This section discusses the various network components which make up a network. Its discusses cabling types, patch panels, connectors and network components like bridges, routers and ethernet switches.
Brief Guide to BUILDING WIRING
The following is a very brief
overview of the components that make up the wiring standards for commercial
buildings. The objectives of such standards is to:
A building wiring system covers a number of different elements
Some general features of the horizontal wiring scheme are:
Unshielded Twisted Pair cable used in Category 5 looks like: 
Category 5 cable uses 8 wires. The length of exposed wires is very
critical, the standard limits this to less than 1/2" an inch. The various jack
connectors look like: 
The patch cord which connects the workstation to the wall jack looks like:
In 10BaseT, each PC is wired back to a central hub using its own cable.
There are limits imposed on the length of drop cable from the PC network card
to the jack, the length of the horizontal wiring, and from the jack to the
wiring closet. 
This is obviously a physical STAR configuration, in that each PC is wired back to a central point (the Hub).
Ethernet 10Base-T wiring specifies an 8 position jack, but uses only two
pairs. 
TWISTED PAIR ETHERNET HORIZONTAL WIRING (Solid 24Awg)
|
Pin |
Colour |
Signal |
|
1 |
White/orange |
Tx data + |
|
2 |
Orange/white |
Tx data - |
|
3 |
White/green |
Rx data + |
|
4 |
Blue/white |
-- |
|
5 |
White/Blue |
-- |
|
6 |
Green/white |
Rx data - |
|
7 |
White/brown |
-- |
|
8 |
Brown/white |
-- |
Each run < 80m from nearest hub
Patch Cables
Patch cables come in two varieties, straight
through or reversed. One application of patch cables is for
patching between modular patch panels in system centers. These are the
straight through variety. Another application is to connect workstation
equipment to the wall jack, and these could be eitherstraight through or
reversed depending upon the manufacturer.
Reversed cables are normally used for voice systems.
How to determine the type of patch cable
Align the ends of the
cable side by side so that the contacts are facing you, then compare the
colors from left to right. 
If the colors are in the same order on both plugs, the cable is straight
through. If the colors appear in the reverse order, the cable is reversed.
Thin coaxial cable [RJ-58AU], as used in ethernet LAN's, looks like
The connectors used in thin-net Ethernet LAN's are T connectors (used to
join cables together and attach to workstations) and terminators (one at each
end ofthe cable). The T-connectors and terminators look like 
Fibre optic is often used to overcome distance limitations. It can be used tojoin two hubs together, which normally could not be connected due to distancelimitations. In this instance, a UTP to Fibre convertor is necessary.
Fibre obtic cable looks like 
In addition, fibre optic patch panels are used to interconnect fibre
cables. These patch panels look like 
NETWORK SEGMENTS
A network segment
Large networks are made by combining several individual network segments together, using appropriate devices like routers and/or bridges.
When network segments are combined into a single large network, paths exist between the individual network segments. These paths are called routes, and devices like routers and bridges keep tables which define how to get toa particular path. When a packet arrives, the router/bridge will look at thedestination address of the packet, and determine which network segment thepacket is to be transmitted on in order to get to its destination.
In the above diagram, a packet
arrives whose destination is segment B. The bridge forwards this incoming packet
from segment A to the B segment.
SPANNING TREE ALOGRITHM
Switches and
bridges generally learn about the segments they are connected to.As packets
arrive, they build up a table which lists the network address used on the
various network segments.
Sometimes, a loop would be created which caused the wrong packets to be sent on incorrect segments. These packets could loop around the network, being forwardedon, eventually arriving back, only to be forwarded on, etc. This quickly floodsthe network.The spanning tree algorithm is a software algorithm which defines how switchesand bridges can communicate and avoid network loops.
Packets are exchanged between bridges/switches, and they establish a singlepath for reaching any particular network segment. This is a continous process,so that if a bridge/switch fails, the remaining devices can reconfigure therouting tables to allow each segment to be reached.
To be effective, ensure that the bridges/switches in use in your network supportthis protocol.
REPEATERS
Repeaters connect multiple network
segments together. They amplify the incomingsignal recieved from one segment and
send it on to all other attached segments.This allows the distance limitations
of network cabling to be extended. Thereare limits on the number of repeaters
which can be used. The repeater counts as a single node in the maximum node
count associated with the ethernet standard [30 for thin coax].
Repeaters also allow isolation of segments in the event of failures or faultconditions. Disconnecting one side of a repeater effectively isolates theassociated segments from the network.
Using repeaters simply allows you to extend your network distance
limitations. It does not give you any more bandwidth or allow you to transmit
data faster. 
It should be noted that in the above diagram, the network number assigned tothe main network segment and the network number assigned to the other side of the repeater are the same. In addition, the traffic generated on one segment is propagated onto the other segment. This causes a rise in the total amount of traffic, so if the network segments are already heavily loaded, its not a good idea to use a repeater.
Summary of features
BRIDGES
Originally designed to interconnect
ethernet segments together. Most bridgestoday support filtering and forwarding,
as well as Spanning Tree Algorithm.
The IEEE 802.1D specification is the
standard for bridges.
During initialisation, the bridge learns about the network and the routes. Packets are passed onto other network segments based on the MAC layer. Each time the bridge is presented with a frame, the source address is stored. Thebridge builds up a table which identifies the segment to which the device islocated on. This internal table is then used to determine which segment incoming frames should be forwarded to. The size of this table is important, especially ifthe network has a large number of workstations/servers.
