Ethernet Technologies

802.3 Ethernet and the OSI Model

Diving deep in the OSI model is not the goal of this chapter, but you do need to focus on
Layer 2, the data link layer, to put Ethernet technologies into perspective. The data link layer has two sublayers, as illustrated in Figure 1-1:

• Data link sublayer— Also known as the MAC layer, this sublayer focuses on topology specific implementations. For example, 802.5 Token Ring networks have a different MAC than 802.3 Ethernet networks.
  

• Logical link (LLC) sublayer— Standard across all 802-based networks, this sublayer provides a simple frame protocol that provides connectionless frame delivery. There is no mechanism to notify the sender that the frame was or was not delivered.

The focus of the subsequent sections surrounds the MAC layer. This layer is unique to 802.3 networks and as such provides a reference point as you progress through the chapters on the wireless MAC.


The 802.3 Frame Format

Figure 1-2 depicts an Ethernet frame.

As Figure 1-2 illustrates, the Ethernet frame consists of the following fields:

• Preamble— The preamble is a set of 7 octets (an octet is a set of 8 bits) totaling 56 bits of alternating 1s and 0s. Each octet has the following bit pattern: 10101010. The preamble indicates to the receiving station that a frame is being transmitted on the medium. It is important to note that Ethernet topologies subsequent to 10 Mbps Ethernet still include the preamble but do not require one.
  

• Start of frame delimiter (SFD)— The SFD is an 8-bit field that has a bit pattern similar to the preamble, but the last 2 bits are both 1s (10101011). This pattern indicates to the receiving station that the frame's contents follow this field.
  

• Destination MAC address— The destination address field is a 48-bit value that indicates the destination station address of the frame.
  

• Source address— The source address field is a 48-bit value that indicates the station address of the sending station.

Type/length value (TLV)— The TLV field uses 16 bits to indicate what type of higher layer protocol is encapsulated in the data or payload field. The value contained in this field is also referred to as the Ethertype value. Table 1-1 lists some common Ethertype values.

Table 1-1. Some Common Ethernet Ethertypes

• Payload or data— The data or payload field carries upper-layer packets and must be a minimum of 46 bytes and a maximum of 1500 bytes in length. The minimum data or payload size is required to allow all stations a chance to receive the frame. This topic is discussed further in the section, "Ethernet Network Diameter and Ethernet Slot Time." If the data or payload is less than 46 bytes, the sending station pads the payload so it meets the minimum 46 bytes.
  

• Frame check sequence (FCS)— The FCS field contains a cyclic redundancy check (CRC) value calculated against the bit pattern of the frame. When the receiving station receives the frame, it calculates a CRC and compares it to what is in the FCS field. If the values match, the frame is considered error free (see Figure 1-3).

Ethernet Addressing

Ethernet addresses are 48-bit values that uniquely identify Ethernet stations on a LAN. Ethernet addresses are in part issued by a global authority, the IEEE, and in part by device vendors. The IEEE assigns unique 24-bit organizational unique identifiers (OUIs) to vendors. The OUI is the first 24 bits of the Ethernet address. The vendors themselves assign the remaining 24 bits. This process ensures that every Ethernet address is unique, and any station can connect to any network in world and be uniquely identified. Because this addressing describes a physical interface, it is also referred to as MAC addressing. For the most part, MAC addresses are expressed in hexadecimal form, with each byte separated by a dash or colon, or with every 2 bytes delimited with a period. For example, the following is an Ethernet address from a Cisco router:

00-03-6b-48-e9-20

You can also represent this value as 00:03:6b:48:e9:20 or 0003.6b48.e920

The IEEE has assigned the first 24 bits, 00-03-6b, to Cisco. The remaining 24 bits, 48-e9-20, have been assigned by Cisco to the device. The OUI of 00-03-6b allows the vendor to assign a range of addresses starting from 00-03-6b-00-00-00 to 00-03-6b-ff-ff-ff. This provides the vendor a total of 224 or 16,777,216 possible addresses.


CSMA/CD Architecture

The Ethernet networking standard is based on the CSMA/CD architecture. CSMA/CD is a halfduplex architecture, meaning only one station can transmit at a time. You can compare the CSMA/CD architecture to people communicating in a conference-call meeting:

• Each participant doesn't know when the other person is going to speak.
• A participant wanting to say something has to wait for the phone line to become quiet before she can start speaking.
• When the phone line becomes quiet, it is possible for two or more participants to start speaking at the same time.
• If two people speak at the same time, it is difficult for listeners to understand, so the speakers must stop talking and again wait for the line to become quiet before trying to speak again.

Consider Figure 1-4 where two stations are at extreme ends of the broadcast domain:

• Station transmits a frame and that is smaller than 512 bits.
• At the same moment, Station B begins transmitting a frame.
• Station A transmits the last bit of its frame.
• Station A does not detect a collision during transmission and discards the frame from its transmit buffer.
• Station A assumes that the destination station of its frame received the frame.
• Station A's frame collides with Station B's frame.
• Station A has already discarded the frame from its transmit buffer, so Station A has no frame to retransmit.