The following are factors that affect LAN response time;
- Bandwidth of transmission line
A network has a finite amount of bandwidth, and if there are too many devices or applications connected to the network, they compete for the available bandwidth, resulting in slow data speeds and overall poor performance. Many routers offer dual-band or tri-band technology, which frees up bandwidth by offering multiple channels for spreading the workload. The more people in an office utilizing a LAN network at the same time, the more bandwidth is needed.
- Queuing at notes or hosts
Network congestion in data networking and queuing theory is the reduced quality of service that occurs when a network node is carrying more data than it can handle. Typical effects include queuing delay, packet loss or the blocking of new connections. A consequence of congestion is that an incremental increase in offered load leads either only to a small increase or even a decrease in network throughput.
Network protocols that use aggressive retransmissions to compensate for packet loss due to congestion can increase congestion, even after the initial load has been reduced to a level that would not normally have induced network congestion. Such networks exhibit two stable states under the same level of load. The stable state with low throughput is known as congestive collapse.
All the links on a network are joined together by routers. These forward packets arriving on incoming links to the appropriate outgoing links, to ensure that the packet is routed to its intended destination. Figure 1 shows the basic architecture of a router.
The router is connected to incoming links and outgoing links. and may be different, although usually equals . In most situations, input and output links are paired to form either a full-duplex channel where data can flow in both directions simultaneously, or a half-duplex channel where data can flow in only one direction at a time.
Incoming packets are buffered in the input buffers. Once in the buffer, they are selected by the Packet Selection Function to be passed to the Routing Function. The Routing Function determines on to which outgoing link a packet must be placed for it to get closer to its destination: this is done with a routing table which, as described earlier, is semi-static. When the correct link for the packet is found, the Routing Function passes the packet to the Packet Dropping Function for queuing on the output buffer for the link. When the packet reaches the other end of the queue, it is transmitted over the link to the next router, or to the final destination.
The Packet Selection Function can choose any of the packets queued in the input buffers for routing by the Routing Function. Normally this would be done in a First-In First-Out method, but other selection methods may be useful in a congested environment.
There are two fundamental bottlenecks in the given router architecture. Firstly, there is a minimum time needed for the router to decode the network header of the
Packet, determine the route for the packet, and pass the packet to an outgoing link for transmission. There is also a delay for the packet to be transmitted on the outgoing link: this may just be the packet’s transmission time for a full-duplex link, or there may be an extra delay for the link to become clear on a half-duplex link. These delays form one bottleneck.
The second bottleneck indicates that the router must be prepared to buffer output packets, to prevent them from being lost while the router waits for an outgoing link to become clear. The two bottlenecks together indicate that the router must buffer incoming packets, to prevent them from being lost if they arrive too quickly for the router to process.
By definition, the router’s input and output buffers are finite. If a buffer becomes full, then no more packets can be queued in the buffer, and the router has no choice but to discard them. This causes data loss in the data flow between a source and destination, and usually causes the source to retransmit the data.
Although a router cannot queue a packet if the corresponding output buffer is full, it can choose either to discard the unqueued packet, or to discard a packet already in the output queue, and then queue the unqueued packet. This choice is performed by the Packet Dropping Function. This cannot be done for the input buffers, as the packet has not been placed in the router’s internal memory until it has been queued in the input buffers. Thus, packets may be lost on input, and the router has no control over which packets are lost.
Finally, the router’s buffers may share a common memory space, or they may have individual memory spaces. With the former, no buffer can become full until the router’s entire memory space is allocated to buffers, at which point all buffers become full. With the latter, any buffer’s utilisation has no influence on any other buffer’s utilisation.
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