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ETHERNET TECHNOLOGY

Part 1: Frames, Collisions, and 10 Mbps LANs

by James Antonakos

Start ý Ethernet Frame Format ý The Interframe Gap ý Collision Or No Collision ý Detecting Errors ý Random Waiting Period ý 10-Mbps Ethernet ý 10BaseFý Errors In Ethernet LANS ý More To Come ý Sources and PDF

ETHERNET FRAME FORMAT

Figure 1 shows the format of an 802.3 frame. A separate, and practically identical, frame format called Ethernet II has an 8-byte preamble of identical 10101010 patterns (no 10101011 SFD) and a two-byte Type field in place of the Length field. Because the Data field of an 802.3 Ethernet frame is limited to 1500 bytes, a value larger than 1500 in the Type/Length field indicates an Ethernet II frame. Many companies, such as Xerox, DEC, and Novell, use Ethernet II frames in their networks. All the data is packaged into one or more frames for transmission over an Ethernet LAN. Letýs look at each portion of the Ethernet frame, which consists of the preamble (7 bytes), start frame delimiter (1 byte), destination address (6 bytes), source address (6 bytes), length (2 bytes), data (46ý1500 bytes), and frame check sequence (4 bytes).

The preamble contains seven bytes with the identical pattern 10101010. This alternating sequence of ones and zeros are the first bytes transmitted. Theyýre provided to help listening stations synchronize quickly on the new datastream. The start frame delimiter (SFD) is the one-byte pattern 10101011. Note that the LSB is high, whereas all the bytes of the preamble had their LSB low.

The destination MAC address is the address of the station that is to receive the frame. Every Ethernet device manufactured (NIC, hub or switch, router) contains a unique 48-bit MAC address assigned by the manufacturer. An example of a MAC address is 00-C0-F0-27-64-E2. The first bit is used to distinguish individual addresses from group addresses, and the second bit separates globally- and locally-administered addresses. These are followed by the organizationally unique identifier (OUI, a 22-bit field assigned by the IEEE) and the organizationally unique address (OUA, a 24-bit number assigned by the manufacturer). The Windows WINIPCFG utility can be used to determine the MAC address, called the adapter address. A MAC address of FF-FF-FF-FF-FF-FF (all ones) is reserved for use as a broadcast address.

The source MAC address is the MAC address of the source station transmitting the frame. The length field indicates the number of bytes in the data field. And, the data area is where all the data from the upper networking layers is carried. Upper layer networking protocols (such as TCP/IP protocols) are encapsulated and stored in the data area.

Finally, the frame check sequence (FCS) is a 32-bit cyclic redundancy check (CRC) value used to determine the validity of the received Ethernet frame. The FCS is found using a 33-bit generator polynomial defined mathematically as:

G(x) = x32 + x26 + x23 + x22 + x16 + x12 + x11 + x10 + x8 + x7 + x5 + x4 + x2 + x + 1

This is equivalent to the binary value 100000100110000010001110110110111, or 104C11DB7 hexadecimal. The ones in the pattern indicate where exclusive OR gates are used in a recirculating shift register circuit that is able to generate/check a valid stream of data of variable length. If even a single bit in the frame is received in error, the shift register will not generate the proper output and the error will be discovered. All bits in the frame, except those in the FCS, are transmitted from LSB to MSB.

From the numbers provided in Figure 1, the minimum and maximum sizes of an Ethernet frame are 72 and 1526 bytes, respectively. Many individuals refer to the smallest Ethernet frame as 64 bytes by not including the 8 bytes of Preamble and SFD. The largest frame would then be 1518 bytes. For the purposes of the discussion here, the Preamble and SFD will be included in the length of the frame.

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