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Wireless Networking Handbook
(Publisher: Macmillan Computer Publishing)
Author(s): Jim Geier
ISBN: 156205631x
Publication Date: 09/01/96

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The IEEE 802.3, 802.5, and 802.11 specifications are part of the overall IEEE standards hierarchy (see fig. 8.5). IEEE defines a LAN with three layers of functionality: Logical Link Control (LLC), Medium Access Control (MAC), and physical layers. The LLC, which is IEEE 802.2, provides link synchronization, and the MAC layer is responsible for medium access. The physical layer defines electrical characteristics of the signal and medium. Thus, 802.3, 802.5, and 802.11 not only specify the medium access method but also the type of medium. The LAN Backbone section in this chapter provides more details on possible physical layers offered by 802.3 and 802.5.


Figure 8.5  The IEEE 802 standards hierarchy.

IEEE 802.3 Carrier Sense Multiple Access

The IEEE 802.3, released in 1980, is the most popular access method. It operates at 10 or 100 Mbps, depending on the type of physical layer used. The use of this standard satisfies most performance requirements, and, because of its high degree of proliferation, products are low cost compared to token ring and FDDI. Therefore, consider IEEE 802.3 as your wired network medium access standard, unless you require real-time data transfers or a high bandwidth. For those needs, you may want to consider IEEE 802.5 or FDDI.

IEEE 802.3 is based on the ethernet protocol developed by Xerox Corporation’s Palo Alto Research Center (PARC) in the 1970s. Shortly after the release of the 802.3 standard, Digital Equipment Corporation, Intel Corporation, and Xerox Corporation jointly developed and released a very similar ethernet specification (Version 2.0). Today, most organizations use the IEEE 802.3 specification, which is commonly called ethernet, as a basis for their LANs. As shown in figure 8.6, IEEE 802.3 and the ethernet specification describe a slightly different frame header; therefore, these protocols are not compatible.


Figure 8.6  The frame headers of IEEE 802.3 and ethernet.

The following describe each of the 802.3 and ethernet frame header fields:

  Preamble. Both ethernet and IEEE 802.3 frames begin with an alternating pattern of ones and zeros called a preamble, which tells receiving stations that a frame is coming.
  Start-of-Frame (SOF). The SOF delimiter ends with two consecutive one bits, which serve to synchronize the frame reception portions of all stations on the LAN.
  Destination and Source Address. The Destination and Source Address fields are 6 bytes long and refer to the addresses contained on the ethernet and IEEE 802.3 network interface cards. The first 3 bytes of the addresses are specified by the IEEE on a vendor-dependent basis, and the last 3 bytes are specified by the ethernet or IEEE 802.3 vendor. The Source Address is always a unicast (single node) address. The Destination Address may be unicast, multicast (group), or broadcast (all nodes).
  Type. Ethernet frames have a 2-byte Type field that specifies which upper-layer protocol will receive the data after ethernet processing is complete.
  Length. The Length field in IEEE 802.3 frames indicates the number of bytes of data in the Data field.
  Data. The Data field contains the actual data carried by the frame that will eventually be given to an upper-layer protocol at the destination computer. With IEEE 802.3, the upper-layer protocol must be defined within the data portion of the frame if necessary.
  Frame Check Sequence (FCS). The 4-byte FCS field contains a Cyclic Redundancy Check (CRC) value so the receiving device can check for transmission errors.
IEEE 802.5 Token Ring

The IEEE 802.5 standard specifies a 4 and 16 Mbps token ring LAN. Stations connected to the LAN take turns sending information to other stations by utilizing a token as explained in Chapter 2 within the section “Point-To-Point Infrared LAN System.” Because of the token access method, 802.5 supports heavier traffic under more stable conditions than 802.3 ethernet. In addition, 802.5 supports deterministic access to the medium, which enables it to handle synchronous type information transfers. IEEE 802.5 is the second most popular LAN medium access technique and is more expensive to implement than ethernet. But, as mentioned earlier, use token ring in cases where you need better performance.

The first token-ring network was developed by IBM in the 1970s; then IEEE wrote the IEEE 802.5 specification based on IBM’s work. Today, IBM Token Ring and IEEE 802.5 networks are compatible, although there are minor differences. For instance, IBM’s Token Ring network specifies a star configuration with all end stations attached to a device called a multistation access unit (MSAU). IEEE 802.5 does not specify a topology, but most 802.5 implementations are also based on a star configuration. Also, IEEE 802.5 does not specify a media type, but IBM Token Ring identifies the use of twisted-pair wire.

The following section explains the purpose of each field.

Tokens

  Start Delimiter. The Start Delimiter alerts each station that a token (or data frame) is arriving. This field includes signals that distinguish the byte from the rest of the frame by violating the encoding scheme used elsewhere in the frame.
  Access Control. The Access Control byte contains the priority and reservation fields, as well as a token bit that is used to differentiate a token from a data or command frame. The monitor bit is used by the active monitor to determine whether a frame is endlessly circling the ring.
  End Delimiter. The End Delimiter identifies the end of the token or data/command frame. It also contains bits to indicate a damaged frame, as well as the last frame in a logical sequence.

Data/Command Frames

  Frame Control. The Frame Control byte indicates whether the frame contains data or control information. In control frames, the Frame Control byte specifies the type of control information.
  Destination and Source Address. As with IEEE 802.3, the Destination and Source Address is 6 bytes long and designates the source and destination stations.
  Data. The Data field contains the data being sent from source to destination. The length of this field is limited by the token holding time, which defines the maximum time a station may hold the token.
  Frame Check Sequence (FCS). The 4-byte FCS field contains a Cyclic Redundancy Check (CRC) value so the receiving device can check for transmission errors.
  End Delimiter. The End Delimiter identifies the end of the data/command frame. It also contains bits to indicate a damaged frame, as well as the last frame in a logical sequence.


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