Ethernet
Ethernet is a Network Architecture
Standards + Topology + Protocols = Network Architecture
Ethernet is a network architecture. If I say that my network uses Ethernet, then you will know its topology and some of the protocols it uses.
Ethernet is very important because it is the most common system for use in PC networks. All of the HCT networks use Ethernet.
Data Link Layer + Physical Layer = Ethernet
Ethernet = The IEEE 802.3 standard
Note that Ethernet is a low level standard, dealing only with the Data Link layer and Physical layer. We can use whatever protocols we wish for the higher layers. So we could have Novell Netware’s SPX/IPX running on Ethernet or Windows NT’s NWLink protocol running on Ethernet.
Ethernet Facts
| Topology: | Bus, Star or Star Bus |
| Access method: | CSMA/CD |
| Defining document: | IEEE 802.3 |
| Speed: | 10 Mbps or 100 Mbps |
| Cable Type: | Thinnet, Thicknet, Twisted Pair or Fiber Optic |
Different Types of Ethernet
There are several different types of Ethernet. they are:
10BaseT
10BaseT
This is the most widely used type of Ethernet.
10 Base T
10BaseT is Ethernet using twisted pair cable.
Category 3, 4 or 5 cable is used, with RJ-45 plugs. Two twisted pairs - 4 conductors - are used in each cable.
As twisted pair cable is only ever used with hubs, so a 10BaseT network must use a star or star bus topology.
The topology of a 10BaseT network is hub based, but the method of transmitting frames is the same as for a linear bus.
When a station sends a frame it first goes down the cable to the hub connecting that station. That hub then copies the frame to all other stations connected to the hub, and passes the frame onto any other connected hubs. They, in turn, pass copies of the frame to all connected stations and hubs.
This system works in a similar way to a simple linear bus, but gives us the added fault tolerance of a star topology.
(In such a system a hub can be referred to as a multiport repeater. Whatever goes into one socket is copied out through all of the other sockets.)
The method of deciding when to send a frame is CSMA/CD:
1. When a PC wishes to send a frame it listens to the cable. If the cable is free it sends. If not it waits a while then tries again.
2. When a PC has sent a frame it listens to check that there was not a collision with another frame. If there was, it waits a random amount of time, then sends the frame again.
3. All frames are received by all stations on the network. The station to whom the frame is addressed reads it - the others ignore it.
In a star bus network we could use UTP cable throughout.
This would be a pure 10BaseT network.
Often Thicknet cable is used as a backbone between hubs. Such a network is no longer 10BaseT - it is a hybrid.
The following limitations apply to 10BaseT
| Maximum length of a single piece of cable | 100 m |
| Maximum number of computers | 1024 |
10Base2
This is Ethernet implemented using Thinnet coaxial cable, BNC connectors, and a simple bus topology.
The maximum total cable length between repeaters is 185 m. There can be up to 30 stations on one 185 meter section.
10Base5
This is Ethernet implemented using Thicknet cable. Each station is connected to the Thicknet backbone by a vampire tap and a short length of thinner cable.
10Base5 networks can be spread over greater distances than 10BaseT or 10Base2. An end-to-end distance of 2,500 m is possible.
10BaseFL
This is 10 Mbps Ethernet using fiber optic cables. The normal reason for using this system is to allow long cable runs, for example, between buildings. A segment can be up to 2000 meters in length.
New, 100 Mbps Architectures
These new systems are 10 times faster than traditional Ethernet. There are two systems:
100VG-AnyLan
This system can use the ordinary 10BaseT cables (but not NICs or hubs) so is sometimes described as ‘Ethernet’. This is misleading, as the mode of operation is different. For example, CSMA/CD is not used.
100BaseX - Fast Ethernet
This system is closer to Ethernet in operation - it uses CSMA/CD.
There are several kinds of 100BaseX:
uses Category 5 UTP cable, using two pairs of conductors - just like 10BaseT. To convert a 10BaseT network to 100BaseTX all that is needed are new NICs and new hubs.
uses category 3, 4 or 5 UTP cable but uses all four pairs of conductors. To convert a 10BaseT network to 100BaseTX all that is needed are new NICs and new hubs.
uses two strands of fiber optic cable.
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