Unit 3      

 Internet Protocol: TCP/IP

33.1   Packet of data

Definition - What does Data Packet mean?

A data packet is a unit of data made into a single package that travels along a given network path. Data packets are used in Internet Protocol (IP) transmissions for data that navigates the Web, and in other kinds of networks.

Structure of a Data Packet

The structure of a packet depends on the type of packet it is and on the protocol. Read further below on packets and protocols. Normally, a packet has a header and a payload.

The header keeps overhead information about the packet, the service, and other transmission-related data. For example, data transfer over the Internet requires breaking down the data into IP packets, which is defined in IP (Internet Protocol), and an IP packet includes:

• The source IP address, which is the IP address of the machine sending the data. 
• The destination IP address, which is the machine or device to which the data is sent.
• The sequence number of the packets, a number that puts the packets in order such that they are reassembled in a way to get the original data back exactly as it was prior to transmission. 
• The type of service
• Flags
• And some other technical data
• The payload, which represents the bulk of the packet (all the above is considered as overhead), and is actually the data being carried. 

33.2   Circuit switching vs. packet switching

Circuit Switching

In circuit switching network dedicated channel has to be established before the call is made between users. The channel is reserved between the users till the connection is active. For half duplex communication, one channel is allocated and for full duplex communication, two channels are allocated. It is mainly used for voice communication requiring real time services without any much delay. 

 
As shown in the figure 1, if user-A wants to use the network; it need to first ask for the request to obtain the one and then user-A can communicate with user-C. During the connection phase if user-B tries to call/communicate with user-D or any other user it will get busy signal from the network.

Packet Switching

In packet switching network unlike CS network, it is not required to establish the connection initially. The connection/channel is available to use by many users. But when capacity or number of users increases then it will lead to congestion in the network. Packet switched networks are mainly used for data and voice applications requiring non-real time scenarios. 
 
As shown in the figure 2, if user-A wants to send data/information to user-C and if user-B wants to send data to user-D, it is simultaneously possible. Here information is padded with header which contains addresses of source and destination. This header is sniffed by intermediate switching nodes to determine their route and destination.

In packet switching, station breaks long message into packets. Packets are sent one at a time to the network. Packets are handled in two ways, viz. datagram and virtual circuit.

In virtual circuit, preplanned route is established before any packets are transmitted. The handshake is established using call request and call accept messages. Here each packet contains virtual circuit identifier(VCI) instead of the destination address. In this type, routing decisions for each packet are not needed.