Specifications

RADring
Installation & Operation Manual Token Ring Desing Considerations
3/11/98 12:26 Token Ring Physical Design 6-3
Design Rules -
Basics
Attenuation
The total attenuation of the ring electrical signal in a LAN without repeaters,
under the worst-case scenario, must not exceed 26dB. The maximum
allowable attenuation is increased by 26dB for each repeater, and can be
translated into distance according to the attenuation of each type of cable
(See Table 6-1 and Table 6-2).
Explanation: The transmit level of the token-ring components is nominally
4 volts peak-to-peak. The sensitivity of a token-ring receiver is also limited,
to avoid random errors caused by noise (usually 0.2 volts peak-to-peak).
These two factors limit the maximum attenuation permitted between any
transmitter and the next downstream receiver to a maximum of 26dB.
This value of maximum allowable attenuation, 26dB, is called "power (or
loss) budget".
the maximum attenuation is exceeded, the signal must be amplified by
means of a repeater (a special device that is not a ring node, and has only
the function of regenerating the ring signals). The repeater restores the signal
to its nominal transmit levels. However, to be effective, a repeater must be
positioned at a location where signal levels are low, e.g., in a long ring
segment.
Jitter
The jitter limits the maximum number of stations that can be connected in a
ring, because each device on a token ring acts as a repeater that receives
signals from an upstream neighbor and retransmits these signals to the next
active downstream neighbor on the ring. During this process, small timing
errors ("jitter") occur in the regenerated signal.
Timing errors introduced during regeneration accumulate. This effect
increases with the number of devices (stations and repeaters) on the ring.
Since there is a limit to the jitter a station, and in particular the active
monitor of the ring, can tolerate, there is a limit to the number of devices on
a ring. The limit depends on cable characteristics:
•
For a ring using IBM Type 1 (STP) cable or equivalent, the limit is 260
devices.
•
For a ring using IBM Type 3 (UTP) cable or equivalent, the limit is 72
devices.
•
For a ring using super UTP cable (e.g., AT&T level 4 cable 1061A/2061A),
the limit is 104 devices.