Communication Cost Essay
In some cases, the commercial cost of data transmission may be more important the time cost. Commercial organisations often prefer to transmit data over low capacity lines which they own as opposed to using public, high capacity lines that have usage charges. The routing algorithms do not have to use just one metric to determine the optimum route; rather it is possible to choose the optimum route based on multiple metrics. In order for the optimum path to be chosen by the routers between the data source and the data destination, the routers must communicate information about the relevant metrics with other routers.
This nature of this communication process is also defined by the routing algorithm and the transmission time is linked to the time required for the routers to have the necessary information about the states of the surrounding routers. The time required for all the routers to agree on the state of the network, the network topology, is known as the convergence time and when all routers are aware of the network topology, the network is said to have converged. Some of the common routing algorithm types can indeed affect the convergence of the network.
Some of the different algorithms characteristics that must be chosen when designing are static or dynamic routing, single path or multi-path routing and link state or distance vector routing. Static Routing. Static routing is done by use of a static list of attributes describing the network topology at the initiation of the network. This list, called a routing table, is used by the routers to decide the optimum routes for each type of data transmission and can only be changed manually. Therefore, if anything changes in the network, such as a cable breaking or a router crashing, the viability of the network is likely to be compromised.
The advantage is that there is no communication required between routers, thus the network is always converged. Dynamic Routing. In contrast to static routing, dynamic routing continually updates the routing tables according to changes that might occur in the network topology. This type of real time information processing allows the network to adjust to variations in data traffic and component reliability, but does require communication between the routers and thus there is a convergence time cost associated with this solution.
Single Path vs Multi-path Routing. Single path and muli-path routing are accurate descriptive terms regarding the use of either a single line to send multiple packets of data from a given source to a given destination as opposed to using multiple paths to send all the data packets from the source to the destination. Multiple path algorithms achieve a much higher transmission rate because of a more efficient utilization of available resources. Link State vs Dynamic Routing Protocols.
Link-state algorithms are dynamic routing algorithms which require routers to send routing table information to all the routers in the network, but only that information which describes its own operational state. Distance-vector algorithms, however, require each router to send the whole of its router table, but only to the neighbouring routers. Because the link-state algorithms require small amounts of information to be sent to a large number of routers and the distance vector algorithm requires large amounts of information sent to a small number of routers, the link state algorithm will converge faster.
However, link state algorithms require more system resources (CPU time and memory). There is a new type of algorithm developed by CISCO which is a hybrid of the link-state algorithm and the distance vector algorithm .. This proprietary algorithm converges faster than the typical distance-vector algorithm but provides more information to the routers than the typical link-state algorithm. This is because the routers are allowed to actively query one another to obtain the necessary information missing from the partial tables communicated by the link-state algorithms.
At the same time, this hybrid algorithm avoids communication of any superfluous information exhibited in the router communications of the full tables associated with distance-vector algorithm. Switching. The distance vector, link state or hybrid algorithms all have the same purpose, to insure that all of the routers have an updated table that gives information on all the data transmission paths to a specific destination. Each of these protocols requires that when data is transmitted from a source to a destination, the routers have the ability to ‘switch’ the address on the data transmission.
When a router receives a data packet from a source with the destination address, it examines the address of the destination. If the router has a path to that destination in the routing table, then the router determines the address of the next router the data packet will ‘hop’ to and changes the physical address of packet to that of the next hop, and then transmits the packet. This process of physical address change is called ‘switching’. It will be repeated at each hop until the packet reaches the final destination.
Although the physical address for the forwarding transmission of the data packet changes as the packet moves across the Internet, the final destination address remains associated with the packet and is a constant. The internet is divided up into hierarchical groups that are useful in the description of the switching process. At the bottom of this hierarchy are network devices without the capability to switch and forward packets between sub-networks, where an AS is a sub-network.
These network devices are called end systems (ESs), because if a packet is transmitted there, it cannot be forwarded and has come to the end. At the top of the hierarchy are the network devices that can switch physical addresses are called intermediate systems (ISs). An IS which can only forward packets within a sub-network are referred to as intra-domain ISs while those which communicate either within or between sub-networks are called intra-domain ISs.