Chapter 1 - Local Area Network (Part2)
4. Topology of LANs
We have two kind of topologies :
- Physical topology.
- Logical topology.
The first one define the physical connections of hosts in a computer network
Local Area Networks (LANs) can be connected in various topologies (bus, ring, star, mesh…). The selection of a topology for a network depends on its specific needs and requirements since each topology has its own set of advantages and disadvantages. Here are the most common topologies (Figure1.3):
Figure1.3- Different topologies of LANs.
Ø Bus topology : The bus topology connects all devices to a single cable known as the bus. Data is transmitted in both directions along the cable, and each device receives all the data transmitted on the bus. The bus topology is simple, easy to install, susceptible to data collisions, and difficult to troubleshoot.
Ø Ring topology : Data is transmitted in a single direction along the ring in the topology where devices are connected. Each device receives data from the device before it and passes data to the device after it. This topology is reliable and efficient, but it can be expensive to install, and a single device failure can disrupt the entire network.
Ø Star topology : In this topology, a central hub or switch connects all devices. Data is transmitted from the device to the hub, and then the hub broadcasts the data to all other devices on the network. The star topology is easy to install and manage, and a single device failure does not disrupt the entire network. However, installing it can be expensive, and the hub can become a single point of failure.
Ø Mesh topology : in this topology devices are interconnected in a decentralized manner. Instead of relying on a central hub or switch, each device connects directly to multiple other devices, forming a mesh-like structure.
The second one. Which is logical topology, a describes how the data is actually sent from one computer to the next.
We have two logical topologies.
Ø Broadcast topology : allows a host to send data to all other hosts available in that network.
Ø Token-passing topology : shares data with various hosts through access to an electronic token.
In the following, we have three LAN technologies. The first one, which is Ethernet, is the most widely used Technologie. The reason is that Ethernet is easy to understand, implement, maintain and allows low cost implementation.
|
|
Ethernet |
Token Ring |
FDDI |
|
Standard |
IEEE 802.3 |
IEEE 802.5 |
IS9314 |
|
Speed |
Basic Ethernet speed up to 10Mbps. Fastest versions exist (Fast Ethernet 100Mbps, Gigabit Ethernet, 10 Gigabit Ethernet) |
Become less speed if we add nodes |
Speed is very high 200Mbps |
|
Cable |
Basic Ethernet-> coaxial cable Fast Ethernet and Gigabit Ethernet-> twisted pair cable or Fiber-optic.
|
Type 1 and Type 6 STP(Shielded twisted pair) as well as Type 3 UTP(Unshielded twisted pair) |
Optical fiber |
|
Maximum coverage |
2.5 km |
Configuration dependent |
200 km |
|
Maximum distance between node |
2.5km |
300m (Recommended 100m) |
2km (multimode fiber) 40km (single mode fiber) |
|
Logical Topology |
Broadcast topology With CSMA/CD mechanism to handle collision |
Token passing topology |
Token passing topology |
|
Physical topology |
Any physical topology can be used. Ethernet is flexible but generally Bus topology is used. |
Physical ring, bus ring, star ring topology is used |
Ring, star, tree. |
|
Determinism |
Not deterministic |
Deterministic |
deterministic |
|
Collision |
Great number of collision if trafic is important. In fact, the performance of the ethernet network degrades as network traffic increase. |
Data collision does not occur. Insensitive to traffic intensity. Support heavy network traffic and maintains the network performance. |
Data collision does not occur |
|
Bandwidth efficiency |
Efficiency up to 40% |
Efficiency up to 90% |
|
|
collapse |
In physical Bus topology, if the bus break down, the entire network collapse |
If links between nodes does not work well or if a node break down.
|
|
|
Transmission |
Device transmit as soon as the medium is free. |
Every station that has the token is allowed to transmit data. Devices transmit at specific time.
|
Every station that has the token is allowed to transmit data. Devices transmit at specific time.
|
|
Alternative link |
No one |
No one |
Alternative link to transmit data |
|
Direction of transmission |
No special direction |
Data transmission is through single direction. Clock sense. |
Duals rings (primary and secondary) transmit in opposite directions |
|
Cost |
Less expensive. Cable and network equipments are not very expensive |
The network setup and maintenance is more expensive than Ethernet. |
The network setup and maintenance is more expensive than Ethernet. |
|
Errors |
Able to correct after a collision. |
Able to detect and correct errors |
Able to detect and correct errors. Second ring is used for automatic repair. |
|
Network adaptator |
One Network adapter is needed per computer |
Two Network adapters is needed per computer |
Two Network adapters is needed per computer |
Reference
Andrew S. Tanenbaum, and David J. Wetherall, Computer Networks, 5th Edition, China Machine Press, 2011.