Ethernet is a type of local-area network (LAN) that operates at 10 Mbps. It originated out of experimental work done by Xerox Corporation in the 1970s. The co-inventor of Ethernet was Dr. Robert Metcalf, who founded 3Com Corp. in 1979. During 3Com’s 20-year history, Ethernet has become the industry’s most widely deployed computer connectivity solution. Much of the original Ethernet design was incorporated into the 802.3 standard developed in 1980 by the Institute of Electrical and Electronic Engineers (IEEE).
Ethernet is based on a bus topology that is contention-based, meaning that stations compete with each other for access to the network, a process that is controlled by a statistical arbitration scheme. Each station “listens” to the network to determine if it is idle. Upon sensing that no traffic is on the line, the terminal is free to transmit. If the channel is already in use, the station backs off and tries again.
Figure 39
A comparison of Ethernet frame formats: “pure” Ethernet (top) and IEEE 802.3 Ethernet (bottom).
Frame Format
The IEEE 802.3 standard defines a multifield frame format, which differs only slightly from that of pure Ethernet (Figure 39):
PREAMBLE The frame begins with an eight-byte field called a preamble, which consists of 56 bits having alternating 1 and 0 values. These are used for synchronization and to mark the start of the frame. The same bit pattern is used for the pure Ethernet preamble as in the IEEE 802.3 preamble, which includes the one-byte start-frame delimiter field.
START FRAME DELIMITER The IEEE 802.3 standard specifies a start-frame delimiter field, which is really a part of the preamble. This indicates the start of a frame.
ADDRESS FIELDS The destination address field identifies the station(s) that are to receive the frame. The source address field identifies the station that sent the frame. If addresses are locally assigned, the address field can be either two bytes (16 bits) or six bytes (48 bits) in length. A destination address can refer to one station, a group of stations, or all stations. Pure Ethernet uses 48-bit addresses, while IEEE 802.3 permits either 16- or 48-bit addresses.
LENGTH COUNT The length of the data field that follows is indicated by the two-byte length count field, which determines the length of the information field when a pad field is included in the frame.
PAD FIELD To detect collisions properly, the frame that is transmitted must contain a certain number of bytes. The IEEE 802.3 standard specifies that if a frame does not meet this minimum length, a pad field must be added to make up the difference.
TYPE FIELD Pure Ethernet does not support length and pad fields, as does IEEE 802.3. Instead, two bytes are used for a type field. The value specified in the type field is meaningful only to higher network layers and is not defined in the original Ethernet specification.
DATA FIELD The data field is passed by the client layer to the data link layer in the form of eight-bit bytes. The minimum frame size is 72 bytes, while the maximum frame size is 1,526 bytes, including the preamble. If the data to be sent uses a frame that is smaller than 72 bytes, the pad field stuffs the frame with extra bytes. In defining a minimum frame size, there are less problems to contend with in collision handling. If the data to be sent uses a frame that is larger than 1,526 bytes, it is the responsibility of the higher layers to break it into individual packets in a procedure called fragmentation. The maximum frame size reflects practical considerations related to adapter-card buffer sizes and the need to limit how long the medium is tied up in transmitting a single frame.
FRAME CHECK SEQUENCE A properly formatted frame ends with a frame check sequence, which provides the means to check for errors. When the sending station assembles a frame, it performs a cyclical redundancy check (CRC) calculation on the bits in the frame. The sending station stores the result of this calculation in the four-byte frame check sequence field before sending the frame. At the receiving station, an identical CRC calculation is performed and compared with the original value in the frame check sequence field. If the two values do not match, the receiving station assumes that a transmission error has occurred and requests that the frame be retransmitted. In pure Ethernet, there is error correction; if the two values do not match, a notification of the error is simply passed to the client layer.
Media Access Control
Several key processes are involved in transmitting data across the network, among them data encapsulation/decapsulation and media access management.
DATA ENCAPSULATION/DECAPSULATION Data encapsulation is performed at the sending station. This process entails adding information to the beginning and end of the data unit to be transmitted. The added information is used to perform the following tasks:
■ Synchronize the receiving station with the signal
■ Indicate the start and end of the frame
■ Identify the addresses of sending and receiving stations
■ Detect transmission errors
The data encapsulation function is responsible for constructing a transmission frame in the proper format. A CRC value for the frame check sequence field is calculated and the frame is constructed. When a frame is received, the data decapsulation function is performed. The receiving station recognizes the destination address of the frame, performs error checking, and then removes the control information that was added by the data encapsulation function at the sending station. If no errors are detected, the frame is passed up the protocol stack to the client application.
MEDIA ACCESS MANAGEMENT A process called media access management or link management is responsible for several functions, starting with collision avoidance and collision handling, which are required for the proper operation of contention networks.
COLLISION AVOIDANCE Collision avoidance entails monitoring the line for the presence or absence of a signal (carrier). This is the “carrier sense” portion of CSMA/CD. The absence of a signal indicates that the channel is not being used and that it is safe to transmit. Detection of a signal indicates that the channel is busy and that transmission must be withheld. If no collision is detected during the period of time known as the collision window, the station acquires the channel and can safely transmit data.
COLLISION HANDLING When two or more frames are offered for transmission at the same time, a collision occurs, which triggers a sequence of bits called a “jam.” All stations recognize the jam as a collision. At that point, all transmissions in progress are terminated. Retransmissions are attempted at staggered intervals. If there are repeated collisions, link management “backs off,” which involves increasing the retransmission wait time following each successive collision.
On the receiving side, link management is responsible for recognizing and filtering out fragments of frames resulting from a transmission that was interrupted by a collision. Any frame less than the minimum size is assumed to be a collision fragment and is not reported to the client layer as an error.
New methods have been developed to improve the performance of Ethernet by reducing or totally eliminating the chance for collisions without having to segment the LAN into smaller subnetworks. Special algorithms sense when frames are on a collision course and temporarily block one frame, while allowing the other to pass.
Last Word
Ethernet is the most popular type of local-area network, and its success has spawned some interesting innovations. 10BaseT Ethernet, for example, allows LANs to operate over unshielded twisted pair (UTP) wiring instead of thick (5BaseT) or thin (10Base2) coaxial cable. There are also high-speed versions of Ethernet, including Fast Ethernet at 100 Mbps and Gigabit Ethernet at 1 Gbps.
