All data in an Ethernet network travels in structures called frames

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Frames

All data in an Ethernet network travels in structures called frames. An Ethernet frame has defined fields for data and other information to help the data get to its destination and to help the destination computer determine whether the data has arrived intact. The Ethernet controller’s hardware places information to be sent in frames for transmitting, and extracts and stores the information in received frames. Table 1-1 shows the fields in an IEEE 802.3 Ethernet frame. The fields add synchronizing bits, addressing information, an error-checking sequence, and additional identifying information to the data being sent. Preamble and Start Frame Delimiter The Preamble and Start Frame Delimiter fields function together. They provide a predictable bit pattern that enables the interfaces on a 10-Mb/s network to synchronize to, or match the timing of, a new frame being transmitted. In any data link, the receiving interface needs to know when to read the bits in the transmitted data. Some interfaces, such as I2C, are synchronous interfaces that include a clock line shared by all of the devices. With I2C, the transmitting device writes bits when the clock is low, and a receiving device reads the bits when the clock is high.

Other interfaces, such as Ethernet, are asynchronous, which means that the interfaces don’t share a clock. RS-232 and other serial interfaces that use a UART (universal asynchronous receiver transmitter) are asynchronous. Each transmitted word begins with a Start bit. The receiver uses the leading edge of the Start bit as a timing reference to predict when to read each of the bits that will follow. An RS-232 character typically has eight or nine bits that follow the Start bit. In contrast, a single Ethernet frame may contain over 1000 bits. Detecting a single voltage change at the beginning of a frame isn’t enough to enable the interface to reliably predict when to read all of the bits that follow. For 10-Mb/s Ethernet, the solution is to begin each frame with a known bit pattern that contains many transitions. Receiving interfaces use the pattern to synchronize to, or lock onto, the transmitted frame’s clock. The Preamble and Start of Frame Delimiter fields provide this pattern. The Preamble consists of seven identical bytes, each with the value 10101010. The Start Frame Delimiter follows the Preamble, and consists of the byte 10101011. After detecting the first transition in the Preamble, a receiving interface uses the transitions of the following bits to synchronize to the timing of the transmitting interface. The final two bits in the Start Frame Delimiter indicate the end of the Preamble.

The faster Ethernet interfaces use different methods to synchronize, but include the Preamble for compatibility. In the earlier DIX standard, the Preamble frame is 64 bits and includes the Start-of-Frame byte, while the 802.3 standard defines the Start of Frame as a separate field. The transmitted bit patterns are the same in both cases.

Destination Address

Every Ethernet interface has a 48-bit physical, or hardware, address that identifies the interface on the network. The Destination Address field contains the physical address of the intended receiver of the frame. The receiver may be an individual interface, a group of interfaces identified by a multicast address, or a broadcast address to all interfaces in the network. Every interface in the network reads the destination address of a received frame. If the address doesn’t match the interface’s physical address or a multicast or broadcast address the interface has been configured to accept, the interface ignores the rest of the frame. The first two transmitted bits in the address have special meanings. The first bit is 0 if the address is for a single interface, and 1 if the address is a multicast or broadcast address. A broadcast address is all 1s and is directed to every interface in the network. Multicasting provides a way for an interface to communicate with a selected group of interfaces. The interfaces in the multicast group are configured to accept frames sent to a specific multicast address. The second bit of the destination address is zero if the address was assigned by the manufacturer of the interface, which is the usual case. In the 802.3 standard, the second bit is 1 if the address is administered locally. In the DIX standard, the second bit is always zero.

Source Address

The Source Address field contains the 48-bit physical address of the transmitting interface. See Destination Address above for more about Ethernet addresses.

Length/Type

The Length/Type field is 16 bits that can have one of two meanings. The field can indicate the number of bytes of valid data in the data field or the protocol used by the data in the field that follows. If the value is less than or equal to 1500 decimal (5DCh), the value indicates length. The data field must contain between 46 and 1500 bytes. If there are less than 46 bytes of valid, or usable, data, the length field can indicate how many of the bytes are valid data. If the value is greater than or equal to 1536 decimal (600h), the Length/Type field indicates the protocol that the contents of the data field use. On on-line database at the Internet Assigned Numbers Authority’s Web site (www.iana.org) specifies values for different protocols. The value for the Internet Protocol (IP) is 800h. Values from 1501 to 1535 decimal are undefined. The DIX standard defined this field as a type field only. The original IEEE 802.3 standard defined the field as a length field only. The current 802.3 standard allows either use.

Data

The contents of the data field are the reason why the frame exists. The data is the information that the transmitting interface wants to send. The data field must be between 46 and 1500 bytes. If there are fewer than 46 bytes of data, the field must include pad bytes to increase the size to 46 bytes. If the transmitting interface has more than 1500 bytes to send, it uses multiple frames. As explained earlier in this networking tutorial, the data field often contains additional information besides the raw data being sent. This information is typically in headers that precede the data. The Ethernet frame doesn’t care what’s in the data field, as long as it meets the length requirements. Another term for the contents of the data field is the message. The data payload, or message body, is the message minus any headers or other supplemental information in the data field. Frames with a full 1500 data bytes are the most efficient because they have just 26 bytes, or less than 2 percent, of overhead. At the other extreme, a frame with just one data byte plus 26 bytes of headers and the required 45 bytes of padding has 71 bytes of overhead. An Ethernet frame must be at least 512 bits (64 bytes) not including the Preamble and Start-of-Frame bits. This is the size of a frame with the minimum 46 data bytes. Receiving interfaces ignore frames that are shorter than this minimum size.

Frame Check Sequence

The Frame Check Sequence (FCS) field enables the receiving interface to detect errors in a received frame. Electrical noise or other problems in the network can corrupt a frame’s contents. A receiving interface can detect corrupted data by using the 32-bit cyclic redundancy check (CRC) value in the frame check sequence field. The transmitting interface performs a calculation, called the cyclic redundancy check, on the bytes to be sent and places the result in the frame check sequence field. The receiving interface performs the same calculation on the received bytes. If the results match, the frame’s contents are almost certain to be identical to what was sent. The Ethernet controller’s hardware typically performs the CRC calculations on both ends. On detecting an error in a received frame, the controller typically sets a bit in a status register.