NETWORKS

Network Topology's
A network topology describes the configuration of a network (how the network components are connected together). There are FOUR main topology's.

1. Star
   The star topology uses a central hub through which all components are connected. In a computer network, the central hub is the host computer, and at the end of each connection is a terminal.

   
    Star Topology

   A star network uses a significant amount of cable (each terminal is wired back to the central hub, even if two terminals are side by side several hundred meters away from the host). All routing decisions are made by the central hub, and all other workstations can be simple.

   An advantage of the star topology is failure in one of the terminals does not affect any other terminal, how-ever, failure of the central hub affects all terminals.

   This type of topology is frequently used to connect terminals to a large time-sharing host computer.

2. Ring
   The ring topology connects workstations in a closed loop. Each terminal is connected to TWO other terminals (the next and the previous), with the last terminal being connected to the first. Data is transmitted around the ring in one direction only, each station passing on the data to the next station till it reaches its destination.

   
    Ring Topology

   Faulty workstations can be isolated from the ring. When the workstation is powered on, it connects itself into the ring. When power is off, it disconnects itself from the ring and allows the information to bypass the workstation.

   Information travels around the ring from one workstation to the next. Each packet of data sent on the ring is prefixed by the address of the station to which it is being sent to. When a packet of data arrives, the workstation checks to see if the packet address is the same as its own. If it is, it grabs the data in the packet. If the packet does not belong to it, it sends the packet to the next workstation in the ring.

   Ring systems use 4 pair cables (separate send/receive). The common implementation of this topology is token ring. A break in the ring causes the entire network to fail. Individual workstations can be isolated from the ring.

3. Bus
   The bus topology connects workstations using a single cable. Each workstation is connected to the next workstation in a point to point fashion. All workstations connect to the same cable.

   
   Fig 8.3: Bus Topology

   If one workstation goes faulty, all workstations are affected. Workstations share the same cable for the sending and receiving of information. The cabling costs of bus systems is the least of all the different topology's. Each end of the cable is terminated using a special terminator.

   The common implementation of this topology is Ethernet. A message transmitted by one workstation is heard by all the other workstations.

4. Mesh
   The mesh topology connects all computers to each other. The cable requirements are high, but there are redundant paths built in. Any failure of one computer allows all others to continue, as they have alternative paths to other computers.

   
   Fig 8.4: Mesh Topology

   Mesh topology's are used in critical connection of host computers (typically telephone exchanges). Alternate paths allow each computer to balance the load to other computer systems in the network by using more than one of the connection paths available.

Network Protocols
This section describes the protocols used in different network topology's. Remember that a protocol defines the rules for sending data from one point to another.

• Carrier Sense Multiple Access with Collision Detection (CSMA/CD)
  This protocol is commonly used in bus (Ethernet) implementations. Multiple access refers to the fact that in bus systems, each station has access to the common cable. Carrier sense refers to the fact that each station listens to see if no other station is transmitting before sending data. Collision detection refers to the principle of listening to see if other stations are transmitting whilst we are transmitting.

  In bus systems, all stations have access to the same cable medium. It is therefore possible that a station may already be transmitting when another station wants to transmit. Rule 1 is that a station must listen to determine if another station is transmitting before initiating a transmission. If the network is busy, then the station must back off and wait a random interval before trying again.

  Rule 2 is that a station which is transmitting must monitor the network to see if another station has begun transmission. This is a collision, and if this occurs, both stations must back off and retry after a random time interval. As it takes a finite time for signals to travel down the cable, it is possible for more than one station to think that the network is free and both grab it at the same time.

  CSMA/CD models what happens in the real world. People involved in group conversation tend to obey much the same behavior.

• Token Ring
  This protocol is widely used in ring networks for controlling station access to the ring. A short message (called a token ) is circulated around the ring, being passed from station to station (it originates from a controller or master station which inserts it onto the ring).

  A station which wants to transmit waits for the token to arrive. When it arrives, it changes it from a token to a connector message, and appends its message on the end. This is then placed on the outgoing side of the ring.

  Each station passes on received tokens if they have nothing to transmit. They monitor connector messages to see if the message is addressed to them. If connector messages are addressed to them, they copy the message, modify it to signify its receipt, then send it on around the ring. Connector messages which are not addressed to them are passed directly onto the next station in the ring.

  When the connector message travels full circle and arrives at the original sending station, it checks the message to see if its been received. It then discards the message and replaces it with a token.

• Integrated Services Data Network (ISDN)
  ISDN refers to a new form of network which integrates voice, data and image in a digitized form.

  It offers the customer a single interface to support integrated voice and data traffic. Rather than using separate lines for voice (PABX system) and data (leased lines), ISDN uses a single digital line to accommodate these requirements.

• Local Area Network
  A LAN is a group of microcomputers or other workstation devices located in the same general area and connected by a common cable. The characteristics of LAN's are
  • physically limited < 2Km
  • high bandwidth > 1Mhz
  • inexpensive cable media
  • data and hardware sharing amongst users
  • owned by the user

  A LAN is used to share resources amongst a group of individuals or company employees. These resources are typically

  • files
  • application software
  • printers
  • computing power
  • modems
  • fax machines
• Wide Area Network
  A Wide area network is similar to a LAN, but geographically spread over a wider area. The different segments of the WAN are interconnected by communication links (high speed serial, X.25 etc). In fact, a WAN often comprises a number of interconnected LAN's at various sites.

  The characteristics of a WAN are

  • geographically expansive
  • large number of computers and multiple host machines
  • sophisticated support devices like routers and gateways for interconnecting the various segments

LAN Connections
A LAN connects resources together so they can be shared by users. In today's terms, this often means linking PC's together. As discussed earlier, the PC's can be linked together using a bus, ring or star topology.

Each PC is equipped with a network interface card, which fits into an available expansion slot. Appropriate driver software provides an interface between the PC operating system and the network interface card.

Once the network software is loaded, access to provided to other machines on the network. There are THREE main types of network access provided.

• Peer to Peer Services
  Each PC in the network can directly communicate and share resources with any other machine in the network. Examples of this are
  • Lantastic
  • Lan Manager
  • Windows For WorkGroups
  • Windows NT
  • OS/2
  • UNIX

    Software programs are executed either locally, or on the peer computer. A user can take advantage of a faster computer by specifying that the program run on the faster machine, rather than their own.

• File Server Services
  A number of machines are designated as HOST servers. Users login to the host machines and access files and application software stored on them. Users have no access to other non host machines. Examples of this are
  • Novell NetWare
  • (Also LAN Manager, Windows NT, and LANtastic)
    
    Software is executed on the users machine. The file server only provides file access; the file is downloaded from the file server into the workstation, where it is then executed.
• Client/Server Services
  This is used extensively in database applications, where a front end software program sends commands to a server for execution, with the results being displayed on the users machine. It utilizes the high speed of the server machine to do number crunching and complex operations.
  

The major problems associated with using Networks are

• user training
• support (user and system)
• configuration

Comparison of LAN and MAN Characteristics