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
(No Transcript)
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.