ensures their correct delivery.  It adds some bits at the beginning and end of each frame plus the checksum. Receiver of the frame verifies the checksum and requests retransmission if the checksum is not correct.  This layer consists of two sublayers. Institute for Computer Science, University of Freiburg Western Australian Interactive Virtual Environments Centre (IVEC)
  • 4. Mobile and Wireless Computing Logical Link Control and Medium Access Control  The LLC defines how data is transferred over the network medium (e.g., cables) and provides data link service to the higher layers.  The Medium Access Control (MAC) layer defines who can use the network medium when multiple computers are trying to access it simultaneously.  For example, token passing or CSMA/CD for Ethernet. Institute for Computer Science, University of Freiburg Western Australian Interactive Virtual Environments Centre (IVEC)
  • 5. Mobile and Wireless Computing Network Layer  This layer is concerned with the transmission of packets. It takes routing decisions, e.g., the best path to send a packet.  The network layer may be quite complex in a large network, e.g., the Internet.  Most protocols are connection oriented (packets are sent when the destination accepts a connection) or connection-less, like IP. Institute for Computer Science, University of Freiburg Western Australian Interactive Virtual Environments Centre (IVEC)
  • 6. Mobile and Wireless Computing Transport Layer  This layer ensures reliable service and deals with lost messages.  It breaks the messages into smaller packets, assigns sequence number and sends them. Reliable transport connections are built on top of X.25 or IP.  Examples are TCP (Transfer Control Protocol), TCP/IP, UDP etc. Institute for Computer Science, University of Freiburg Western Australian Interactive Virtual Environments Centre (IVEC)
  • 7. Mobile and Wireless Computing Sessions Layer  Very few applications use this layer. It is mostly an enhanced version of the transport layer.  It facilitates dialog control and synchronization.  Many network implementations do not support it. For example, Internet suite does not support this layer. Institute for Computer Science, University of Freiburg Western Australian Interactive Virtual Environments Centre (IVEC)
  • 8. Mobile and Wireless Computing Presentation Layer  Very few applications use this layer. This layer is concerned with the semantics of the bits.  It defines records and fields in them. The sender can tell the receiver about the format.  It makes machines with different internal representations to communicate. It is the best layer for implementing cryptographic services. Institute for Computer Science, University of Freiburg Western Australian Interactive Virtual Environments Centre (IVEC)
  • 9. Mobile and Wireless Computing Application Layer  This is a collection of miscellaneous protocols for high level applications.  Typical services are electronic mail, file transfer, connecting remote terminals etc.  FTP, telnet, HTTP, NFS etc. Institute for Computer Science, University of Freiburg Western Australian Interactive Virtual Environments Centre (IVEC)
  • 10. Mobile and Wireless Computing Multiplexing  In a mobile and wireless network, the wireless medium is shared by many nodes.  Hence, multiple use of a shared medium is a major challenge in wireless networking.  Most decisions for accessing the wireless medium is made in the MAC layer. Institute for Computer Science, University of Freiburg Western Australian Interactive Virtual Environments Centre (IVEC)
  • 11. Mobile and Wireless Computing Multiplexing  The wireless channels can be multiplexed (used by multiple machines) in four dimensions : – Space (s) – Time (t) – Frequency (f) – Code (c) Institute for Computer Science, University of Freiburg Western Australian Interactive Virtual Environments Centre (IVEC)
  • 12. Mobile and Wireless Computing Space multiplexing : Cellular Networks  Same frequency can be reused when the base stations are separated in space.  The reuse of frequencies depend on signal propagation range.  Example : fixed frequency assignment for reuse with distance 2. Institute for Computer Science, University of Freiburg Western Australian Interactive Virtual Environments Centre (IVEC)
