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PMI Sublayer Functions

The PMI sublayer is responsible for performing four key functions before passing data to the PMI sublayer. Those functions are quartet channeling, data scrambling, 5BGB encoding, and the addition of a preamble and “start” and “end” frame delimiter to frames, which prepares them for transmission by the lower sublayer. Exhibit 4 illustrates the functions performed at the PMI and PMD sublayers.

Quartet Channeling. Quartet channeling is the process of first sequentially dividing MAC frame data octets into 5-bit data quintets. Next, each 5-bit quintet is distributed sequentially among four channels. The rationale for the use of four channels is that they represent a transmission pair for a 4-UTP demand-priority network.

As indicated in Exhibit 4, channel 0 data is transmitted on twisted-pair wires 1 and 2, channel 1 data winds up being transmitted on wires 3 and 6, and so on. When 2-pair or fiber optic cable is used, 100VG-AnyLAN specifies the use of a multiplexing scheme that is incorporated at the PMD sublayer. Through the use of multiplexing, the four channels illustrated in the lower portion of Exhibit 4 are converted into two channels for transmission on 2-pair wire, or to one channel for transmission on fiber optic cable. Thus, the addition of multiplexing tailors the PMD sublayer to the physical medium used by the network.


Exhibit 3.  IEEE 802.12 Reference Model Compared to OSI Reference Model..


Exhibit 4.  PMI and MPD Sublayer Functions..

Data Scrambling. The scrambler used at the PMI level reduces the potential effect of radio frequency interference and signal crosstalk between cable pairs. To accomplish this, the scramblers randomize the bit patterns on each transmission pair, eliminating the potential for long repetitious strings of 0s and 1s. Each of the four scramblers uses a different scrambling mechanism that ensures the randomness of the resulting data.

5BGB Encoding. The mapping of scrambled 5-bit data quintets into 6-bit symbols is performed by the 5BGB encoders shown in Exhibit 4. The encoding process results in the creation of a balanced data pattern that contains equal numbers of 0s and 1s, providing guaranteed clock-transition synchronization for receiver circuits. In addition, the 5BGB encoding process provides an added error-checking capability. This results from the fact that 5BGB encoding supports the use of only 16 symbol patterns. Thus, invalid symbols can be detected as error conditions.

Data Addition. The last function performed by the PMI layer is adding the preamble and starting and ending frame delimiters to each channel. This preprocesses the data into a format that can be transmitted across the network. The actual placement of data onto the network is performed by the PMD sublayer.

PMD Sublayer

PMD sublayer functions include NRZ encoding, link medium operation, and link status control. In addition, if the transmission medium is 2-pair or fiber optic cable, the PMD sublayer will also perform channel multiplexing. The remainder of the functions discussed in this section use 4-pair unshielded twisted- pair cable cabling.

NRZ Encoding. NRZ encoding is a two-level signaling mechanism (0 and + voltage) used to represent the values of data transmitted on the copper 4-pair unshielded twisted-pair cable. Under NRZ encoding, successive 1 bits are represented by a continuous + voltage level. Thus, to differentiate one “1” from a succeeding “1,” NRZ encoding requires the use of clocking circuitry.

Link Medium Operation. Link medium operation permits a 4-UTP 100VG-AnyLAN network to operate in both full- and half-duplex. F-DX communications use two channels for transmission from the hub to a node, and the remaining two channels are used for transmission from the node to the hub.

F-DX communication is required when link-status information is transmitted between a hub and a node. In comparison, normal data flow is accomplished via a half-duplex operation, where all four channels are used to transmit data from the node to the hub or from the hub to the node.

Exhibit 5. PMD Link Status Control Signaling.
Tone Pattern Meaning when Received by a Node Meaning when Received by a Hub

1-1 Idle Idle
1-2 Incoming Data Packet Normal-Priority Request
2-1 Reserved High-Priority Request
2-2 Link Training Request Link Training Request

Link Status Control. Link status control requires a full-duplex transmission mode of operation. When operating in F-DX, two frequency tones — referred to as tone 1 and tone 2 — are used to communicate the link status between the hub and the node. In actuality, the tones are generated by producing a pattern of 1s and 0s at a specific signaling rate to produce a tone. For example, tone 1 is generated by transmitting a 30MHz alternating pattern of sixteen 1s followed by sixteen 0s, resulting in a frequency of approximately 0.9375 MHz. In comparison, tone 2 is generated by transmitting a 30MHz alternating pattern of eight 1s followed by eight 0s, resulting in a frequency of approximately 1.875 MHz.

Through the use of a combination of tones, control signals are transmitted by the hub and the node. Exhibit 5 lists the link status control signals supported at the PMD sublayer.

The “idle” status, when received by a node, indicates that the hub has no pending packets. When received by the hub, an idle status indicates no requests are pending. An “incoming data packet” status indicates to a node that data may be destined to that port from the hub. This instructs the node to stop sending control tones on channels 2 and 3 so it can receive the packet. The normal-priority request indicates to the hub that the node is requesting to transmit a normal-priority packet. In comparison, the high-priority request indicates to the hub that the node is requesting to send a high-priority packet. The last signal permissible, link training request, indicates to the node or hub that link initialization is being requested.

CONCLUSION

100VG-AnyLAN network adapter cards are expected to be available for under $400, providing a cost-effective, high-speed LAN transmission infrastructure. In comparison, ATM adapter cards are many years away from commercial production and are expected to cost in excess of $1,250 per network adapter card when they become available. Because of the affordability of 100VG-AnyLAN and its ability to directly interface with existing Ethernet and Token Ring networks (and indirectly to FDDI and ATM), it is a very attractive method of satisfying high-speed organizational LAN communications requirements. Thus, network managers should place 100VG-AnyLAN on the top of their list of emerging technologies to explore for satisfying organizational requirements.


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