Basics Of Data Communication: Part 7

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Introduction to LAN, MAN and MAC

There are two types of Network Technology: Switch Networks and Broadcast Networks. In switch network, systems are interconnected by means of point-to-point transmission lines, multiplexers and switches. In switching Networks, the transfer of packet across networks requires routing, to direct packets from source to destination, as the source and destination stations will not be connected by a single transmission link most of the cases. But in Broadcast Networks, all the system are connected to common transmission medium, which acts as a broadcast medium making routing unnecessary. Since the transmission medium is shared by all the connected systems, there is a need for additional layer, consisting of Medium Access Control (MAC) Protocol to orchestrate the transmission from various systems. The role of MAC protocol is to co-ordinate the access to the transmission medium so that packets transmitted from the systems do not interface with other packets. There are two networks based on broadcast medium that is LAN and MAN. The definition of LANs and MANs taken from IEEE 802 standards.

IEEE Standards

A set of network standards developed by the IEEE. They include:

  • IEEE 802.1: Standards related to network management.
  • IEEE 802.2: General standard for the data link layer in the OSI Reference Model. The IEEE divides this layer into two sublayers -- the logical link control (LLC) layer and the media access control (MAC) layer. The MAC layer varies for different network types and is defined by standards IEEE 802.3 through IEEE 802.5.
  • IEEE 802.3: Defines the MAC layer for bus networks that use CSMA/CD. This is the basis of the Ethernet standard.
  • IEEE 802.4: Defines the MAC layer for bus networks that use a token-passing mechanism (token bus networks).
  • IEEE 802.5: Defines the MAC layer for token-ring networks.
  • IEEE 802.6: Standard for Metropolitan Area Networks (MANs).

LAN (Local Area Network)

A LAN is a high-speed data network that covers a relatively small geographic area. It typically connects workstations, personal computers, printers, servers, and other devices. LANs offer computer users many advantages, including shared access to devices and applications, file exchange between connected users, and communication between users via electronic mail and other applications.

LAN data transmissions fall into three classifications: unicast, multicast, and broadcast. In each type of transmission, a single packet is sent to one or more nodes.

  1. In a unicast transmission, a single packet is sent from the source to a destination on a network. First, the source node addresses the packet by using the address of the destination node. The package is then sent onto the network, and finally, the network passes the packet to its destination.
  3. A multicast transmission consists of a single data packet that is copied and sent to a specific subset of nodes on the network. First, the source node addresses the packet by using a multicast address. The packet is then sent into the network, which makes copies of the packet and sends a copy to each node that is part of the multicast address.
  4. A broadcast transmission consists of a single data packet that is copied and sent to all nodes on the network. In these types of transmissions, the source node addresses the packet by using the broadcast address. The packet is then sent on to the network, which makes copies of the packet and sends a copy to every node on the network.
LAN Architecture

The 3 most common types of LAN architectures are:
  • Ethernet
  • Token Ring
  • ArcNet
These architectures are sometimes referred to as "lower-level protocols" because they represent the specifications for the IEE802 model which encompasses the physical (1st) and data link (2nd) layers of the OSI model.
  1. Ethernet is a popular, relatively inexpensive, easy-to-install LAN architecture with the following characteristics:
    • Uses the CSMA/CD media access control.
    • Data transmission normally occurs at 10Mbps.
    • Typically implemented in a bus or star topology.
    • Ethernet LANs are normally distinguished by the type of cable they use (Thinnet, Thicknet, or Twisted Pair).

    The Ethernet architecture conforms to most but not all of the IEEE 802.3 specification (the physical layers are identical but the MAC layers are somewhat different).

  2. Token ring is a relatively expensive LAN architecture that is strongly influenced by IBM. It is very stable and can be expanded without a significant degradation in network performance. Token ring uses the token passing media access control. Data transmission normally occurs at 4 or 16 Mbps depending on the cable. Token ring is normally implemented in a logical ring/physical star topology with a MAU (Multistation Access Unit) as the hub. The maximum number of stations on one ring is 260 for shielded twisted pair and 72 for unshielded twisted pair (UTP). There can be up to 33 MAUs per ring. Token Ring LANs normally use shielded twisted pair (STP) but may also use unshielded twisted pair (UTP) or fiber-optic cable. The maximum distance to the MAU from the workstation depends on the cable and varies from 45 meters for UTP to 100 meters for STP. The Token Ring architecture conforms generally to the IEEE's 802.5 specification
  3. ArcNet (Attached Resource Computing Network) is a relatively inexpensive, reliable, and easy-to-install LAN architecture with the following characteristics:
    • Additional workstations are easily added.
    • ArcNet is a baseband, token passing media access control architecture
    • ArcNet is relatively slow. Data transmission occurs at 2.5 Mbps. (20 Mbps for ArcNet Plus)
    • ArcNet can be implemented in a bus or star topology.
    • ArcNet LANs normally use coaxial cable but can also use twisted pair or fiber-optic cable. Maximum cable segment depends on the type of cable and hub connection (120m -600m)

     The ArcNet architecture conforms very loosely to the IEEE's 802.4 specification. ArcNet is a baseband star or bus and 802.4 defines a broadband bus.
LAN Topologies

A topology refers to the manner in which the cable is run to individual workstations on the network. The dictionary defines topology as: the configurations formed by the connections between devices on a local area network (LAN) or between two or more LANs
There are three basic network topologies (not counting variations thereon): the bus, the star, and the ring.
It is important to make a distinction between a topology and architecture. A topology is concerned with the physical arrangement of the network components. In contrast, an architecture addresses the components themselves and how a system is structured (cable access methods, lower level protocols, topology, etc.).