  • 13. Mobile and Wireless Computing Frequency Division Multiplex (FDM)  The whole spectrum is separated into smaller frequency bands.  A band is allocated to a channel for the whole f time.  This is somewhat inflexible if the traffic is non-uniform.  An example is radio or TV broadcast. The bandwidth is wasted if a station is off the air. t Institute for Computer Science, University of Freiburg Western Australian Interactive Virtual Environments Centre (IVEC)
  • 14. Mobile and Wireless Computing Time Division Multiplex (TDM)  A channel gets the whole frequency spectrum for a certain amount of time.  Only one user for the f medium at a time.  Usually the throughput is high even with many users.  However, no two users should use the medium at the same time. Precise synchronization is t needed. Institute for Computer Science, University of Freiburg Western Australian Interactive Virtual Environments Centre (IVEC)
  • 15. Mobile and Wireless Computing An example of TDM  Ethernet uses a protocol called CSMA/CD Carrier Sense Multiple Access with Collision Detection  When a node wants to broadcast, it checks whether any other node is broadcasting (senses the carrier).  A node broadcasts when no other node is broadcasting. Otherwise, it tries later at a random interval. Institute for Computer Science, University of Freiburg Western Australian Interactive Virtual Environments Centre (IVEC)
  • 16. Mobile and Wireless Computing CSMA Problems in Wireless Medium  Collision detection is easy in wired networks but difficult in wireless medium.  With only one antenna, nodes can only listen or send.  Full duplex radios are extremely expensive.  CSMA gives rise to hidden terminal and exposed terminal problems. Institute for Computer Science, University of Freiburg Western Australian Interactive Virtual Environments Centre (IVEC)
  • 17. Mobile and Wireless Computing Hidden Terminal Problem  Wireless transmission is usually short range. Even if the medium is free near the transmitter, it may not be free near the intended receiver. A B C Collision at B Institute for Computer Science, University of Freiburg Western Australian Interactive Virtual Environments Centre (IVEC)
  • 18. Mobile and Wireless Computing Exposed Terminal Problem  Even if the medium is busy near the transmitter, it may be free near the intended receiver. A B C D C cannot transmit because B is transmitting. Institute for Computer Science, University of Freiburg Western Australian Interactive Virtual Environments Centre (IVEC)
  • 19. Mobile and Wireless Computing Message Loss due to Collision  Using CSMA in wireless medium results in message loss and requires retransmission of lost messages.  A node spends much more energy while receiving or transmitting messages. Hence, retransmission wastes a lot of energy.  The other alternative is to use a reservation based TDM protocol. Institute for Computer Science, University of Freiburg Western Australian Interactive Virtual Environments Centre (IVEC)
  • 20. Mobile and Wireless Computing Demand Assignment Multiple Access (DAMA)  In a DAMA protocol, nodes first reserve slots which they intend to use for broadcasting.  Each round of broadcast is preceded by a reservation round.  DAMA protocols are widely used in satellite communication and increasingly being used in wireless networking. Institute for Computer Science, University of Freiburg Western Australian Interactive Virtual Environments Centre (IVEC)
  • 21. Mobile and Wireless Computing Code Division Multiplex (CDM)  Each channel uses a unique code for c transmitting.  All channels use the same frequency spectrum at the same time. t f  However, signal regeneration is very complex and requires complex HW/SW support. Institute for Computer Science, University of Freiburg Western Australian Interactive Virtual Environments Centre (IVEC)
  • 22. Mobile and Wireless Computing Code Division Multiplexing  CDMA has ben adopted for the 3G mobile phone technology.  CDMA is not very suitable for ad hoc networking as we cannot expect specialized hardware/software support at the nodes.  TDMA and its variations are most suitable for ad hoc networking. Institute for Computer Science, University of Freiburg Western Australian Interactive Virtual Environments Centre (IVEC)
  • 23. Mobile and Wireless Computing The Organization of the Course  We will mainly study routing protocols in this course.  Routing messages is the most important issue in any network. Conventional protocols for wired networks do not work well for ad hoc networks.  We will study routing protocols both for ad hoc mobile networks and mobile IP. Institute for Computer Science, University of Freiburg Western Australian Interactive Virtual Environments Centre (IVEC)