Technology Performance Basis

Having taken a review of the various access technologies which can be implemented for rural connectivity. It is also imperative that there be basis upon which the technology can be analyzed according to their performance.

Parameters such as round trip time, download time/latency and throughput can be used to measure the performance of a technology.

• Round Trip Time (RTT): Round Trip Time is a measure of the time it takes for a packet to travel from a computer, across a network to another computer, and back. RTT is computed by the sending side recording the clock when it transmits a packet, and then records the clock again when an acknowledgment or a reply arrives. Subtracting the two values, yields a single estimate of the round trip time. A collection of these values for a set period of time give the average RTT. RTT shows the IP network total path delay that can be caused by many factors such as; packet capturing delay, queuing delay, switching/routing delay and light-speed propagation delay.

Packet capture delay is the time required to receive the entire packet before processing and forwarding it through the router. This delay is determined by the packet length and transmission speed. Using short packets over high-speed trunks can easily shorten the delay but potentially decreases network efficiency.

Latency is normally composed of queue delay, path delay and transmission delay. Path delay is the delay in transmitting frames between end nodes. Queue delay is delay required at the input and output ports of a packet switch due to the statistical multiplexing nature of IP networks and to the asynchronous nature of packet arrivals. This delay is a function of the traffic load on a packet switch, the length of the packets, and the statistical distribution over the ports. Designing very large router and link capacities can reduce but not completely eliminate this delay. Switching/routing delay is the time the router takes to switch the packet. This time is needed to analyze the packet header, check the routing table, and route the packet to the output port. This delay depends on the architecture of the route engine and the size of the routing table. Hence

additional headers increase this type of delay. New IP switches can significantly speed up the routing process by making routing decisions and forwarding the traffic via hardware as opposed to software processing (International Engineering Consortium, 2005).

Propagation delay is the time required for a digital signal to travel from one point to another, usually from source node to the destination node in a computer network. It largely depends on the underlying network topology. Increased RTT induced by additional overhead due to more processing time and larger frame sizes affects the QoS of the network, especially in real-time applications such as video conferencing and voice over IP (VoIP). Transmission delay is the time it takes for the TCP Layer to transmit the segment onto the communications link. This is determined purely by the TCP segment's size and the bandwidth of the link.

=

Where t = return trip time of ݅

n = be the number of trips recorded Adopted from Mujinga, 2005

• Download Time: Download Time is the time it takes to download a file or a webpage from the remote server or computer to the local computer. This can be calculated by subtracting the time the first frame of the downloaded file is received from the server and the time that last frame is received.

Where the request for a web page is sent from the client computer to the web server

the time the last fragment is received on the client from the web server

n the number of times the process is performed

Adopted from Mujinga, 2005

• Throughput: Throughput is the amount of digital data per time unit that is delivered over a physical or logical link, or that is passing through a certain network node. This can be taken as the amount of data that is delivered to a certain network terminal or host computer, or between two specific computers. Throughput is measured in bits per second (bits/s or bps), occasionally in data packets per second or data packets per timeslot.

Throughput = (3.3)

• Ping: Ping is an application that tests host responses over a network connection. Ping uses the network layer to send packets to a remote address. If there are network connectivity problems or the host has problems, the ping will fail, indicating a problem exists. Additional tests may be needed at that point to determine the cause of the problem.

The Ping is an application on OSI layer 7 designed to use echo request and echo reply to calculate and display round trip times for each request/reply pair to a host over a network. Ping utilizes raw ICMP sockets to send packets to a remote host, calculates the difference in time between transmission and receipt of the packet and 10 8 produces

printed, human-readable results of these measurements in milliseconds for each request/reply pair. Any delay anywhere in the chain will create what is referred to as propagation delay or more simply latency.

3.4 Conclusions

Table 3.2 provides a summary of access technologies with their key features for rural applications. In this chapter I discussed the various access technologies currently available and others planned for future Internet connectivity in South Africa. Some of the technologies are able to deliver the mission of rural Internet connectivity. With Neotel set to be the second network operator there is hope coverage will soon improve in remote disadvantage areas of South Africa. It is important that we test these technologies for rural deployment, especially in terms of their applicability in rural areas taking into consideration the performance parameters.