Categories of Computer Network

Networks can also be categorized depending on their physical size. Major categories of computer networks are

1. Local area network
2. Metropolitan area network
3. Wide area network

Local Area Network (LAN)

• The IEEE 802 LAN is a popularly used shared medium peer-to-peer communications network that broadcasts information for all stations to receive.
• The LAN enables stations to communicate directly using a common physical medium on a point-to-point basis without any intermediate switching node being required.
• A LAN is a system composed of computer hardware and transmission media and software.
• LANs are privately owned networks within a single building or campus of upto few km in range.
• It generally use only one type of transmission media.
• Depends upon application and cost, various topology used in LAN. (e.g. star, bus, ring)
• The basic idea of a LAN is to provide easy access to data terminal equipment (DTEs) within the office. These DTEs are not only computers but other devices, files and databases.
• LAN can provide users

1. Flexibility
2. Speed
3. Adaptability
4. Reliability
5. Security
6. Transparent interface
7. Access to the other LAN and WAN
8. Hardware and software sharing
9. Centralized management
10. Private ownership of the LAN

• Attributes of LAN

1. The LAN transmits data among user stations.
2. The LAN transmission capacity is more than 1 Mbps.
3. The LAN channel is typically privately owned by the organization using the facility.
4. The geographical coverage of LANs is limited to areas less than 5 square kilometers.

• LANs are typically identified by the following properties-

1. Multiple systems attached to shared medium
2. High total bandwidth (-10 mbps)
3. Low delay
4. Low error rate
5. Broadcast/multicast capability
6. Limited geography (1-2 km)
7. Limited number of stations
8. Peer relationship between stations
9. Confined to private property

• The low level protocols used in such environments are different from those used in wide area networks.
• The common forms of LAN are those described by the IEEE standard 802. This standard describes operation upto and including OSI layer 2. Individuals may build what they like on top of these basic protocols.
• A common set of higher level protocols is called TCP/IP which provides OSI layer 3 and 4 functionality, on the top of this may be found a set of protocols commonly called telnet protocols.
• At the lowest level, the IEEE 802 specifications split into 3 corresponding to three different but common LAN structures. These are 802.3, 802.4, 802.5 standards for topology.
• LANs are capable of transmitting data at very fast rates, much faster than data can be transmitted over a telephone line : but the distances are limited and there is also a limit on the number of computers that can be attached to a single LAN.
• The following characteristics differentiate one LAN from another :

1. Topology : The geometric arrangement of devices on the network. For example, devices can be arranged in a ring or in a straight line.
2. Protocols : The rules and encoding specifications for sending data. The protocols also determine whether the network uses a peer-to-peer or client/ server architecture.
3. Media : Devices can be connected by twisted-pair wire, co-axial cables or fiber optic cables. Some networks do without connecting media altogether, communicating instead via radio waves.

Metropolitan Area Network (MAN)

• A MAN, while larger than LAN is limited to city or group nearby corporation offices. It uses similar technology of LAN.
• The metropolitan area network standards are sponsored by the IEEE,ANSI and the regional bell operating companies. The MAN standard is organized around a topology and technique called Distributed Queue Dual Bus (DQDB).
• MAN provides the transfer rates from 34 to 150 Mbps.
• MAN is designed with two unidirectional buses. Each bus is independent of the other in the transfer of traffic. The topology can be designed as an open bus or a closed configuration.
• MANs are based on fiber optic transmission technology and provide high speed interconnection between sites. It can support both data and voice.
• MAN as a special category is that a standard has been adopted for them and this standard is now being implemented. It is called IEEE 802.6.

Wide Area Networks (WAN)

• A WAN provides long distance transmission of data and voice.
• A network that covers a larger area such as a city, state, country or the world is called Wide Area Network.
• The WAN contains host and collection of machines. User program is installed on the host and machines. All the host are connected by each other through communication subnet. Subnet carries messages from host to host.
• Subnet consists of transmission lines and switching elements. The transmission line is used for data transfer between two machines. Switching elements are used for connecting two transmission lines. Switching elements are specialized computers. It selects the proper outgoing line for incoming data and forward the data on that line.
• The switching elements are basically computers and they are called packet switching nodes, intermediate systems and data switching exchanges. These switching elements are also called routers.
• In the relationship between hosts, subnet and routers in WAN, each host is connected to a LAN on which a router is present. Sometimes the host can be directly connected to the router. The interconnection of routers forms the subnet.
• In the WAN, when the packet is sent from one router to another via one or more intermediate routers, the packet is received at each intermediate router in the entirety. This packet is stored in that router until the required output line is free. The subnet which uses this principle is called point to point, store and forward or packet switched subnet.
• Almost all the WANs use store and forward subnets.
• If the packet are small and of same size, they are also called cells.
• In the point-to-point subnet, the router interconnection topology becomes important. WANs can also use satellite or ground radio system. The routers have antenna, through which they can send or receive data, they can listen from satellite.
• WAN uses hierarchical addressing because they facilitate routing. Addressing is required to identify which network is to be connected to which network output.

Global Area Network

• Global Area Network (GAN) is a network that is composed of different interconnected computer networks and covers an unlimited geographical area. It refers to any network that is composed of different interconnected computer networks(WANs) and also covers an unlimited geographical area. GAN operate from 1.5 Mbps to 100 Gbps and cover thousands of miles.

Building backbone

• A building backbone is a network connection that normally carries traffic between department LAN, within a single organization.

Interconnection of networks

• In a LAN, when number of users or stations ae more and if the stations are located at a longer distances forming bigger LAN is not always possible due to following limitations:

1. Constraint of maximum number of stations which can be connected on a single LAN.
2. Constraint of maximum physical size of a LAN.
3. Network security.
4. Investment already done and further to be done.

• If LANs which are geographically separated or if they are located in different corners of a city or in different cities, are needed to be connected or communicated, again these LANs may not be of the same type i.e. one of these may be CSMA/CD LAN and other may be FDDI LAN. The different LANs can be interconnected.
• The LANs and switched data subnetworks are interconnected to allow the users to communicate across several subnetworks. This extended network so formed is called as internetwork.
• Subnetworks interconnected may differ in technologies, topologies, protocols and their services. Therefore, some special intermediatary devices are required to sort out the differences. Thus enabling the users of one subnet to communicate with users on another subnet. The interconnected devices such as bridges, routers are used to connect the two or more LANs or subnets.
• The functions of intermediatary devices are-

1. Matching the protocols of the peer layers.
2. Matching the services offended by different layers.
3. Relaying the data units from one subnet to another.
4. Routing of data units from one subnet to other.

Each intermediary device has different capabilities and functions.

Significance of internetworking

1. Provides a link between network.
2. Provides routing and delivery of data between process on different network.
3. Provides an accounting service that keeps track of the various networks and routers and that maintains status information.
4. Internetworking facility accomodate a number of differences among networks which includes different addressing schemes, different maximum packet size, different access mechanisms, error recovery, status reporting, routing techniques etc.