Ethernet was originally developed to run on a long coaxial cable that connected all the computers on the network. This type of network topology is called a bus. When one station transmitted data, all the other stations heard it. Ethernet was designed assuming that all stations would hear these broadcasts to the segment of wire used to connect them. This is where the terms 'wire segment' and 'broadcast domain' come from. A broadcast domain includes all the wire and computers that can hear each other whenever one of the computers is transmitting. A wire segment is the piece of wire used to connect two devices.
Because Ethernet networks are composed of broadcast domains and there is no clock signal on the wire, as serial connections often have. Instead, Ethernet systems must determine if the wire is in use, and if not, send enough data to enable the remote station to allow it to synchronize properly. This synchronization mechanism combined with the ability to detect other computers attempting to access the wire is a formalized protocol called Carrier Sense Multiple Access-Collision Detect (CSMA/CD).
IEEE 803.2 / 802.2
| 7 bytes | 1 byte | 2 or 6 bytes | 2 or 6 bytes | 2 bytes |
4-1500 bytes | 4 bytes | |||
| Preamble | Start Frame Delimiter |
Dest. MAC address |
Source MAC address |
Length |
(Data / Pad) | FCS | |||
| DSAP | SSAP | CTRL | NLI | ||||||
- Preamble
- This is a stream of bits used to allow the transmitter and reciever to synchronize their communication. The preamble is an alternating pattern of binary 56 ones and zeroes. The preamble is immediately followed by the Start Frame Delimter.
- Start Frame Delimter
- This is always 10101011 and is used to indicate the beginning of the frame information.
- Destination MAC
- This is the MAC address of the machine receiving data. When a network interface card (NIC) is listening to the wire is checking this field for it's own MAC address.
- Source MAC
- This is the MAC address of the machine transmitting data.
- Length
- This is the length of the entire Ethernet frame in bytes. Although this field can hold any value between 0 and 65,534, it is rarely larger than 1500 as that is usually the maximum transmission frame size for most serial connections. Ethernet networks tend to use serial devices to access the Internet.
- Data/Padding (a.k.a. Payload)
- The data is inserted here. This is where the IP
header and data is placed if you are running IP
over Ethernet. This field contains IPX information
if you are running IPX/SPX (Novell). Contained within the data/padding section
of an IEEE 803.2 frame
are four specific fields:
DSAP - Destination Service Access Point
SSAP - Source Service Access Poiont
CTRL - Control bits for Ethernet communication
NLI - Network Layer Interface - FCS
- This field contains the Frame Check Sequence (FCS) which is calculated using a Cyclic Redundancy Check (CRC). The FCS allows Ethernet to detect errors in the Ethernet frame and reject the frame if it appears damaged.