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	--

In addition, fibre optic patch panels are used to interconnect fibre cables. These patch panels look like

NETWORK SEGMENTS
A network segment
-is a length of cable
-devices can be attached to the cable
-it has its own unique address
-it has a limit on its length and the number of devices which can be attached to it

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.

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
-only the first few bytes of the packet is read to obtain the source and destination addresses
-the packets are then passed through to the desination segment without checking the rest of the packet for errors
- invalid packets can still be passed onto other segments
- there is little delay involved in packet throughput

Cut through switches use either a cross-bar or cell-backplane architechure.

Cross-bar switches
--read the destination address then immediately forward
--acts as a simple repeater once the path is established
--can introduce delay, if the destination port is busy, it may need to bufferthe packet
Cell-backplane switches
--break the frame into small fixed cell lengths
--each cell is labelled with special headers which contain the address(s) ofthe destination port
--the cells are buffered at the destination port
--the cells are then reassembled and transmitted
--the data rate on the backplane is significantly greater than the aggregatedata rate of the ports
--in heavily overloaded networks, cell-backplane switching offers better performance than cross-bar switching

Store-Forward Switches
--they examine the entire packet
--each incoming packet is buffered, then examined
--the switch filters out any bad packets it detects
--good packets are forwarded to the correct segment
--detect more errors than the cut-through variety
--impose a small delay in packet throughput

Features of Switches
Switch Benefits

Other Forms of Ethernet

100 Megabit Ethernet

Summary of features
--based on ANSI FDDI over copper
--uses MLT-3 signalling
--retains CSMA/CD
--uses two pairs
--uses ethernet packet format
--requires no changes to exisiting network components, cabling, bridges
--need category 5 cabling

Summary of features
--uses voice grade cable
--supported by Microsoft, Apple, Novell, etc
--signal is split over 4 pairs
--data is half duplex, one direction at a time
--handles time sensitive data like voice and video
--CSMA/CD replaced with Demand Priority Protocol

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
--supported by Sun, Synoptics and 3Com
--uses CSMA/CD
--uses ethernet frame format
--uses new signalling scheme using 3 voltage levels

Reference Material
3COM: Fast Ethernet Strategies
3COM: Ethernet Backbone Cabling
Intel: Networking Products

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