Describe the impact of applications (VoIP and Video Over IP) on a network

Exam: 200-120 - Interconnecting Cisco Networking Devices: Accelerated (CCNAX)


The network of your company can be called the transportation system of your data on which many different types of applications run. The different types of applications have different impact on your network in terms of their availability. Today most businesses depend on the high availability of applications. Some different types of applications are email applications, OLTP (online transaction processing programs) applications, and VoIP (Voice over Internet Protocol)/ Video Over IP applications.

The low quality of service for all these types of applications may cause business losses. However, with proper network planning, the high availability of all the different types of applications can be ensured.

Types of Network Applications

The network applications can be broadly divided into three types:

  • Applications that require high bandwidth and high latency (not affected by latency): An example of such applications is File download applications. These applications require high bandwidth to download large chunks of data to manage fast download of files but do not care about latency. The irregularity of responses does not make much difference. The TCP and its Windowing helps in fast file download.
  • Applications that require medium bandwidth and medium latency: An example of such applications is VoIP applications. These applications require moderate bandwidth and moderate latency. However, they require a consistent jitter, which means variations that occur in packet transmissions due to network congestion. The high jitter leads to loss of packets, which results in low quality audio. The compression of audio files reduces the bandwidth requirement. For video over IP applications, the bandwidth is also important for the video part. The audio part has same constraints.
    • Applications that require low bandwidth and low latency: An example of such applications is gaming applications. The gaming applications require constant updates, which can be done by sending a small amount of information on the network. The gaming applications however require low latency rate keep up the performance requirements of the application.

    Impact of VoIP and VideoOver IP Applications on a network

    The VoIP application frames mostly have to travel on an unreliable medium and if some frames need to be dropped due to network traffic congestion, the loss results in low performance. While other types of applications such as email applications recover from the loss through retransmission of lost frames. The retransmission in case of VoIP applications cause additional delay and make the conversation unintelligible.

    The IP networks also impose additional delays because of streaming and network overloading even if packets are not dropped. The RTP (Real Time Protocol) reduce delays by accumulating voice samples for up to 30 ms. In addition, jitter may be caused on the network because the successive packets may follow the different route from a source to a destination.

    Thus the most common problems or unique impairments related to VoIP applications on IP networks are:

    • Jitter that leads to variation in packet transmission due to packet discards that occur due to network congestion.
    • Packet loss that happens due to link failures or Random Early Detection (RED) in routers and route changes.
    • Delay or the lag time that leads to two way conversational difficulty.

    For VoIP applications, we are mostly concerned with the latency of the link. The impact of VoIP applications can be judged on the network by judging the impact of latency on the network. The latency can be caused by the cables, the Path taken by the signal, and the time taken to convert an optical signal to electrical signal and vice versa.

    Both fiber optic and UTP cables allow signals to travel with the speed of light. The light takes 5 ms to travel 1000 miles. The signals on UTP and fiber optic cables are almost as fast as light. The refraction makes them slow than light. They require 7.5 ms to travel 1000 miles. In one second a signal on UTP and fiber optic cables can navigate seven times on the circumference of the globe.

    However the signal may degrade when the latency gets above a few 10s of milliseconds and the delay becomes noticeable. The delay becomes more noticeable when the signals are sent through satellite that causes the delay of 10ths per second to travel to satellite and come back to ground because the satellites are far away from earth.

    The second important element that causes latency on a network is the path taken by the signal. The devices used on the path taken by the signal causes latency. The devices such as hubs, switches, and routers cause different levels of latency.

    The hubs take no extra time to process the signals. They just act like simple electrical connections. The switches cause a little high latency because they need to process signals. However they process signals very fast. Their processing time can be measured in nanoseconds. To process a packet they strip off their headers, insert new header, and then forwards them to the port.

    The routers take a little longer to process signals as compared to switches. The routers use ACLs (access control list) to permit or deny packets to the network based on the source and destination IP addresses. The delays increase when ASIC (Application-specific integrated circuit) forward the entire frame to the CPUs for processing. The CPUs process the frames by running routing process on the frames and create forwarding tables to route the packets.

    The last cause of latency is the time taken to convert an optical signal to an electrical signal and an electrical signal to an optical signal. The time taken to convert the signals is very little but the lag is caused when there are too many hops for this conversion.

    Designing of a network to cater to low latency traffic requirements

    A good network design can take care of the network applications such as VoIP that require low latency. While designing a network, you can take care about keeping short cables in your network design and keeping less number of switches and hubs. The strength of the signal decreases,when a network signal travels through long cables. The use of start topology can help in designing a network that uses short cables to connect devices. The number of hubs and switches should be kept less on a network to keep the processing time low for signals and thus maintaining low latency level.

    Example Question

    Which of the following is required by the VoIP application on a network?

    • High bandwidth and high latency
    • Medium bandwidth and medium latency
    • Low bandwidth and low latency

    Answer: B

    The VoIP applications do not need a high bandwidth but they need medium or low latency to keep the conversation going. The loss of packets cause performance degradation in VoIP applications and make the conversation unintelligible.


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