The advantages of bridges are
The disadvantages of bridges are
Transparent bridges (also known as spanning tree, IEEE 802.1 D) make all routing decisions. The bridge is said to be transparent (invisible) to the workstations. The bridge will automatically initialize itself and configure itsown routing information after it has been enabled.
Bridges are ideally used in environments where there a number of well definedworkgroups, each operating more or less independant of each other, with occassional access to servers outside of their localised workgroup or network segment. Bridges do not offer performance improvements when used in diverse or scattered workgroups, where the majority of access occurs outside of the local segment.
The diagram below shows two separate network segments connected via a bridge.
Note that each segment must have a unique network address number in order for
the bridge to be able to forward packets from one segment to the other.
Ideally, if workstations on network segment A needed access to a server, the best place to locate that server is on the same segment as the workstations, as this minimises traffic on the other segment, and avoids the delay incurred by the bridge.
Summary of features
ROUTERS
Packets are only passed to the network
segment they are destined for. They work similar to bridges and switches in that
they filter out unnecessary network traffic and remove it from network segments.
Routers generally work at the protocol level.
Routers were devised in order to separate networks logically. For instance, aTCP/IP router can segment the network based on IP subnets. Filtering at thislevel (on IP addresses) will take longer than that of a bridge or switch whichonly looks at the MAC layer.
Most routers can also perform bridging functions. A major feature of routers, because they can filter packets at a protocol level, is to act as a firewall. This is essentially a barrier, which prevents unwanted packets either entering or leaving the network.
Typically, and organisation which connects to the Internet will install a router as the main gateway link between their network and the outside world. Byconfiguring the router with access lists (which define what protocols and what hosts have access) this enforces security by restricted (or allowing) access to either internal or external hosts.
For example, an internal WWW server can be allowed IP access from external networks, but other company servers which contain sensitive data can be protected, so that external hosts outside the company are prevented access (you could even deny internal workstations access if required).
Summary of features
HUBS
Provide full bandwidth to each client,
unlike BUS networks where the bandwidth is shared. Often include buffering of
packets, and filtering, so that unwanted packets (or packets which contain
errors) are discarded.
In standard ethernet, all stations are connected to the same network segment inbus configuration. Traffic on the bus is controlled using the CSMA protocol, andall stations share the available bandwidth.
Hubs dedicate the entire bandwidth to each port (workstation). Theworkstations attach to the hub using UTP. The hub provides a number of ports, which are logically combined using a single backplane, which often runs at amuch higher data rate than that of the ports.
Ports can also be buffered, to allow packets to be held in case the hub or portis busy. And, because each workstation has their own port, they do not contendwith other workstations for access, having the entire bandwidth available fortheir exclusive use.
The ports on a hub all appear as one single ethernet segment. In addition, hubs can be stacked or cascaded (using master/slave configurations) together, to add more ports per segment. As hubs do not count as repeaters, this is a better option for adding more workstations than the use of a repeater.
Hub options also include an SNMP (Simple Network Management Protocol) agent. This allows the use of network management software to remotely administer and configure the hub. Detailed statistics related to port usage and bandwidth are often available, allowing informed decisions to be made concerning the state of the network.
In summary, the advantages of hubs are,
ETHERNET SWITCHES
Ethernet switches increase
network performance by decreasing the amount of extraneous traffic on individual
network segments attached to the switch. They also filter packets a bit like a
router does.
When a packet arrives, the header is checked to determine which segment the packet is destined for, and then its forwarded to that segment.If the packet is destined for the same segment that it arrives on, thepacket is dropped and not retransmitted. This prevents the packet being"broadcasted" onto unnecessary segments, reducing the traffic.
Nodes which inter-communicate frequently should be placed on the same segment. Switches work at the MAC layer level.
Cut-Through Switches
Cut through switches use either a cross-bar or cell-backplane
architechure.
Store-Forward Switches
Features of Switches
Switch
Benefits
Other Forms of Ethernet
100 Megabit Ethernet
Summary of features
All four pairs are used to send the packet to a hub. If more than one packet arrives at the hub at the same time, the highest priority packet is servicedfirst. Typically use VIDEO with high priority.
Ethernet Frame - yes CSMA/CD - no Voice grade UTP - yes
Uses two-level NRZ output, 5 data bits converted into 6 transmit bits. Half-duplex.
Summary of features
In order to achieve the high 100Mbps rate, the collision timing associated with CSMA/CD is reduced. This means the total lengths of cabling from a workstation to another via two hubs cannot exceed 205 meters. If using fiber, a maximum distance of 405 meters from workstation to hub is allowed.
100BaseT devices are auto sensing, which means they can run at either speed. On startup, the card advertises their speed with a series of Fast Link Pulses. If a hub supports this, it will detect the FLP's, and then negotiate with the card for the highest possible speed. This can be over-ridden by network managers, who can force the slower 10Mbps mode.
Summary of features
The advantages of 100BaseVG-AnyLan over 100BaseT are,
Reference Material
3COM: Fast Ethernet
Strategies
3COM: Switches and
Routers Overview
Intel: Deploying
Fast Ethernet
Intel: Fast
Ethernet Overview
Intel Express
100BASE-TX Stackable Hub