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IEEE 802.12 Demand Priority Access Method and Physical Layer Specifications The standards committees of the IEEE have been unable to choose a single standard for fast LANs. Instead the IEEE 802.3 committee is working on standards for fast (100M-bps) CSMA/CD/Ethernet networks as discussed earlier in this chapter. In another committee, Hewlett-Packard is proposing a different version of fast LANs, called IEEE 802.12 Demand Priority Method and Physical Layer Specifications. The proposal advocates the use of a central controller/repeater for communications with data terminal equipment (DTE) (i.e., work stations). The repeater arbitrages between transmission requests from connected DTEs depending on the designated priority. Two priority levels are available. Exhibit 1-1-35 shows the architecture of the repeater.
The intention is to have 802.12 support both connectionless and connection-mode transmissions. Specifically, it will be able to handle and interface with both 802.3 CSMA/CD and 802.5 Token Ring frame formats. However, the bit order in the octets of the 802.3 MAC format differs from that in the 802.5 MAC format. Basically, the bits are read in opposite directions. One consequence of this is that the same 802.12 devices cannot be used for 802.3 and 802.5 at the same time. P802.12 supports the link formats listed in Exhibit 1-1-36.
With four unshielded twisted pairs (UTP) of category 3 or better wire, distances to 100 m between the nodes are possible. Each pair can carry 30M baud for a total of 120M baud. Non-return-to-zero (NRZ) encoding will be specified resulting in the bit rate of 120M bps. Two UTPs of category 5 can cover 150 m, and two shielded twisted pairs (STPs) 200 m. Alternatively, one optical fiber operating at a wavelength of 1,300 nm can support a distance of 2 km. P802.12 can support all existing 802.3 and 802.5 topologies without bridges. For instance, it can support a 2,500-km network with as many as five hubs between nudes. P802.3u limits the distance to about 210 m with at most two hubs between nodes. Larger 802.3u topologies require bridges. The two priority levels in 802.12 allow multimedia traffic to be serviced with very short bounded delays, which will make possible such interactive multimedia services as training and videoconferencing (both local and over a WAN). The CSMA/CD access method used in 802.3 is subject to very long delays when the network becomes congested. Because of the probabilistic nature of the access method, very wide variations of delay occur. This does not happen with the demand priority access method of 802.12. IEEE 802.14 Cable-TV Based Broadband Communication Network (Hybrid Fiber/Coaxial Cable Systems) The cable TV industry is deploying hybrid fiber/coaxial (HFC) networks, e.g. networks with fiber optic cables from headends to distribution points and coaxial cables the last distance to the subscribers. Lately, telephone companies have started installing similar networks. In 1994 a 802.14 committee was spun off from the 802.6 (MAN) project with the mandate to establish a standard for digital broadband communications over HFC cable networks. This standard will cover the following types of traffic: constant bit rate (CBR), variable bit rate (VBR); connection-oriented (CO) and connectionless (CL) data. Sources both in the headend and within the network will be provided for and one-to-one, one-to-many, many-to-one and many-to-many type of traffic will be covered. By specifying the media access control and physical layers the standard will be architecturally compatible with other 802-type LAN standards. At this early point the 802.14 committee plans to make the final standard compatible with emerging technologies such as video compression and ATM. SUMMARY This chapter has discussed the major current LAN standards. Adherence to these standards provides the basis for the formation of LANs comprising equipment from many different vendors. Standards, however, are not perfect. Some problems remain, and even when these problems are resolved, LAN designers must continue to exercise a degree of caution to ensure compatible LAN implementations.
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