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.
Glossary:
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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