of sources. The Frame Check Sequence (FCS) field contains a number that is calculated by the source node based on the data in the frame. This FCS is then added to the end of the frame that is being sent. When the destination node receives the frame the FCS number is recalculated and compared with the FCS number included in the frame. If the two numbers are different, an error is assumed, the frame is discarded, and the source is asked to retransmit. There are three primary ways to calculate the Frame Check Sequence number:

Cyclic Redundancy Check (CRC) – performs calculations on the data.
Two-dimensional parity – adds an 8th bit that makes an 8 bit sequence have an odd or even number of binary 1s.
Internet checksum – adds the values of all of the data bits to arrive at a sum.

The node that transmits data must get the attention of other devices, in order to start a frame, and to end the frame. The length field implies the end, and the frame is considered ended after the FCS. Sometimes there is a formal byte sequence referred to as an end-frame delimiter. Web Links 802.3 Frame (packet) http://www.usyd.edu.au/is/comms/networkcourse/ USydNet_mod1_introduction& osimodel.html#toc8023

Content 6.1 Ethernet Fundamentals 6.1.6 Ethernet frame structure At the data link layer the frame structure is nearly identical for all speeds of Ethernet from 10 Mbps to 10,000 Mbps. However, at the physical layer almost all versions of Ethernet are substantially different from one another with each speed having a distinct set of architecture design rules. In the version of Ethernet that was developed by DIX prior to the adoption of the IEEE 802.3 version of Ethernet, the Preamble and Start Frame Delimiter (SFD) were combined into a single field, though the binary pattern was identical. The field labeled Length/Type was only listed as Length in the early IEEE versions and only as Type in the DIX version. These two uses of the field were officially combined in a later IEEE version, as both uses of the field were common throughout industry. The Ethernet II Type field is incorporated into the current 802.3 frame definition. The receiving node must determine which higher-layer protocol is present in an incoming frame by examining the Length/Type field. If the two-octet value is equal to or greater than 0x600 (hexadecimal), then the frame is interpreted according to the Ethernet II type code indicated. Web Links Working with Ethernet Frames http://howto.lycos.com/lycos/step/ 1,,5+30+34556+34588+34592,00.html

Content 6.1 Ethernet Fundamentals 6.1.7 Ethernet frame fields Some of the fields permitted or required in an 802.3 Ethernet Frame are:

Preamble
Start Frame Delimiter
Destination Address
Source Address
Length/Type
Data and Pad
FCS
Extension

The Preamble is an alternating pattern of ones and zeroes used for timing synchronization in the asynchronous 10 Mbps and slower implementations of Ethernet. Faster versions of Ethernet are synchronous, and this timing information is redundant but retained for compatibility. A Start Frame Delimiter consists of a one-octet field that marks the end of the timing information, and contains the bit sequence 10101011. The Destination Address field contains the MAC destination address. The destination address can be unicast, multicast (group), or broadcast (all nodes). The Source Address field contains the MAC source address. The source address is generally the unicast address of the transmitting Ethernet node. There are, however, an increasing number of virtual protocols in use that use and sometimes share a specific source MAC address to identify the virtual entity. The Length/Type field supports two different uses. If the value is less than 1536 decimal, 0x600 (hexadecimal), then the value indicates length. The length interpretation is used where the LLC Layer provides the protocol identification. The type value specifies the upper-layer protocol to receive the data after Ethernet processing is completed. The length indicates the number of bytes of data that follows this field. If the value is equal to or greater than 1536 decimal (0600 hexadecimal), the value indicates that the type and contents of the Data field are decoded per the protocol indicated. The Data and Pad field may be of any length that does not cause the frame to exceed the maximum frame size. The maximum transmission unit (MTU) for Ethernet is 1500 octets, so the data should not exceed that size. The content of this field is unspecified. An unspecified pad is inserted immediately after the user data when there is not enough user data for the frame to meet the minimum frame length. Ethernet requires that the frame be not less than 46 octets or more than 1518 octets. A FCS contains a four byte CRC value that is created by the sending device and is recalculated by the receiving device to check for damaged frames. Since the corruption of a single bit anywhere from the beginning of the Destination Address through the end of the FCS field will cause the checksum to be different, the coverage of the FCS includes itself. It is not possible to distinguish between corruption of the FCS itself and corruption of any preceding field used in the calculation. Web Links TechFest Ethernet Technical Summary http://www.techfest.com/networking/lan/ ethernet2.htm

Content 6.2 Ethernet Operation 6.2.1 Media Access Control (MAC) MAC refers to protocols that determine which computer on a shared-medium environment, or collision domain, is allowed to transmit the data. MAC, with LLC, comprises the IEEE version of the OSI Layer 2. MAC and LLC are sublayers of Layer 2. There are two broad categories of Media Access Control, deterministic (taking turns) and non-deterministic (first come, first served). Examples of deterministic protocols include Token Ring and FDDI. In a Token Ring network, individual hosts are arranged in a ring and a special data token travels around the ring to each host in sequence. When a host wants to transmit, it seizes the token, transmits the data for a limited time, and then forwards the token to the next host in the ring. Token Ring is a collisionless environment as only one host is able to transmit at any given time. Non-deterministic MAC protocols use a first-come, first-served approach. CSMA/CD is a simple system. The NIC listens for an absence of a signal on the media and starts transmitting. If two nodes transmit at the same time a collision occurs and none of the nodes are able to transmit. Three common Layer 2 technologies are Token Ring, FDDI, and Ethernet. All three specify Layer 2 issues, LLC, naming, framing, and MAC, as well as Layer 1 signaling components and media issues. The specific technologies for each are as follows:

Ethernet – logical bus topology (information flow is on a linear bus) and physical star or extended star (wired as a star)
Token Ring – logical ring topology (in other words, information flow is controlled in a ring) and a physical star topology (in other words, it is wired as a star)
FDDI – logical ring topology (information flow is controlled in a ring) and physical dual-ring topology (wired as a dual-ring)

Web Links Media Access Control http://howto.lycos.com/lycos/step/ 1,,pop-26166+ 25845+18046,00.html

Content 6.2 Ethernet Operation 6.2.2 MAC rules and collision detection/backoff Ethernet is a shared-media broadcast technology. The access method CSMA/CD used in Ethernet performs three functions:

Transmitting and receiving data packets
Decoding data packets and checking them for valid addresses before passing them to the upper layers of the OSI model
Detecting errors within data packets or on the network

In the CSMA/CD access method, networking devices with data to transmit work in a