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Communications: Network

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Title: Communications: Network


1
CommunicationsNetwork
2
Communication Networks
  • Computers in a DS are interconnected through a
    computer communication network
  • Computer can exchange messages with other
    computers and access data stored at another
    computer through this network
  • Layered protocols are commonly used for
    communication purpose

3
Layered Protocols
4
Layered Protocol
5
The OSI Model
  • A widely accepted structuring technique is
    layering
  • The communications functions are partitioned into
    a hierarchical set of layers
  • Each layer performs a related subset of the
    functions required to communicate with another
    system
  • The resulting OSI architecture has seven layers

6
The OSI Model
  • Physical layer
  • The physical layer is responsible for handling
    both the mechanical and electrical details of the
    physical transmission of a bit stream
  • This layer is implemented in the hardware of the
    network device.
  • Data-link layer
  • Provides for the reliable transfer of data across
    the physical link
  • sends blocks of data (frames) with the necessary
    synchronization, error detection and error
    control.

7
The OSI Model
  • Network layer
  • The network layer is responsible for providing
    connections and for routing packets in the
    communication network
  • includes handling the address of outgoing
    packets, decoding the address of incoming
    packets, and maintaining routing information for
    proper response to changing load levels.
  • Transport layer
  • The transport layer is responsible for two level
    accesses to the network and for transfer of
    messages between the clients
  • includes partitioning the messages into packets,
    maintaining packet order, controlling flow, and
    generating physical addresses

8
The OSI Model
  • Session layer
  • Session layer is responsible for implementing
    sessions, or process to process communications
    protocols
  • Typically these protocols are the actual
    communications for remote logins and for file and
    mail transfers.
  • Presentation layer
  • Presentation layer performs transformations on
    data to provide a standardization application
    interface and provide common communications
    services
  • examples encryption, text compression,
    reformatting

9
The OSI Model
  • Application layer
  • The application layer is responsible for
    interacting directly with the users
  • This layer deals with file transfer, remote login
    protocol, and electronic mail. 

10
Topology
  • The sites in the system can be connected
    physically in a variety of ways. Each
    configuration has advantages and disadvantages
  • We describe briefly the most common
    configurations implemented, and compare them with
    respect to the following criteria
  • Basic Cost How expensive is it to link the
    various sites in the system?
  • Communication Cost How long does it take to send
    a message from site A to site B?
  • Reliability If a link or a site in the system
    fails, can the remaining sites still communicate
    with one another?

11
 Partially Connected Networks
  • Direct links exist between some, but not all,
    pairs of sites
  • Hence, the basic cost of this configuration is
    lower than that of the fully connected network
  • A message from one site to another may have to be
    sent through several intermediate sites,
    resulting in slower communication.

12
Partially Connected Networks
13
Hierarchical Networks
  • In a hierarchical network, the sites are
    organized as a tree
  • This organization is commonly used for corporate
    networks
  • Individual offices are linked to the local main
    office
  • Main offices are linked to regional offices
    regional offices are linked to corporate
    headquarters

14
Hierarchical Networks
15
Star Networks
  • In a star network, the workstations are directly
    wired to the server or to a central wiring hub
  • Ethernet networks which comply with the current
    10Base-T standard may be configured in a physical
    hierarchical star topology
  • star topology gives each workstation a dedicated
    line between itself and server.

16
Star Networks
17
 Ring Networks
  • The ring network topology is much like the bus in
    that each workstation and file server is attached
    to a central cable
  • the workstations and file server are connected
    together to form a ring
  • The workstations and file servers take turns
    passing information from one to another until the
    information reaches its final destination

18
 Ring Networks
19
Bus Topology
  • Most common of physical network layouts
  • Associated with the Ethernet protocol
  • Cabling in a bus topology consists of a single or
    central cable to which the workstations and
    servers are connected
  • This topology can use coaxial and twisted-pair
    cabling.

20
Bus Topology

21
Network Types
  • There are basically two types of networks
  • local-area networks and
  • wide-area networks.
  • Main difference between is the way in which they
    are geographically distributed
  • Local-area networks are composed of processors
    that are distributed over small geographical
    areas, such as a single building or a number of
    adjacent buildings
  • Wide-area networks, on the other hand, are
    composed of a number of autonomous processors
    that are distributed over a large geographical
    area (such as the United States).
  • These differences imply major variations in the
    speed and reliability of the communications
    network, and are reflected in the distributed
    operating-system design.

22
Local-Area Networks
  • LANs emerged in the early 1970s as a substitute
    for large mainframe computer systems
  • It had become apparent that, for many
    enterprises, it is more economical to have a
    number of small computers each with its own
    self-contained applications, rather than a single
    large system
  • Each small computer is likely to need a full
    complement of peripheral devices (such as disks
    and printers), and because some form of data
    sharing is likely to occur in a single enterprise

23
 Wide-Area Networks
  • Wide-area networks allows hardware and software
    to be shared conveniently and economically by a
    wide community of users
  • The first WAN to be designed and developed was
    the Arpanet. Work on the Arpanet began in 1968
  • The Arpanet has grown from a four-site
    experimental network to a worldwide network of
    networks, the Internet, comprising thousands of
    computer systems
  • Recently, several commercial networks have also
    appeared on the market. The Telenet system is
    available within the continental United States
    The Datapac system is available in Canada
  • These networks provide their customers with the
    ability to access a wide range of hardware and
    software computing resources.

