Ethernet Addressing Framing and Error Detection
 


Addressing -

The heart of the Ethernet system is the Ethernet frame, which is used to deliver data between computers. The frame consists of a set of bits organized into several fields. These fields include address fields, a variable size data field that carries from 46 to 1,500 bytes of data, and an error checking field that checks the integrity of the bits in the frame to make sure that the frame has arrived intact.

The first two fields in the frame carry 48-bit addresses, called the destination and source addresses. The IEEE controls the assignment of these addresses by administering a portion of the address field. The IEEE does this by providing 24-bit identifiers called "Organizationally Unique Identifiers" (OUIs), since a unique 24-bit identifier is assigned to each organization that wishes to build Ethernet interfaces. The organization, in turn, creates 48-bit addresses using the assigned OUI as the first 24 bits of the address. This 48-bit address is also known as the physical address, hardware address, or MAC address.

A unique 48-bit address is commonly pre-assigned to each Ethernet interface when it is manufactured, which vastly simplifies the setup and operation of the network. For one thing, pre-assigned addresses keep you from getting involved in administering the addresses for different groups using the network. And if you've ever tried to get different work groups at a large site to cooperate and voluntarily obey the same set of rules, you can appreciate what an advantage this can be.

As each Ethernet frame is sent onto the shared signal channel, all Ethernet interfaces look at the first 48-bit field of the frame, which contains the destination address. The interfaces compare the destination address of the frame with their own address. The Ethernet interface with the same address as the destination address in the frame will read in the entire frame and deliver it to the networking software running on that computer. All other network interfaces will stop reading the frame when they discover that the destination address does not match their own address.

A multicast address allows a single Ethernet frame to be received by a group of stations. Network software can set a station's Ethernet interface to listen for specific multicast addresses. This makes it possible for a set of stations to be assigned to a multicast group which has been given a specific multicast address. A single packet sent to the multicast address assigned to that group will then be received by all stations in that group.

There is also the special case of the multicast address known as the broadcast address, which is the 48-bit address of all ones. All Ethernet interfaces that see a frame with this destination address will read the frame in and deliver it to the networking software on the computer.

MAC Address -

A Media Access Control address (MAC address) is a unique identifier assigned to network interfaces for communications on the physical network segment. MAC addresses are used for numerous network technologies and most IEEE 802 network technologies, including Ethernet. Logically, MAC addresses are used in the Media Access Control protocol sub-layer of the OSI reference model.

MAC addresses are most often assigned by the manufacturer of a network interface card (NIC) and are stored in its hardware, the card's read-only memory, or some other firmware mechanism. If assigned by the manufacturer, a MAC address usually encodes the manufacturer's registered identification number and may be referred to as the burned-in address. It may also be known as an Ethernet hardware address (EHA), hardware address or physical address. A network node may have multiple NICs and will then have one unique MAC address per NIC.

MAC addresses are formed according to the rules of one of three numbering name spaces managed by the Institute of Electrical and Electronics Engineers (IEEE): MAC-48, EUI-48, and EUI-64. The IEEE claims trademarks on the names EUI-48 and EUI-64, in which EUI is an acronym for Extended Unique Identifier.

An MAC address -

Error Detection -

Each Ethernet frame contains a trailer with a cyclic redundancy check (CRC) of the frame contents. After reception of a frame, the receiving node creates a CRC from the bits in the frame to compare to the CRC value in the frame trailer. If these two CRC calculations match, the frame can be trusted as being received without error. The trusted frame will then be processed. A frame received with bad, nonmatching CRC's will be discarded.