Handbook of Local Area Networks, 1998 Edition:LAN Basics
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IEEE 802.3 broadband CSMA/CD allows a segment length of 3.75 km; however, 5.5 km is achievable. Historically, the major technological problem was the allocation of the frequency channels. Because no standard existed for frequency allocations, applications assigned to one channel by one vendor often overlapped with the frequencies of another, causing interference or cross talk on other channels. However, by adhering to the recommendations of the 802.7 technical advisory group, frequency overlap should not pose a serious threat.
In many implementations of CSMA/CD over cable television-type cable, two separate channels are used. A node that has packets to transmit listens for transmission activity on the downlink channel. If the downlink is idle, the station transmits its packet on the uplink channel while performing collision detection handling and enforcement. If the node senses that the downlink is busy, indicating a transmission in progress, the station waits before proceeding to transmit on the uplink channel. The station performs collision detection by comparing the received downlink transmission, bit by bit, with the data transmitted on the uplink. Collision detection is concurrent with packet transmission and is usually performed for an interval known as the collision window. When a transmitting station detects a bit mismatch at any time during the window, it knows that a collision has occurred. In accordance with the CSMA/CD algorithm, the sending node first initiates a collision enforcement, then retries the channel after the back-off interval.
New trends in the topology of LANs include star topology networks, which have a hub or a switch at their center. Most hubs are actually concentrators; future switches will use the asynchronous transfer mode (ATM). For the latter, ATM converters are required at the terminals and stations. Terminals and stations connected to a hub can use the 802.3, or other, protocols. The 802.3 standards committee has decided not to address switching as a separate issue, claiming that existing 802.3 standards can be applied.
IEEE 802.3u—100Base-T Fast Ethernet
100Base-T is a low cost, easy-to-manage protocol for networks that require high-bandwidth support for data traffic. It was developed by the 802.3u high-speed study group within IEEE.
Media Specification 4T+ for 100Base-T
This proposal recommends four pairs of twisted wires in telephone grade cable of category 3 or better (i.e., either category 4 or category 5). In the designation, the number 4 refers to the four pairs, the letter T to the telephone cable, and the +-sign to “or better.”
By using four pairs of wire the clock and bit rate on each wire can be kept lower, which improves reliability. One of the four pairs is reserved for collision detection. Ternary (3-level) signaling and 8B6T code (with 8 binary bits that are coded into 6 ternary symbols) are used for data transmission, which further increases the reliability. 100Base-T is as robust as 10Base-T. Maximum allowable distance to the hub is 100 m. Either half or full-duplex mode can be applied. In full-duplex (i.e., simultaneous transmit and receive) the full bit rate can be used in both directions simultaneously.
Proposed 100Base-X
This method uses two pairs of category 5 unshielded twisted pair (UTP) or shielded twisted pair (STP), or fiber, over a maximum distance of 100 m between nodes and repeaters and a maximum network extent of 250 m. The recommendation supports full-duplex transmission.
NWay Autodetect
The many different versions of CSMA/CD Ethernet networks are and will cause interoperability problems. As a solution National Semiconductor is proposing to standardize on an autodetection and configuration mechanism in the 802.3 specification referred to as NWay. It will allow two stations with N different common modes of operation to find a single “way” to establish a common mode of operation.
The proposed NWay will negotiate between two nodes to detect a common mode of possible operation and to configure the nodes for that mode. The proposal suggests that the existing link pulse mechanism employed by 10Base-T nodes be used to communicate between nodes.
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