Introduction to Networking

Network Classifications
Scope
- Local area network (LAN)
- Metropolitan area (MAN)
- Wide area network (WAN)

Ownership
 - Closed
- Open

Topology (configuration)
- Peer to peer
- Bus (Ethernet)
- Star (Wireless networks with central Access Point)
- Ring
- Mesh
- Hybrid

Network Topologies
A topology refers to the manner in which the cable is run to individual workstations on the network. the configurations formed by the connections between devices on a local area network (LAN) or between two
or more LANs

There are three basic network topologies (not counting variations thereon): p to p, the bus, the star, the ring and the mesh. It is important to make a distinction between a topology and an architecture.

A topology is concerned with the physical arrangement of the network components. In contrast, an architecture addresses the components themselves and how a system is structured (cable access methods, lower level protocols, topology, etc.). An example of an architecture is 10baseT Ethernet which typically uses the star topology.

Bus Topology
- A bus topology connects each computer (node) to a single segment trunk. A ‘trunk’ is a communication line, typically coax cable, that is referred to as the ‘bus.’  The signal travels from one end of the bus to the other. A terminator is required at each end to absorb the signal so it does not reflect back across the bus.

- In a bus topology, signals are broadcast to all stations. Each computer checks the address on the signal (data frame) as it passes along the bus. If the signal’s address matches that of the computer, the computer processes the signal. If the address doesn’t match, the computer takes no action and the signal travels on down the bus.

- The bus topology is passive. In other words, the computers on the bus simply ‘listen’ for a signal; they are not responsible for moving the signal along.  

Advantages of bus topology:
- Easy to implement and extend
- Well suited for temporary networks that must be set up in a hurry
- Typically the cheapest topology to implement
- Failure of one station does not affect others  

Disadvantages of bus topology:
- Difficult to administer/troubleshoot
- Limited cable length and number of stations
- A cable break can disable the entire network; no redundancy
- Maintenance costs may be higher in the long run - Performance degrades as additional computers are added

Star Topology
- All of the stations in a star topology are connected to a central unit called a hub.The hub offers a common connection for all stations on the network. Each station has its own direct cable connection to the hub. In most cases, this means more cable is required than for a bus topology. However, this makes adding or moving computers a relatively easy task; simply plug them into a cable outlet on the wall.

- If a cable is cut, it only affects the computer that was attached to it. This eliminates the single point of failure problem associated with the bus topology. (Unless, of course, the hub itself goes down.)

Advantages of star topology: 
   - Easy to add new stations
 - Easy to monitor and troubleshoot
- Can accommodate different wiring  

Disadvantages of star topology:
- Failure of hub cripples attached stations
- More cable required (more expensive to wire a building for networking)

Ring Topology
- A ring topology consists of a set of stations connected serially by cable. In other words, it’s a circle or ring of computers. There are no terminated ends to the cable; the signal travels around the circle in a clockwise (or anticlockwise) direction.
- Under the ring concept, a signal is transferred sequentially via a "token" from one station to the next. When a station wants to transmit, it "grabs" the token, attaches data and an address to it, and then sends it around the ring. The token travels along the ring until it reaches the destination address. The receiving computer acknowledges receipt with a return message to the sender. The sender then releases the token for use by another computer.
- Each station on the ring has equal access but only one station can talk at a time.
- In contrast to the ‘passive’ topology of the bus, the ring employs an ‘active’ topology. Each station repeats or ’boosts’ the signal before passing it on to the next station.  

Advantages of ring topology: 
  - Growth of system has minimal impact on performance
- All stations have equal access

Disadvantages of ring topology: 
  - Most expensive topology
- Failure of one computer may impact others
- Complex

Choosing a Topology
The following factors should be considered when choosing a topology:
-Installation
-Maintenance
- troubleshooting
-Expected growth
-Distances
-Infrastructure
-Existing network
-Security
-Speed

Cabling 
Coaxial Cable Thinnet looks like regular TV cable. It is about 1/4 inch in diameter and is very flexible and easy to work with. Thicknet is about 1/2 inch in diameter and not very flexible. Thicknet is older and not very common anymore except as a backbone within and between buildings. Coax transmits at 10 Mbps..   

Twisted Pair. Twisted pair looks like telephone wire and consists of insulated strands of copper wire twisted together. There are two versions of twisted pair cable: Shielded Twisted Pair (STP). STP is commonly used in Token Ring networks Unshielded Twisted Pair (UTP). UTP is used in Ethernet networks. Transmission rates vary between 10-100-1000-10000 Mbps.

Fiber-Optic Cable. Fiber-optic cable consists of a thin cylinder of glass surrounded by glass cladding, encased in protective outer sheath.   Fiber-optic cable is very fast (over 1Gbps).  It can transmit over long distances (2 km +) but is expensive.

Connecting Networks  
Repeater: Extends a network
Bridge: Connects two compatible networks, doesn’t necessarily pass all the messages across the connection
Switch: Connect several compatible networks, allowing it to connect several busses rather than two.