11.6
LAN ACCESS PROTOCOLS
303
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
6
m/sec (Ref. 3).
11.6.2.1.1
Transmission Requirements
. 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
c.
4
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
link segments.
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
Figure 11.9
Maximum transmission path.
4
Where
c = velocity of light in a vacuum.