The datalink layer in Networking
The datalink layer in Networking
Computer scientists are usually not interested in exchanging bits between two hosts. They prefer to write software that deals with larger blocks of data in order to transmit messages or complete files. Thanks to the physical layer service, it is possible to send a continuous stream of bits between two hosts. This stream of bits can include logical blocks of data, but we need to be able to extract each block of data from the bit stream despite the imperfections of the physical layer. In many networks, the basic unit of information exchanged between two directly connected hosts is often called a frame. A frame can be defined has a sequence of bits that has a particular syntax or structure.
To enable the transmission/reception of frames, the first problem to be solved is how to encode a frame as a sequence of bits, so that the receiver can easily recover the received frame despite the limitations of the physical layer. If the physical layer were perfect, the problem would be very simple. We would simply need to define how to encode each frame as a sequence of consecutive bits. The receiver would then easily be able to extract the frames from the received bits. Unfortunately, the imperfections of the physical layer make this framing problem slightly more complex. Several solutions have been proposed and are used in practice in different network technologies. Framing The framing problem can be defined as : “How does a sender encode frames so that the receiver can efficiently extract them from the stream of bits that it receives from the physical layer”.A first solution to this problem is to require the physical layer to remain idle for some time
after the transmission of each frame. These idle periods can be detected by the receiver and serve as a marker to delineate frame boundaries. Unfortunately, this solution is not acceptable for two reasons. First, some physical layers cannot remain idle and always need to transmit bits. Second, inserting an idle period between frames decreases the maximum bit rate that can be achieved. Note: Bit rate and bandwidth Bit rate and bandwidth are often used to characterize the transmission capacity of the physical service. The original definition of bandwidth, as listed in the Webster dictionary is a range of radio frequencies which is occupied by a modulated carrier wave, which is assigned to a service, or over which a device can operate. This definition corresponds to the characteristics of a given transmission medium or receiver. For example, the human ear is able to decode sounds in roughly the 0-20 KHz frequency range. By extension, bandwidth is also used to represent the capacity of a communication system in bits per second. For example, a Gigabit Ethernet link is theoretically capable of transporting one billion bits per second. Given that multi-symbol encodings cannot be used by all physical layers, a generic solution which can be used with any physical layer that is able to transmit and receive only bits 0 and 1 is required. This generic solution is called stuffing and two variants exist : bit stuffing and character stuffing. To enable a receiver to easily delineate the frame boundaries, these two techniques reserve special bit strings as frame boundary markers and encode the frames so that these special bit strings do not appear inside the frames. Bit stuffing reserves the 01111110 bit string as the frame boundary marker and ensures that there will never be six consecutive 1 symbols transmitted by the physical layer inside a frame. With bit stuffing, a frame is sent as follows. First, the sender transmits the marker, i.e. 01111110. Then, it sends all the bits of the frame and inserts an additional bit set to 0 after each sequence of five consecutive 1 bits. This ensures that the sent frame never contains a sequence of six consecutive bits set to 1. As a consequence, the marker pattern cannot appear inside the frame sent. The marker is also sent to mark the end of the frame. The receiver performs the opposite to decode a received frame. It first detects the beginning of the frame thanks to the 01111110 marker. Then, it processes the received bits and counts the number of consecutive bits set to 1. If a 0 follows five consecutive bits set to 1, this bit is removed since it was inserted by the sender. If a 1 follows five consecutive bits sets to 1, it indicates a marker if it is followed by a bit set to 0. The table below illustrates the application of bit stuffing to some frames.