Understanding the network.pdf - Back to Home

connectors. The 100Base-FX PMD operates over a pair of multimode
(850-nanometer short wavelength light on 62.5/125 micron) fiber optic cables
using SC type connectors.
The MII interface hands off 4-bit data blocks called "nibbles" to the PCS, which
encodes them using 4B/5B encoding. Depending on the PMD, the PMA transmits the
4B/5B symbols serially over 125MHz data paths. Each path is capable of 100Mbps
operation. One is used for data transmission and collision resolution, and the other
is used for data reception and collision detection. With the 100Base-FX PMD, the
4B/5B symbols are sent by the PMA across the medium using NRZI signaling.
Category 5 UTP, however, can only provide 100MHz of bandwidth, so additional
signaling must be used to reduce the bandwidth requirement. The data is processed
further by the PMA and encoded with MLT-3 line encoding, which operates at 100
Mbps on only 32MHz of bandwidth. The MTL-3 signal is then handed to the PMD for
transmission over the medium.
100Base-TX supports dual 10/100 operation and auto-negotiation. This standard,
like 100Base-T4, can support hub-to-end-station cable segment distances up to
100m. Also like 100Base-T4, the maximum cable segment length permitted
between end-stations is 205m. 100Base-FX can be implemented in a point-to-point
topology between end-stations utilizing segment lengths up to 400M. A 100Base-FX
hub can also be utilized which reduces the hub-to-end-station segment length to
300m. 100Base-FX supports full-duplex operation, but not auto-negotiation. In
full-duplex mode, the point-to-point segment distance increases substantially, to
2,000m.
100Base-T Repeaters
Whereas 10Base-T and 100Base-T differ in their repeater implementations, they
operate using the same collision detection mechanisms and the same slot-periods
(512 bit-periods). With 10Base-T, the network size is limited, so a collision can be
detected within one slot-period. This reduced size enables a station sending the
smallest legal packet size to acknowledge a collision if one occurred with a station
located at the farthest possible distance from itself. However, 100Base-T's
transmission rate is 10 times faster than 10Base-T's. This has the effect of reducing
the actual bit-period by a factor of 10. Therefore, in order for the original CSMA/CD
collision mechanism to work, the collision domain needs to be reduced so a collision
can still be detected within one slot-period.
To accommodate this reduced collision domain of 205m (compared to 10Base-5's
2,500m and 10Base-T's 500m domain), 100Base-TX can only use, at the most, two
repeaters per segment. There are two types of repeaters used for Fast Ethernet:
Class 1 (the most common) and Class 2. Class 1 repeaters have a certain advantage
over Class 2 repeaters, because Class 1 repeaters provide translation services