October 03, 2005
Voice quality and QoS in a VoIP network - part 2
IP traffic is classified in the following ways:
• Diffserv/TOS bits: These are used for prioritizing the traffic. RFC 791 describes the second byte in the header of an IP frame as the Type of Service (TOS) byte. This byte is used for assigning priority to the packet. The same byte is called the Differentiated Services (DS) field by RFC 2474. The second byte is used to signal the edge devices and to mark the high-priority traffic for the router to recognize it.
• RSVP signaling: Is used for checking and reserving resources such that the QoS requirements regarding jitter and delay are met. The intermediate routers receive instructions from the IP control flows that the RSVP introduces from end to end. RSVP works well in a private WAN where the applications use it to contact their TCP/IP stacks.
• Port numbers and addresses: These lead to a better handling of the applications marked by the destination port numbers. It is a simple IP QoS technique used by several edge devices to check the port numbers and addresses.
• RTP header information: Useful for managing audio data packets. The RTP header helps the receiver to gauge the timing of the original data and to manage out-of-sequence data packets. This protocol is useful in prioritizing streams of audio and video.
• Data content: Useful for quick transmission of binary data. It can analyze URLs to classify web traffic in a better manner.
• Data rate: Used for better management of low-volume traffic by applying the Weighted Fair Queuing technique. The dispatch of data can also be controlled by the applications either on every API Send call or at each connection.
• Buffer Size: For prioritizing the frames based on their size. It can be used to prioritize small frames over large frames; it works on the assumption that small frames are more time-critical than the larger frames. The size of the buffer can be controlled for each API send and receive call. For TCP and UDP, the default buffer size is 32K bytes and 8K bytes, respectively.
Devices such as traffic shapers and bandwidth managers classify the network at its edges and provide a central point of administration. The traffic can also be classified in the middle of the network by devices such as routers but such connections are unable to provide much information about the traffic.
The different classes of traffic are managed on the basis of the flow rate, paths, and queuing. These help in taking decisions regarding the reservation of bandwidth and the fixing of latency. RSVP, WFQ, LFI, LLQ, RED, and WRED are examples of queue-based traffic management techniques. The traffic shapers and bandwidth managers at the edge of a network manage the traffic on the basis of the flow rate. They can be used to limit the throughput of traffic on a given route. MPLS is an example of path-based traffic management, in which the traffic is identified at the network’s edge and allotted either a preferred path or a “best effort” path. The following things should be taken care of in order to maintain the desired QoS in a network:
• Look up for relevant information on QoS as it is a new and fast evolving technology.
• Classifying traffic should be need based and not driven by any other rationale.
• Given the numerous QoS schemes that exist, IT employees need to be aware of the parameters and tuning requirements associated with each.
• QoS should only be measured against the backdrop of a heavy load to confirm its configuration and if the desired classes are getting better handling.
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