Local Area Network Technology: Part 2 - Hubs

by Jeffrey L. Carrell
Issue v1n2 - June 1997

Part 1 of this series discussed the first component of the OSI Physical Link Layer - cabling. In LAN technology, the physical layer electronics used have been referred to as: amplifiers concentrators, hubs, and MSAU's, depending on the topology used. Most of the time, "hub" is the operative word.

Hubs come in many variations: from a few ports, to hundreds of ports; from a small enclosure not much bigger than an apple, to larger than a mini-fridge; from under $50 to well in excess of $25K. There are basically three types of hub designs: fixed port configurations, usually with 4-24 ports; fixed port with modular options and/or where multiple hubs can be connected together to form larger port density systems (sometimes called "stackable hubs"); chassis systems that have slots than can support modules with many ports, mixed media, mixed topology speeds, and many other high-end features like routers, switches, bridges, WAN connectivity, etc. One note, most of the hubs today support a connector type known as RJ45, which is an 8P8C style connector. This article assumes that the RJ45 is the default connector type.

In Ethernet, there are three different speeds available: 10Mb/s, 100Mb/s and 1Gb/s (although gigabit Ethernet is not yet a standard). Additionally, there is the capability to support full-duplex speeds which doubles the effective throughput of any of these topologies. Token Ring has three speeds available: 4Mb/s, 16Mb/s and 100Mb/s (although 100Mb/s TR is referred to as FDDI or CDDI). Token Ring can also support full-duplex but only on 16Mb/s and 100Mb/s technologies.

Many hubs have an LED indicator per port known as "link". This indicates that a device has successfully loaded or been powered up, and the connection between the device and the hub is active and OK. However, depending on many issues a link light does not always guarantee a signal is passing. Bad or improperly pinned cabling can allow some pins to provide a link light, but no signal passing; the physical port of the hub may be bad, but still provide a link light, and of course there are always the anomalies associated with electronics, that some things "work" when in fact they are not really working. So just because a link light is on, don't take for granted that everything is "always" OK.

When connecting devices to an Ethernet hub, a straight pinned cable is used (following either T568A or T568B wiring configurations). When connecting two Ethernet hubs together, a cross-over cable must be used (T568A on one end, and T568B on the other), unless one of the hubs has a port that can be configured to "cross-over". Many of the higher priced stand-alone hubs and modular hubs have this feature. When the "cross-over" port is available, a straight pinned cable is used to connect the two hubs together. If a cross-over cable is used to connect a device to a hub, a link state LED may be active/on, but no signal will pass. If a straight pinned cable is used to connect two hubs together, usually no link state LED is on, and therefore no signal will pass (this assumes that neither hub has a "cross-over" port capability or it is not in use).

When connecting devices to a Token Ring hub, a straight pinned cable is used (following either T568A or T568B wiring configurations). When connecting two Token Rings hubs together, two straight pinned cables are used to connect a pair of ports designated as "Ring In/Ring Out" which must be available on each hub. Ring In/Ring Out ports are not device ports, but are special designated ports. Because of the function of the signal is moving in a logical 'ring', the way to extended the ring is to connect the two hubs together so that the ring is maintained and lengthened. You cannot simply connect a device port (IBM referred to this as a "lobe" port) on one hub to another device port on the second hub, no signal will pass. If IBM MAUs or 100% compliant devices are being used, there are special cables available for connecting both devices to the MAU and MAU-to-MAU, since the connector is a special type (referred to as the IBM Data Connector) and not an RJ45 type connector.


b/s - bits per second

Bridge - An OSI Data Link Layer device. Used to connect multiple LANs together.

CDDI - Copper Distributed Data Interface. The copper version of FDDI

Concentrator - see Hub

FDDI - Fiber Distributed Data Interface. 100Mb/s Token Ring functioning in a counter-rotating ring function.

Hub - An electronic device that serves as the center of a star-topology.

MSAU/MAU - IBM's Multi Station Access Unit. (Token Ring hubs).

Port - An interface on a hub (what the devices connect to).

Router - AN OSI Network Layer device. Used to connect multiple local or remote LANs together.

Switch - An OSI Data Link Layer device. Used to connect multiple LANs together.

Topology - The physical arrangement of network devices and media within a LAN structure.

WAN - Wide Area Network

In the next article I'll discuss the active electronics that is installed in host devices…NICs.

Copyright © 1997 Jeffrey L. Carrell. All Rights Reserved.

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