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ETHERNET TECHNOLOGY

Part 1: Frames, Collisions, and 10 Mbps LANs

by James Antonakos

Start ý Ethernet Frame Format ý The Interframe Gap ý Collision Or No Collision ý Detecting Errors ý Random Waiting Period ý 10-Mbps Ethernet ý 10BaseFý Errors In Ethernet LANS ý More To Come ý Sources and PDF

10-MBPS ETHERNET

The first three widely used Ethernet technologies were 10Base5, 10Base2, and 10BaseT. Figure 7 shows the general 10-Mbps architecture of the data-link and physical layers. Beginning with the media (coax, UTP), you first encounter the medium dependent interface (MDI). The MDI is essentially the connection method used with the media, such as a vampire tap or RJ-45 connector. The physical medium attachment (PMA) provides the functions necessary for transmission, reception, and collision detection. Together, the MDI and PMA make up the medium attachment unit (MAU). The attachment unit interface (AUI) may be a transceiver cable up to 50 m in length, connected via a 15-pin AUI connector. The AUI cable may be used, for example, to connect a thick-wire vampire-tap transceiver to the upstream AUI port of a hub or switch. The physical signaling (PLS) is where Manchester encoding is applied to the bit stream. Table 3 shows the properties of each 10-Mbps technology.

The 2.5-m spacing requirement in the 10Base5 is designed to prevent signal distortions from each station from adding together in phase. The number of nodes allowed on a segment is limited by the electrical properties of the cable. You can see that a different coaxial cable in the 10Base2 affects the allowable cable length as well as the number of nodes/segment. And, the number of nodes/segment is misleading in the 10BaseT because the UTP cable requires a point-to-point connection, typically a NIC to a port on a hub, or a hub-to-hub connection.

All three technologies have some common properties, including:

• 10-Mbps data rate
• Manchester encoding
• maximum of 1024 stations in a single collision domain. Repeaters do not count towards this maximum limit
• maximum of four repeaters in longest path through the network

Figure 8ýAn example of Manchester encoding, where each bit causes a signal transition, can be seen here.

Figure 8 shows an example of how a digital signal is Manchester encoded. A logic zero is encoded as a falling edge in the middle of a bit time. A logic one is encoded as a rising edge. This guarantees an edge during every bit time, making the signal easier to synchronize with and decode. The four-repeater limit chosen to maintain CSMA/CD operation is part of a larger set of restrictions, commonly referred to as the 5-4-3 Rule, which has the following properties:

• five segments in the longest path
• four repeaters in the longest path
• three segments with nodes
• Again, these rules apply to a single collision domain.

Figures 9a and b show two ways of applying the 5-4-3 rule. In Figure 9a, a 10Base5/10Base2 system is illustrated. Three of the five segments contain nodes. The longest path between stations (A and Z) is five segments, with four repeaters in between. Figure 9(b) shows a 10BaseT star network. The A and Z stations are separated by five UTP segments with four hubs in between. Each UTP cable from a station to a hub or from a hub to another hub is considered a segment and may be up to 100 m in length.

Collisions are detected by a 10BaseT hub whenever two ports are active at the same time. Even though the transmit and receive wire pairs are different and there is no electrical collision (as you see on the coaxial cable), there is an attempt by two stations to use the network simultaneously, which results in a collision. Older equipment is restricted to half-duplex operation, because any station may send or receive but not at the same time. New, auto-negotiating Ethernet transceivers are capable of operating in full-duplex, sending and receiving frames simultaneously.

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