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o IBM Type 3 cabling, data-grade unshielded twisted-pair
(UTP)—Token Ring operates at 4Mbps on Category 3 and 4, and at
16Mbps on Category 4 and 5 UTP. UTP uses RJ-11 and RJ-45
connectors. The IBM specification states a UTP ring can support a
density of no more than 72 end-stations. The maximum cable
distance between the end-station and the MAU cannot exceed 45m.
A ring can consist of up to 33 MAUs using Type 1 and Type 2 cabling. This count is
reduced to 9 MAUs when you deploy eight ports per MAU for a maximum of 72
stations per UTP ring. Connections between MAUs can be no more than 45m. These
distances can be increased through the use of repeaters.
With Type 3 cabling, which is by far the most common today, there are some limits
with regard to the maximum functional cable length between end-stations and
MAUs, and with MAU-to-MAU interconnections. The IBM specification for
station-to-MAU connections is 45m. There are, however, manufacturers that
support site distances up to three times that amount. It is best to check with your
equipment's manufacturer for the maximum supported cable lengths.
Token Ring Bridges and Dedicated Token Ring
It was mentioned previously that IBM Token Ring supports a type of bridging called
source route bridging (SRB). SRB is part of the IBM Token Ring specification; it is
not part of the IEEE 802.5 specification. 802.5, like all the other 802 specifications.
uses 802.1 spanning tree bridging. Both of these bridging methods will be covered
in Chapter 6. There are two types of bridges used with Token Ring: traditional
two-port bridges and multiport bridges.
The 802.5r Token Ring standard defines the operation of full-duplex or Dedicated
Token Ring (DTR). With DTR, the end-station can transmit and receive tokens
simultaneously, which, when utilized, gives the end-station an effective data
transmission throughput rate of 32Mbps. DTR requires the use of a Token Ring
multiport bridge and a DTR adapter. Token Ring multiport bridges operate the same
as Ethernet multiport bridges. Each bridge port is its own network segment—in this
case, a ring. The bridge enables all its active ports to establish connections between
each other simultaneously. The DTR adapter is different than a traditional Token
Ring adapter. With traditional Token Ring adapters, a repeater loop between the
transmit and receive path facilitates the ring operation. DTR adapters do not have
this loop; the transmit and receive paths are not connected. When the DTR interface
is connected to a Token Ring multiport bridge, the Transmit Immediate Protocol
(TXI) is used to facilitate simultaneous data transmission and reception between the
end-station and the bridge port, without acquiring a token before transmission. If a
non-DTR end-station is directly connected to a Token Ring multiport bridge port, the
station must wait for a token before it can transmit data.