Bus Topology:

A bus topology connects each computer (node) to a single segment trunk. A 'trunk' is a communication line, typically coax cable that is referred to as the 'bus.' The signal travels from one end of the bus to the other. A terminator is required at each end to absorb the signal so it does not reflect back across the bus.

In a bus topology, signals are broadcast to all stations. Each computer checks the address on the signal (data frame) as it passes along the bus. If the signal's address matches that of the computer, the computer processes the signal. If the address doesn't match, the computer takes no action and the signal travels on down the bus.

Only one computer can 'talk' on a network at a time. A media access method called CSMA/CD is used to handle the collisions that occur when two signals are placed on the wire at the same time. The bus topology is passive. In other words, the computers on the bus simply 'listen' for a signal;
they are not responsible for moving the signal along.

A bus topology is normally implemented with coaxial cable.

Advantages of bus topology:
  • Easy to implement and extend
  • Well suited for temporary networks that must be set up in a hurry
  • Typically the least cheapest topology to implement
  • Failure of one station does not affect others
Disadvantages of bus topology:
  • Difficult to administer/troubleshoot
  • Limited cable length and number of stations
  • A cable break can disable the entire network; no redundancy
  • Maintenance costs may be higher in the long run
  • Performance degrades as additional computers are added
Start Topology:

All of the stations in a star topology are connected to a central unit called a hub.

The hub offers a common connection for all stations on the network. Each station has its own direct cable connection to the hub. In most cases, this means more cable is required than for a bus topology. However, this makes adding or moving computers a relatively easy task; simply plug them into a cable outlet on the wall.

If a cable is cut, it only affects the computer that was attached to it. This eliminates the single point of failure problem associated with the bus topology. (unless, of course, the hub itself goes down.)

Star topologies are normally implemented using twisted pair cable, specifically unshielded twisted pair (UTP). The star topology is probably the most common form of network topology currently in use.

Advantages of star topology:
  • Easy to add new stations
  • Easy to monitor and troubleshoot
  • Can accommodate different wiring

Disadvantages of ring topology:

  • Failure of hub cripples attached stations
  • More cable required

Ring Topology:

A ring topology consists of a set of stations connected serially by cable. In other words, it's a circle or ring of computers. There are no terminated ends to the cable; the signal travels around the circle in a clockwise direction.

Note that while this topology functions logically as ring, it is physically wired as a star. The central connector is not called a hub but a Multi-station Access Unit or MAU. (Don't confuse a Token Ring MAU with a 'Media Adapter Unit' which is actually a transceiver.)
Under the ring concept, a signal is transferred sequentially via a "token" from one station to the next. When a station wants to transmit, it "grabs" the token, attaches data and an address to it, and then sends it around the ring. The token travels along the ring until it reaches the destination address. The receiving computer acknowledges receipt with a return message to the sender. The sender then releases the token for use by another computer.

Each station on the ring has equal access but only one station can talk at a time.

In contrast to the 'passive' topology of the bus, the ring employs an 'active' topology. Each station repeats or 'boosts' the signal before passing it on to the next station.

Rings are normally implemented using twisted pair or fiber-optic cable.

Advantages of ring topology:

  • Growth of system has minimal impact on performance
  • All stations have equal access
Disadvantages of ring topology:
  • Most expensive topology

  • • Failure of one computer may impact others
    • Complex
Tree Topology:

Among all the Network Topologies we can derive that the Tree Topology is a combination of the bus and the Star Topology. The tree like structure allows you to have many servers on the network and you can branch out the network in many ways. This is particularly helpful for colleges, universities and schools so that each of the branches can identify the relevant systems in their own network and yet connect to the big network in some way. A Tree Structure suits best when the network is widely spread and vastly divided into many branches. Like any other topologies, the Tree Topology has its advantages and disadvantages. A Tree Network may not suit small networks and it may be a waste of cable to use it for small networks. Tree Topology has some limitations and the configuration should suit those limitations.

Advantages of Tree topology:
  • A Tree Topology is supported by many network vendors ad even hardware vendors.
  • A point to point connection is possible with Tree Networks.
  • All the computers have access to the larger and their immediate networks.
  • Best topology for branched out networks.
Disadvantage of Tree topology:
  • In a Network Topology the length of the network depends on the type of cable that is being used.
  • The Tree Topology network is entirely dependant on the trunk which is the main backbone of the network. If that has to fail then the entire network would fail.
  • Since the Tree Topology network is big it is difficult to configure and can get complicated after a certain point.
The Tree Topology follows a hierarchical pattern where each level is connected to the next higher level in a symmetrical pattern. Each level in the hierarchy follows a certain pattern in connecting the nodes. Like the top most level might have only one node or two nodes and the following level in the hierarchy might have few more nodes which work on the point to point connectivity and the third level also has asymmetrical node to node pattern and each of these levels are connected to the root level in the hierarchy. Think of a tree that branches out in various directions and all these branches need the roots and the tree trunk to survive. A Tree Structured network is very similar to this and that is why it is called the Tree Topology.

Note: Continue in Next Part.


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