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Concentrators attach stations to the ring trunk by means of lobe cabling as shown in Exhibit 1-1-28. The lobes are formed through the interconnection of several trunk coupling units (TCUs), each of which supports a station. Several concentrators can be interconnected in series by means of trunk cable to form a ring topology. In IEEE draft standard 802.5q, passive and active concentrators are specified. An active concentrator regenerates signals to restore their amplitude and shape, and retimes the signals. This increases the lobe cabling length in relation to that supported by a passive concentrator.


Exhibit 1-1-28.  Example of IEEE 803.5 Connections

The allowable lengths of trunk cables depend on the attenuation of each cable, whether the concentrators are passive or active, the number of concentrators, and the bi rate (i.e., 1M, 4M, or 16M bps). In networks with passive concentrators the cable budget, that is, the total cable length, can range from 74 m to 107 m. Similar examples for networks with active concentrators show cable budgets ranging from 69 m to 364 m.

As an option, traffic can move in both directions on the ring simultaneously (i.e., full duplex transmission). To the extent that traffic is available for movement in both directions at the same time, the capacity of the ring can be doubled. Communication off the ring occurs through a bridge.

Ring Access Control. Station entry into the ring is controlled by the station itself. The insertion and ring bypass mechanisms reside in the trunk coupling unit (see Exhibit 1-1-27), which the media interface cable controls using a phantom-circuit technique. The circuit places a dc voltage on the medium interface connector which is transparent to the passage of the station transmitted symbol, or phantom. The technique is used within the trunk coupling unit to guide the switching action that causes the serial insertion of the station into the ring. Cessation of the phantom drive causes a switching action that removes the station from the ring and prepares it for offline self-testing.

IEEE 802.6 Metropolitan Area Network

The metropolitan area networking (MAN) group (IEEE 802.6) was formed to broaden the scope of the LAN standards and identify the major capabilities for MANs. Broadly speaking, a metropolitan area network is a network capable of providing high-speed switching connectivity across distances typically found within a metropolitan area. The committee recognized that a network spanning a metropolitan area may use multiple transmission media (e.g., copper, microwave, and optical fiber). Because MANs may be public offerings with multiple customers, the MAN must address such concerns as maintenance and billing and provide privacy and security. Because of the distance and the multiple media, the access methods for LANs were viewed as having serious deficiencies.

As a result, a new architectural framework and protocols were developed for MANs. The distributed queue dial-bus, or QPSX, proposal from Telecom Australia received the working group’s endorsement. The switch architecture of QPSX is based on two contradirectional buses, as shown in Exhibit 1-1-29. It is configured as a physical ring but behaves as a logical bus to enable the generation of common frames. In case of a bus fault, the network can isolate the fault and close the data buses through the headpoint of the loop.


Exhibit 1-1-29.  QPSX Dual-Bus Architecture

The protocol is based on time division multiple access (TDMA). It uses reservations, where the time is divided into continuous, discrete time segments (frames) of 125 ms (see Exhibit 1-1-30). Each frame is subdivided into a number of cells, depending on the speed of the channel. A cell may be allocated to contain isochronous or nonisochronous traffic.


Exhibit 1-1-30.  Frame Format for QPSX MAN

The operation of the protocol is based on two control bits: a BUSY bit that indicates whether a slot on the network is used, and a REQ, or request, bit that is sent whenever a station has a packet waiting for access. When a station wants to transmit downstream it sends a reservation upstream. Each station maintains a request/countdown for each transmission direction (see Exhibit 1-1-31). When a reservation request passes along the upstream bus, the counter is incremented by one; it is decremented by one when an empty cell goes by on the downstream bus. A non-zero value in the counter means that there are unsatisfied requests for cells in the downstream direction. If the counter has a value of zero, then there are no outstanding requests and the station can transmit in the next vacant cell. By counting the number of requests it receives and nonbusy cells that pass it, each station can determine the number of cells queued ahead of it. This counting establishes a single, ordered queue across the network for access to each bus. Thus, access priority levels can be established by operating a number of queues, one for each level.


Exhibit 1-1-31.  Scheduling Counter

The basic QPSX network is expected to operate at approximately 150M bps, with the streams divided into 53-byte cells (5 bytes overhead, 48 bytes payload) recurrently at the frame rate of 8K bps. A MAN may consist of many QPSX subnetworks interconnected in an approximately hierarchical manner using bridges or gateways.


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