1.2 Ethernet

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Ethernet is a type of LAN that uses a contention-based method of access to allow computers to share resources, send files, print documents, and transfer messages. The Ethernet LAN originated as a result of the experimental work done by Xerox Corporation at its Palo Alto Research Center (PARC) in the mid-1970s and quickly became a de facto standard with the backing of Digital Equipment Corp. (DEC) and Intel Corp. Xerox licensed Ethernet to other companies that developed products based on the specification issued by the three companies. Much of the original Ethernet design was incorporated into the 802.3 standard adopted in 1980 by the Institute of Electrical and Electronic Engineers (IEEE).

Ethernet 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 currently on the line, the station is free to transmit. If the network is already in use, the station backs off and tries again. If multiple stations sense that the network is idle and transmit at the same time, a “collision” occurs and each station backs off to try again at staggered intervals. This media access control scheme is known as carrier sense multiple access with collision detection (CSMA/CD).

1.2.1 Frame Format

The IEEE 802.3 standard defines a multi-field frame format, which differs only slightly from that of the original version of Ethernet, known as “pure” Ethernet (see Figure 1.1):

• Preamble. The frame begins with an 8-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 used in the pure Ethernet preamble is used in the IEEE 802.3 preamble, which includes the 1-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 is used to indicate 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 2 bytes (16 bits) or 6 bytes (48 bits) in length. A destination address can refer to one station, a group of stations, or all stations. The original Ethernet specifies the use of 48-bit addresses, while IEEE 802.3 permits either 16- or 48-bit addresses.

• Length count. The length of the data field is indicated by the 2-byte count field. This IEEE 802.3-specified field is used to determine 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 being assembled for transmission does not meet this minimum length, a pad field must be added to bring it up to that length.

• Type field. Pure Ethernet does not support length and pad fields, as does IEEE 802.3. Instead, 2 bytes are used for a type field. The value specified in the type field is only meaningful to the higher network layers and was not defined in the original Ethernet specification.

• Data field. The data field of a frame is passed by the client layer to the data link layer in the form of 8-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 is used to stuff the frame with extra bytes. In defining a minimum frame size, there are less problems to contend with in handling collisions. 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 the length of time 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 results of the calculation in the 4-byte frame check sequence field before sending the frame. At the receiving station, an identical CRC calculation is performed and a comparison made 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 no provision for error correction; if the two values do not match, notification that an error has occurred is simply passed to the client layer.

1.2.2 Media Access Control

Several key processes are involved in transmitting data across the network; among them, data encapsulation/decapsulation and media access management, which are performed by the media access control (MAC) sublayer of Open Systems Interconnection’s (OSI) data link layer.

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 data unit is received by the MAC sublayer from the logical link control (LLC) sublayer. 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. The destination address, source address, type and information fields are passed to the data link layer by the client layer in the form of a packet. Control information necessary for transmission is encapsulated into the offered packet. The CRC value for the frame check sequence field is calculated, and the frame is constructed.

When a frame is received, the data decapsulation function performed at the receiving station is responsible for recognizing the destination address, performing error checking, and then removing 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 to the LLC sublayer.

Specific types of errors are checked in the decapsulation process, including whether the frame is a multiple of 8 bits or exceeds the maximum packet length. The address is also checked to determine whether the frame should be accepted and processed further. If it is, a CRC value is calculated and checked against the value in the frame check sequence field. If the values match, the destination address, source address, type and data fields are passed to the client layer. What is passed to the client station is the packet in its original form.

Media Access Management

The method used to control access to the transmission medium is known as media access management, which is responsible for several functions, starting with collision handling, which are defined by the IEEE 802.3 standard for contention networks. There are two collision handling schemes: one for detection and one for avoidance.

• With detection (i.e., CSMA/CD), collisions occur when two or more frames are offered for transmission at the same time, which triggers the transmission of a sequence of bits called a “jam.” This is the means whereby all stations on the network recognize that a collision has occurred. At that point, all transmissions in progress are terminated. Retransmissions are attempted at calculated intervals. If there are repeated collisions, a process called “backing off” is used, which involves increasing the retransmission wait time following each successive collision.

• With collision avoidance [i.e., carrier sense multiple access with collision avoidance (CSMA/CA)], the line is monitored for the presence or absence of a signal (carrier), as in CSMA/CD. But with collision avoidance, a broadcast is issued to the network notifying other stations that a data transmission is about to occur. While CSMA/CA is effective at avoiding collisions on a network, it has an additional overhead requirement that CSMA/CD does not. This results in CSMA/CA increasing network traffic because it has to broadcast the intent of the station to transmit before any real data is put onto the cable.

On the receiving side, the management function is responsible for recognizing and filtering out fragments of frames that resulted from a transmission that was interrupted by a collision. Any frame that is less than the minimum size is assumed to be a fragment that was caused by a collision.

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