LAN ACCESS PROTOCOLS
a 500-m segment of 50-
coaxial cable is 2165 nsec, assuming that the velocity of
propagation of this medium is 0
.77 × 300 × 10
m/sec (Ref. 3).
. System model. Propagation time is critical for
the CSMA/CD access method. The major contributor to propagation time is the coaxial
cable and its length. The characteristic impedance of the coaxial cable is 50
± 2 .
The attenuation of a 500-m (1640-ft) segment of the cable should not exceed 8.5 dB
(17 db/km) measured with a 10-MHz sine wave. The velocity of propagation is 0.77
The referenced maximum propagation times were derived from the physical configuration
model described here. The maximum configuration is as follows:
1. A trunk coaxial cable, terminated in its characteristic impedance at each end, consti-
tutes a coax segment. A coax segment may contain a maximum of 500 m of coaxial
cable and a maximum of 100 MAUs. The propagation velocity of the coaxial cable
is assumed to be 0.77
c minimum (c = 300,000 km/sec). The maximum end-to-end
propagation delay for a coax segment is 2165 nsec.
2. A point-to-point link constitutes a link segment. A link segment may contain a max-
imum end-to-end propagation delay of 2570 nsec and shall terminate in a repeater
set at each end. It is not permitted to connect stations to a link segment.
3. Repeater sets are required for segment interconnection. Repeater sets occupy MAU
positions on coax segments and count toward the maximum number of MAUs on
a coax segment. Repeater sets may be located in any MAU position on a coax
segment but shall only be located at the ends of a link segment.
4. The maximum length, between driver and receivers, of an AUI cable is 50 m.
The propagation velocity of the AUI cable is assumed to be 0.65
c minimum. The
maximum allowable end-to-end delay for the AUI cable is 257 nsec.
5. The maximum transmission path permitted between any two stations is five seg-
ments, four repeater sets (including optional AUIs), two MAUs, and two AUIs. Of
the five segments, a maximum of three may be coax segments; the remainder are
The maximum transmission path consists of 5 segments, 4 repeater sets (with AUIs),
2 MAUs, and 2 AUIs, as shown in Figure 11.9. If there are two link segments on the
transmission path, there may be a maximum of three coaxial cable segments on that path.
If there are no link segments on a transmission path, there may be a maximum of three
coaxial cable segments on that path given current repeater technology. Figure 11.10 shows
a large system with maximum length transmission paths. It also shows the application of
link segments versus coaxial cable segments. The bitter ends of coaxial cable segments are
Maximum transmission path.
c = velocity of light in a vacuum.