RSS
 Has 10BASE-T Wiring Run Amuck? By Bill Alderson and J. Scott Haugdahl Q: The majority of our customers with Ethernet are operating over 10BASE-T (10-Mbps, baseband medium, twisted-pair cable). The popularity of 10BASE-T is due in part to its simplicity and flexibility in building a hub-based wiring infrastructure, which lets Ethernets be built in a centralized, hierarchical or linear fashion. Category 5 twisted-pair is the media type of choice, with fiber gaining popularity, especially for router-to-hub or hub-to-hub connections. However, along with this flexibility in Ethernet topology and media types, comes an increase in comple xity. 
Bill:
Unfortunately, it often takes a rocket scientist to perform the complex round-trip bit-delay calculations to ensure that we stay within the "rules" of such highly flexible wiring systems.
Scott: Consequently, we sometimes encounter networks that break the rules. Bill: By monitoring a busy network with a protocol analyzer or other device that can count collisions, short/runt packets and Cyclic Redundancy Check (CRC) errors, we can often see the first warning sign that the network may be out-of-spec. Scott: A "clean" Ethernet will experience a relatively small percentage of collisions with very few short/runt packets or packets with CRC errors. Bill: Thus, the warning sign to look for is numerous short/runt packets, which usually are an indication of collision fragments. Scott: Prior to every Ethernet packet transmission, there is a 9.6-microsecond (ms) gap, or wait time (the equivalent of 96 bits of dead time), followed by a 64-bit preamble. The majority of collisions in an Ethernet are usually within this 64-bit preamble. Bill: Since the first byte of data is actually part of the destination data link control (DLC) address, collisions that hit this or later bytes result in a partial packet buffered into the protocol analyzer. Scott: Let's step back for a moment and discuss 10BASE-T technology in more detail. Bill: Unlike 10BASE-2 (thin coax) and 10BASE-5 (thick coax), 10BASE-T only allows two media attachment units (MAUs) per segment. Thus, there must be a way to connect multiple segments to make a multinode network. Scott: This is accomplished via a multiport repeater, more commonly known as a hub. Bill: The physical size of a 10BASE-T (or mixed media) network is limited by the various components that make up the system. Scott: We must take into account technical factors, such as cable length and associated propagation, repeater and MAU delays, interpacket gap shrinkage and delays with the data terminal equipment (DTE)--our workstations, servers and routers--that contend for the media. Bill: So for us to really understand all of this, we need to read the comprehensive 300-page ISO/IEC 8802-3 (ANSI/IEEE Std 802.3) standard.
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by Art Wittmann FreeWire by Bill Frezza Corporate View by Brian Walsh In The Middle by Nick Gall Updated June 27, 1997 |
