In modern computers data is usually stored in files, that can be small or very, very large. One might assume that, when we transfer a file from one

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2 In modern computers data is usually stored in files, that can be small or very, very large. One might assume that, when we transfer a file from one computer to another, the whole file is sent as a continuous stream of ones and zeros over the telecommunications network. This would be the best way to do it in ideal networks, error free and with infinite communication capacity, but real networks are not like that, they have errors and they have a finite communication capacity, called bandwidth. So, if a computer sends a large amount of data over a network without any pause, it will cause other computers to wait while the data is being moved, disturbing the communication of the rest of users. And when an error happens, a large amount of data will have to be retransmitted, what will add up more traffic to the network. These effects are minimized when large data units are reformatted into smaller packages, what allows better network sharing and better management of errors in transmission. This way, you can mix packets from different users and, if an error occurs, only a small section of data is affected, and, therefore, only a small amount of data must be retransmitted, making it relatively easy to recover from the error. So, in order for many users to transmit at once data quickly and easily across the network, data must be broken into small, manageable chunks. These chunks are normally called packets or frames. The terms "packet" and "frame" are often used interchangeable but there are some differences. The communication process in a computer network is

3 organised in layers, frame is usually used to name the data structures used in lower layers and packet in the layers that sit above them.

4 A packet or frame needs a way to mark its beginning and its ending and to carry some control information. Different communication protocols use different conventions to distinguish between the elements of the packets and to format the data. For example in a lot of Point-to-Point Protocols, the packet is structured in 8-bit bytes, and special characters are used to delimit the different elements. In other protocols, like Ethernet, a special sequence of bytes is used to mark the beginning of a new frame. We have just seen a couple of examples of protocols that organise their packets as bytes, but this is not always like this, some protocols define their packets at bit level instead of at byte level.

5 All packets and frames include a header, a body and a footer. If we speak about an IP packet, the packet header provides the destination address (IP address) of the packet as well as information about the length of the packet, the originating IP address, the packet number in the sequence of packets, the protocol to determine the type of packet being transmitted (streaming audio or video, , Web page, etc.), and information to perform error checking operations. The packet body, also called payload, contains the actual message. The packet footer (that is optional) contains a sequence of bits that indicate the end of the message. It may also contain information to perform error checking operations at the receiving end.

6 Error detection and correction is performed at various layers in the protocol stack. Network packets may contain a checksum, parity bits or cyclic redundancy checks. All of them are mechanisms devised to be able to detect and/or correct errors that occur during transmission by operating with the transmitted data. At the transmitter, a checksum, a parity bit or a Cyclic Redundancy Check code is calculated before the packet is sent. This code is transmitted with the data and, when it is received at the destination, the checksum is recalculated and compared with the one in the packet. If discrepancies are found, the packet may be corrected or discarded. Any packet loss is dealt with by the network protocol. Simpler codes allow only for detection of errors, whilst more complex codes can correct up to a certain number of errors. In some cases, modifications of the network packet may be necessary while routing, in which case checksums are recalculated.

7 Under fault conditions, packets can end up following a closed circuit. If nothing is done to avoid it, the number of packets recirculating could build up until the network was congested to the point of failure. A timeto-live is a field that is decreased by one each time a packet goes through a network node. If the field reaches zero, routing has failed, and the packet is discarded. Ethernet packets have no time-to-live field and so are subject to what is called a broadcast storm in the presence of a switch loop.

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