4Chapter Network Eflements

• Network Elements


  • Different topologies of Computers connectivity
  • Various Network types
  • Various network connecting devices
  • LAN Architecture
• Networking


  Interconnection of computers
• Basic reasons why computers need to be networked

  • To share resources (files, printers, modems, fax machines etc.)
  • To share application software (MS Office, Adobe Publisher etc.)
  • To increase productivity (makes it easier to share data amongst users)
• Local Area Network (LAN)


  A network allowing easy access to other computers or peripherals
• Typical characteristics of a LAN


  • Physically limited distance (< 2km)
  • High bandwidth (> 1mbps)
  • Inexpensive cable media (coax or twisted pair)
  • Data and hardware sharing between users owned by the user

  
• Factors that determine the nature of a LAN

  • Topology
  • Transmission medium
  • Medium access control technique
• Protocol Architecture


  The layered protocol concept can be employed to describe the architecture of a LAN, wherein each layer represents the basic functions of a LAN
• Protocol


  Establishes the rules and encoding specifications for sending data
• Higher layer protocols (layer 3 or 4 and above) are independent of network architecture and are not applicable to LAN. Therefore LAN protocols are concerned primarily with the lower layers of the OSI model.

  
• Functions of the physical layer in the IEEE 802 reference model


  • Encoding/ decoding of signals
  • Preamble generation/ removal (for synchronization)
  • Bit transmission/ reception

  
• Functions of MAC


  • Assembling data into a frame with address and error-detection fields for onward transmission
  • Disassemble frame, perform address recognition and error detection during reception
  • Supervise and control the access to the LAN transmission medium
• Functions of LLC


  Provide an interface to the higher layers and perform flow control and error control
• The separation of MAC and LLC is done for the following reasons: The logic and mechanism required to manage access to a shared- access medium is not found in the conventional layer-2 data link control, and for the same LLC, different MAC options may be provided.
• IEEE802.2 Standards


  • IEEE 802.3 (Ethernet)
  • IEEE 802.4 (Token Bus)
  • IEEE 802.5 (Token Ring)
  • IEEE 802.6 (DQDB)
  • IEEE 802.11 (Wireless LAN)
  • IEEE 802.12 (100VG-AnyLAN)

  
• Physical topology


  The configuration of cables, computers, and other peripherals
• Logical topology


  The method used to pass information between workstations
• Common LAN topologies


  • Bus
  • Tree
  • Ring
  • Star
• Bus topology


  A simple design that utilizes a single length of cable, also known as the medium, with directly attached LAN stations through TAP. All stations share this cable segment.
• Every station on the bus segment sees transmissions from every other station on the cable segment; this is known as a broadcast medium.
• Tree topology


  A logical extension of the bus topology, with the physical (cable) plant known as a branching tree with all stations attached to it with no closed loops. The tree begins at the root, the pinnacle point, and expands to the network endpoints.
• Advantages of a Tree Topology

  • Point-to-point wiring for individual segments
  • Supported by several hardware and software venders
• Disadvantages of a Tree Topology

  • Overall length of each segment is limited by the type of cabling used
  • If the backbone line breaks, the entire segment goes down
  • More difficult to configure and wire than other topologies
• Ring topology


  Every device has exactly two neighbors for communication purposes. All messages travel through a ring in the same direction (either "clockwise" or "counterclockwise").
• A failure in any cable or device breaks the loop and can take down the entire network.
• Star topology


  Each station is directly connected to a common central node. Typically, each station attaches to a central node, referred to as the star coupler, via two point-to point links, one for transmission in each direction.
• Advantages of a Star Topology

  • Easy to install and wire
  • No disruptions to the network then connecting or removing devices
  • Easy to detect faults and to remove parts
• Disadvantages of a Star Topology

  • Requires more cable length than a linear topology
  • If the hub or concentrator fails, nodes attached are disabled
  • More expensive than linear bus topologies because of the cost of the concentrators
• Medium Access Control (MAC)

  Some means of controlling access to the transmission medium to provide for an orderly and efficient use of network's transmission capacity
• Two areas for MAC

  • Where (centralized or distributed)
  • How (round robin, reservation, contention)
• Round Robin


  Each station in turn is given an opportunity to transmit. During that period, the station may decline to transmit or may transmit subject to a specified upper bound.
• Reservation


  For stream traffic, time on the medium is divided into slots, and a station wanting to transmit reserves future slots for an extended or even an indefinite period.
• Contention


  For bursty traffic, no control is required to determine whose turn it is; all stations contend for time.
• Standardized Medium Access Control Techniques

  • Bus Topology: Token Bus (IEEE 802.4), Polling (IEEE 802.11), CSMA/CD (IEEE 802.3)
  • Ring Topology: Token Ring (IEEE 802.5 & FDDI)
  • Switched Topology: Request/Priority (IEEE 802.12)
  • Reservation: DQDB (IEEE 802.6)
  • Contention: CSMA/CD (IEEE 802.3), CSMA (IEEE 802.11)
• MAC Frame Format


  The MAC layer receives a block of data from the LLC layer and is responsible for performing functions related to medium access and for transmitting the data. MAC implements these functions, by making use of protocol data unit at its layer; in this case, the PDU is referred to as a MAC frame.
• Some techniques for medium access control are easy to implement and efficient under light to moderate load, but their performance tends to collapse under heavy load
• Standardized Medium Access Control Techniques

  • Bus Topology
  • Ring Topology
  • Switched Topology
  • Round Robin
  • Token Bus (IEEE 802.4)
  • Polling (IEEE 802.11)
  • Token Ring (IEEE 802.5 & FDDI)
  • Request/Priority (IEEE 802.12)
  • Reservation DQDB (IEEE 802.6)
  • Contention CSMA/CD (IEEE 802.3)
  • Contention CSMA (IEEE 802.11)
• MAC Frame


  The PDU used by the MAC layer to transmit data
• MAC Frame Format


  • MAC control field
  • Destination MAC Address
  • Source MAC Address
  • LLC PDU
  • CRC
• MAC control field


  Contains protocol control information needed for the functioning of the MAC protocol
• Destination MAC Address


  The destination physical attachment point on the LAN for this frame