Multiplexing

Multiplexing:-

When two communicating nodes are connected through a media, it generally happens that bandwidth of media is several times greater than that of the communicating nodes. Transfer of a single signal at a time is both slow and expensive. The whole capacity of the link is not being utilized in this case. This link can be further exploited by sending several signals combined into one. This combining of signals into one is called multiplexing.

Frequency Division Multiplexing (FDM): This is possible in the case where transmission media has a bandwidth than the required bandwidth of signals to be transmitted. A number of signals can be transmitted at the same time. Each source is allotted a frequency range in which it can transfer it's signals, and a suitable frequency gap is given between two adjacent signals to avoid overlapping. This is type of multiplexing is commonly seen in the cable TV networks.

1. Time Division Multiplexing (TDM): This is possible when data transmission rate of the media is much higher than that of the data rate of the source. Multiple signals can be transmitted if each signal is allowed to be transmitted for a definite amount of time. These time slots are so small that all transmissions appear to be in parallel.

Synchronous TDM: Time slots are preassigned and are fixed. Each source is given it's time slot at every turn due to it. This turn may be once per cycle, or several turns per cycle ,if it has a high data transfer rate, or may be once in a no. of cycles if it is slow. This slot is given even if the source is not ready with data. So this slot is transmitted empty.

Asynchronous TDM: In this method, slots are not fixed. They are allotted dynamically depending on speed of sources, and whether they are ready for transmission.

Network Topologies:

A network topology is the basic design of a computer network. It is very much like a map of a road. It details how key network components such as nodes and links are interconnected. A network's topology is comparable to the blueprints of a new home in which components such as the electrical system, heating and air conditioning system, and plumbing are integrated into the overall design. Taken from the Greek work "Topos" meaning "Place," Topology, in relation to networking, describes the configuration of the network; including the location of the workstations and wiring connections. Basically it provides a definition of the components of a Local Area Network (LAN). A topology, which is a pattern of interconnections among nodes, influences a network's cost and performance. There are three primary types of network topologies which refer to the physical and logical layout of the Network cabling. They are:

Star Topology: All devices connected with a Star setup communicate through a central Hub by cable segments. Signals are transmitted and received through the Hub. It is the simplest and the oldest and all the telephone switches are based on this. In a star topology, each network device has a home run of cabling back to a network hub, giving each device a separate connection to the network. So, there can be multiple connections in parallel.   

Advantages

• Network administration and error detection is easier because problem is isolated to central node
• Networks runs even if one host fails
• Expansion becomes easier and scalability of the network increases
• More suited for larger networks

Disadvantages:

• Broadcasting and multicasting is not easy because some extra functionality needs to be provided to the central hub
• If the central node fails, the whole network goes down; thus making the switch some kind of a bottleneck
• Installation costs are high because each node needs to be connected to the central switch

Bus Topology: The simplest and one of the most common of all topologies, Bus consists of a single cable, called a Backbone, that connects all workstations on the network using a single line. All transmissions must pass through each of the connected devices to complete the desired request. Each workstation has its own individual signal that identifies it and allows for the requested data to be returned to the correct originator. In the Bus Network, messages are sent in both directions from a single point and are read by the node (computer or peripheral on the network) identified by the code with the message. Most Local Area Networks (LANs) are Bus Networks because the network will continue to function even if one computer is down. This topology works equally well for either peer to peer or client server.

The purpose of the terminators at either end of the network is to stop the signal being reflected back.

Advantages:

• Broadcasting and multicasting is much simpler
• Network is redundant in the sense that failure of one node doesn't effect the network. The other part may still function properly
• Least expensive since less amount of cabling is required and no network switches are required
• Good for smaller networks not requiring higher speeds

Disadvantages:

• Trouble shooting and error detection becomes a problem because, logically, all nodes are equal
• Less secure because sniffing is easier
• Limited in size and speed

Ring Topology: All the nodes in a Ring Network are connected in a closed circle of cable. Messages that are transmitted travel around the ring until they reach the computer that they are addressed to, the signal being refreshed by each node. In a ring topology, the network signal is passed through each network card of each device and passed on to the next device. Each device processes and retransmits the signal, so it is capable of supporting many devices in a somewhat slow but very orderly fashion. There is a very nice feature that everybody gets a chance to send a packet and it is guaranteed that every node gets to send a packet in a finite amount of time.     

 Advantages:

• Broadcasting and multicasting is simple since you just need to send out one message
• Less expensive since less cable footage is required
• It is guaranteed that each host will be able to transmit within a finite time interval
• Very orderly network where every device has access to the token and the opportunity to transmit
• Performs better than a star network under heavy network load

Disadvantages:

• Failure of one node brings the whole network down
• Error detection and network administration becomes difficult
• Moves, adds and changes of devices can effect the network
• It is slower than star topology under normal load

Generally, a BUS architecture is preferred over the other topologies - ofcourse, this is a very subjective opinion and the final design depends on the requirements of the network more than anything else. Lately, most networks are shifting towards the STAR topology. Ideally we would like to design networks, which physically resemble the STAR topology, but behave like BUS or RING topology.