Network Topology

The physical topology of LAN refers to the way in which the stations are physically interconnected.

Topology is also defined as, the manner in which nodes are geometrically arranged and connected is known as the topology of the network.

Physical topology of a local area network should have the following desirable features.

1. The topology should be flexible to accommodate changes in physical locations of the stations, increase in the number of stations and increase in the LAN geographic coverage.

2. The cost of physical media and installation should be minimum.

3. The network should not have any single point of complete failures.

Network topology refers to the physical layout of the network. Each topology has its own strengths and weaknesses.

• Four types of topologies are commonly used in the network. They are bus, star ring and mesh topology.

Bus Topology

• Bus topology also called horizontal topology.

In bus topology, multiple devices are connected one by one, by means connectors or drop cables.

When one computer sends a signal up (and down) the wire, all the computers or the network receive the information, but only one accepts the information (using address matching). The rest discard the message.

Star Topology

A star topology consists of a number of devices connected by point-to-point links to a central hub.

Easy to control and traffic flow is simple.

Data travels from the sender to central hub and then to the receiver.

Ring Topology

In a ring topology, each computer is connected to the next computer, with the last one connected to the first. The signals travel on the cable in only one direction. Since each computer retransmits what it receives.
Ring is an active network. Termination is not required.
.The mesh topology has a link between each device in the network. It is more difficult to install as the number of devices increases.
Mesh networks are easy to troubleshoot.

Much of the bandwidth available in mesh configuration is wasted.

Most mesh topology networks are not true mesh networks. Rather, they are hybrid mesh networks, which contain some most important sites with multiple links.

Data Link Layer Design Issues

Some important functions of data link layer include well defined service interface to the network layer, framing, flow control, error detection and error control, frame formatting and sequencing. All these are very important functions for reliable communication and plays a vital role in designing data link layer.

Services Provided to the Network Layer

• The primary responsibility of data link layer is to provide services to the network layer. The principle service is transferring data from the network layer on the source machine to the network layer on the destination machine.

• The two data link layer communicates with each other by data link control protocol.

• Following are the important services provided by data link layer to the network


1) Unacknowledged connectionless service.

2) Acknowledged connectionless service.

3) Acknowledged connection-oriented service.

Unacknowledged connectionless service

As the name suggests, it is unacknowledged form of transmission. Here the source machine sends the data to the destination machine without any acknowledgement. For this, no connection is either established or released. If the data is lost due to noise or interference, the lost data is not even recovered by the layer.

 Acknowledged connectionless service

• In acknowledged connectionless service each data frame is acknowledged by the destination machine. If any data frame is lost or not arrived in time the same can be transmitted again. In this service no connection are used.

 Acknowledged connection service

• Acknowledged connection service establishes a connection prior to data transmission. Each frame is numbered before transmission and corresponding acknowledgement is also received. The transmission is carried out in distinct


Framing in the data link layer separates a message from one source to destination or from other messages to other destinations by adding a sender address and a destination address.

• To service the network layer, data link layer uses the service provided to it by the

physical layer.

• Physical layer accepts the raw bit stream and delivers it to the destination. This bit stream may contain error i.e. number of bits received may not be equal to number of bits transmitted. • The data link layer breaks the stream into discrete frames and computes the

checksum for each frame.

• At the destination the checksum is recomputed.

• The breaking of bit stream by inserting spaces or time gaps is called framing.

Since it is difficult and risky to count on timing and mark the start and end of each frame. Fixed-size framing • Frames can be of fixed or variable size. In fixed size framing, there is no need for defining the boundaries of the frames; the size itself can be used as a delimiter.

• ATM is the example of fixed size framing.

Variable Size Framing

In variable size framing, end of the frame and the beginning of the next frame is defined.

. Two methods are used for this purposes.

1. Character oriented 2. Bit oriented

Character Oriented Protocol

In this type, data to be carried are 8-bit characters from a coding system such as ASCII. Header contains source and destination address and other control information are also multiple of 8 bits. Trailer contains error detection or error correction redundant bits are also multiples of 8 bits.

. To separate one frame from the next, an 8-bit flag is added at the beginning and the end of a frame. The flag consists of protocol dependent special characters, signals the start or end of a frame.

. Character oriented framing was suitable only for text data transmission. The flag could be selected to be any character not used for text communication.

. When we send other types of information such as graphs, audio and video, the flag could also be part of the information. So it creates problem for receiver. When receiver encounters this pattern in the middle of the data, thinks it has reached the end of the frame.

• To solve this problem, a byte stuffing was used.  More

Byte stuffing

Aspecial byte is added to the data section of the frame. When there is a character with the same pattern as flag. The data section is stuffed with an extra byte. This byte is usually called the Escape Character (ESC); which has a predefined bit pattern.

Leave a Reply