24
 Communication
  •   The designer of a communication network must
    address four basic issues
  • Naming and Name Resolution How do two processes
    locate each other to communicate?
  • Routing Strategies How are messages sent through
    the network?
  • Packet Strategies Are packets sent individually
    or as a sequence?
  • Connection Strategies How do two processes send
    a sequence of messages?
  • Contention The network is a shared resource, so
    how do we resolve conflicting demands for its
    use?

25
Naming and Name Resolution
  • The first component of network communication is
    the naming of the systems in the network
  • For a process at site A to exchange information
    with a process at site B, they must be able to
    specify each other
  • Within a computer system, each process has a
    process-id, and messages may be addressed with
    the process-id
  • Because networked systems share no memory, they
    initially have no knowledge of the host of their
    target process, or even if the other process
    exists.

26
Routing Strategies
  • The three most common routing schemes are fixed
    routing, virtual routing, and dynamic routing
  • Fixed Routing
  • A path from A to B is specified in advance and
    does not change unless a hardware failure
    disables this path
  • Usually, the shortest path is chosen, so that
    communication costs are minimized.
  • Virtual Circuit
  • A path from A to B is fixed for the duration of
    one session
  • Different sessions involving messages from A to
    B may have different paths
  • A session could be as short as a file transfer
    or as long as a remote-login period.
  • Dynamic Routing
  • The path used to send a message from site A to
    site B is chosen only when a message is sent.
    Because the decision is made dynamically,
    separate messages may be assigned different
    paths
  • Site A will make a decision to send the message
    to site C C, in turn, will decide to send it to
    site D, and so on. Eventually, a site will
    deliver the message to B.

27
Packet Strategies
  • Messages are generally of variable length
  • We commonly implement communication with
    fixed-length messages called packets, frames, or
    datagram's
  • A communication implemented in one packet can be
    sent into its destination in a connectionless
    message
  • A connectionless message can be unreliable,
    meaning that the sender is not guaranteed, and
    cannot tell, if the packet reached its
    destination
  • the packet can be reliable, usually with a packet
    returned from the destination indicating that the
    packet arrived. (Of course the return packet
    could be lost along the way.)
  • If a message is too long to fit within one
    packet, or if the packets need to flow back and
    forth between the two communications, a
    connection needs to be established to allow the
    reliable exchange of multiple packets.

28
Connection Strategies
  • The three most common schemes are circuit
    switching, message switching, and packet
    switching
  • Circuit Switching Communication via circuit
    switching implies that there is a dedicated
    communication path between two stations.
  • That path is a connected sequence of links
    between network nodes. On each physical link, a
    logical channel is dedicated to the connection.

29
Connection Strategies
  • Packet Switching
  • Once logical message may have to be divided into
    a number of packets
  • Each packet may be sent to its destination
    separately, and therefore must include a source
    and destination address with its data
  • Each packet may take a different path through the
    network
  • The physical path between two end points may
    change often because packets take advantage of
    least cost routes or avoid congested area

30
Connection Strategies
31
 Packets
  • A packet is a package of data that is exchanged
    between devices over a data communication link.
    The data exchanged between devices may take the
    following form
  • Messages and commands, such as a request for
    service.
  • Control codes for managing the session, such as
    codes that indicate communication errors and the
    need for retransmission.
  • Data, such as the contents of a file.
  • One of the main reasons for packetizing and
    framing information is that any errors on the
    communication link only affect a small,
    discernible part of the transmission, which is
    easily retransmitted.

32
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33
Contention
  • Depending on the network topology, a link may
    connect more than two sites in the computer
    network,
  • It is possible that several sites will want to
    transmit information over a link simultaneously
  • This difficulty occurs mainly in a ring or
    multiaccess bus network. In this case, the
    transmitted information may become scrambled and
    must be discarded

34
Contention
  • Several techniques have been developed to avoid
    repeated collisions, including collision
    detection, token passing, and message slots.
  • CSMA/CD
  • Before transmitting a message over a link, a site
    must listen to determine whether another message
    is currently being transmitted over that link
  • this technique is called carrier sense with
    multiple access (CSMA). If the link is free, the
    site can start transmitting
  • Otherwise it must wait (and continue to listen)
    until the link is free. If two or more sites
    begin transmitting at exactly the same time (each
    thinking that no other site is using the link),
    then they will register a collision detection
    (CD) and will stop transmitting.

35
Contention
  • Token Passing
  • A unique message type, known as a token,
    continuously circulates in the system (usually a
    ring structure)
  • A site that wants to transmit information must
    wait until the token arrives. It removes the
    token from the ring and begins to transmit its
    messages
  • When the site completes its round of message
    passing, it transmits the token
  • This action, in turn, allows another site to
    receive and remove the token, and to starts its
    message transmission. If the token gets lost,
    then the systems must detect the loss and
    generate a new token
  • They usually do that by declaring an election, to
    elect a unique site where a new token will be
    generated. A token-passing scheme has been
    adopted by the IBM and HP/Apollo systems. The
    benefit of a token-passing network is that
    performance is constant.

36
Contention
  • Message Slots
  • A number of fixed-length message slots
    continuously circulate in the system (usually a
    ring structure)
  • Each slot can hold a fixed-sized message and
    control information (such as what the source and
    destination are, and whether the slot is empty or
    full)
  • A site that is ready to transmit must wait until
    an empty slot arrives. If then inserts its
    message into the slot, setting the appropriate
    control information
  • The slot with its message then continues in the
    network. When it arrives at a site, that site
    inspects the control information to determine
    whether the slot contains a message for this
    site. If not, that site re-circulates the slot
    and message. Otherwise, it removes the message,
    resetting the control information to indicate
    that the slot is empty.
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