Home networks are now transitioning from data-only to include streaming media and there is a big difference between the two--the latter requiring unique quality of service (QoS) considerations on our local networks, wireless or otherwise.

Data networking is not time constrained. The important thing about data networking is that the data arrive fully intact, irrespective of time. E-mail, documents, pictures and all sorts of downloads are examples of data types that are transferred based on accuracy and not time. In the process of transferring data, packets (chunks of the data) are sent and resent as needed until the transfer is confirmed to have been successful. How long this takes will depend on channel conditions of speed, noise and channel load.

Media networking is different. Time is the most important aspect of the delivery. Delivery delay, called latency, and variations in the delivery delay, called jitter, are important. Voice delays contributed by a local network must be kept in the less-than 10 ms region because it is a ‘real-time’ activity in which alternate talking and listening is taking place and because additional delays are added to this beyond the local network.

Streaming audio and video is not perceived to be a real-time activity so delays on the local network can be in the less-than 100 ms range. Memory buffering is used to collect packets and frames of streaming media to swamp out the latency and jitter effects and create a precise lockstep final delivery. The delay caused by buffering is not perceived by the recipient because there is no reference of comparison available. However, the buffer is flushed out at regular intervals and latency must be less than the refresh interval or video and audio will be disrupted.

Streaming media requires sufficient available bandwidth, a very low bit error rate, low latency (with the help of buffering) and priority access to ensure quality of service (QoS). These can be boiled down to just two main elements: channel quality and priority access. Channel quality includes bandwidth, low bit error rate and low latency. Priority access stands alone as perhaps one of the most important QoS requirements.

Priority access implies that a system of traffic prioritization is in place, being used by the network manager, which is a function of the router or some other device. Traffic of all types, including data, voice and streaming media, is classified or prioritized. Local ethernet networks use an 8-level system of prioritization. The eight levels are expressed as a 3-bit binary number that is included in the header of each frame (b000 to b111 = 0 to 7). Seven (7 = b111) is the highest traffic priority and, interestingly, 1 and 2 are treated as the lowest levels, not 0. The following is a list describing the levels of priority, also called ‘class of service (CoS)’:

7 – highest level reserved for network control and configuration data
6 – voice communications
5 – streaming video and audio
4 – controlled load generated by business applications
3 – ‘excellent effort’ data delivery
0 – ‘best effort’ generic LAN data traffic
1 & 2 – background bulk data transfers

Network control, voice and streaming media must have access priority over other data types to help ensure successful transfer without competition or unacceptable delays. The local network channel belongs to traffic marked as 5, 6 and 7. In this way, the available bandwidth is allocated to the higher-level traffic on demand. Lower-priority traffic must share whatever bandwidth remains.

When the network is near full capacity, same-level traffic must be admitted, or allowed to remain, based on ‘established traffic policies’ or a means of contention resolution. Levels 4 and below usually resolve channel-use contention with a numerical ‘roll of dice,’ the highest number wins. Channel access disputes for same-priority-level traffic at level 5 is further resolved by applying some policy based on the traffic’s source address or some other parameter. Level 6 operates on a first-come-first-served policy and there is no dispute at level 7.

Soon, our home networks will carry streaming media from service providers, PVRs and media-enabled PCs along with voice and the usual e-mail, file transfer and download Internet traffic. IEEE 802.11n, ultrawideband wireless and HomePlug AV powerline communications technologies are making that happen. Both wireless and wired networks employ some kind of CoS or type of service (ToS) access prioritization translated from the above Ethernet CoS levels. Keep in mind that our internal network pipes have a limited capacity, most of which will be commandeered by voice and streaming services. So, if your download or file transfer takes ‘forever’ while you are streaming media, you will understand why. Nevertheless, you will continue to enjoy your higher-priority media without interruption.

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