Packet switching : What is a Packet?

Packet switching
Dedicated private lines tie up an entire phone line for the entire month. Circuit switching also ties up an entire phone line, but only for the duration of the call. Packet switching is much more efficient. Packet switching only ties up part of the phone line for the duration of the call. Like circuit switching, if the computers are not sending or receiving data, they stay off the phone lines, keeping them free for other users. But with packet switching technology, even the quiet times within one call will be filled with packets of data from someone else’s call.

The data is broken into packets that are sent from point A to point B. The packets may travel across different paths within the telephone company’s network. At the terminating point, all the packets are reassembled in the correct order.

What is a packet?
A packet is group of bytes of information that are processed independently across packet-switching networks (see Figure 1). Each packet has three parts: header, data, and footer. The header and footer are like a train with an engine in the front and a caboose in the back. They contain important information about the packet, such as the sender’s address, the destination address, the size of the packet, and the type of data contained in the packet, such as voice, data, or video. Voice and video packets are given priority in packet-switching networks, just like passenger trains are given priority on a railroad. Voice and video require a steady stream for the transmission to be smooth.


Figure 1: Packets, frames, and cells.

One reason packet switching is fast is because it does not correct transmission errors within the network. Instead, the data is sent again. The receiving computer tells the sending computer “I didn’t get all the packet … please resend the missing ones.” This is very effective for data communication where slight delays can be tolerated, but most businesses still refuse to move their voice traffic across a packet-switching network, because of the slight delay. Few companies want to sound like the 1980s computerman Max Headroom when they talk to their customers.

Prior to being transmitted across a computer network, a given computer file, such as a simple word processing document will be broken into packets of data. For example, imagine that a man named Sam in Seattle decides to write a love letter to Louise in Long Island. Sam plans to transmit the letter to Louise across a packet-switched network.

Using Microsoft Word, Sam writes a brief letter to Louise expressing his deep love for her. Sam is not a man of many words; his letter contains only 100 words. The size of the computer file containing the letter is only 20 Kb. Sam’s company uses a packet-switching service called ATM. The ATM service breaks Sam’s letter into 377 individual packets and transmits each one to the Long Island office. The whole process takes less than 1 second. In Long Island, the packets are reassembled in order, and the computer file is now accessible. Tears run down Louise face as she reads Sam’s love letter.