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Audio/Visual Systems and Networks:
Riding the IP Wave
by Brian Patrick
IT networks, in the simplest sense, transport data between devices. So can IT networks be used effectively for an audio-visual system? The answer to the question lies in the main functions and methodologies of a network: connectivity and transport of information.
Transportation Methods
Network engineers are familiar with the two most common protocols used: Transport Control Protocol/Internet Protocol (TCP/IP), and User Datagram Protocol (UDP). These protocols allow information to be cut up into small, manageable chunks, called packets. The more nodes (PC’s, AV gear, etc.) on the network sending information, the higher the risk that these packets will “collide,” interfering with each other’s communication and causing the packets to arrive at the destination in a different order or timing sequence than they were sent. For interconnecting electronic devices for the transmission of data, TCP/IP is the recognized world standard. However, for quality AV transmission, different transport protocols are needed.
For large-scale audio systems, a common transport method is Cobranet™ – a multi-channel network audio transport protocol compatible with the Ethernet. Cobranet™-enabled audio processors allow traditional microphone and source audio inputs to travel across an Ethernet local area network (LAN). This is typically implemented in large spaces, such as auditoriums or churches, where many channels of audio need to reach multiple destinations.
For video, common IP transport methods employ compression schemes like MPEG-2, or videoconferencing standards such as H.323. Video is especially bandwidth-intensive and can rob a network of performance and speed if not implemented properly. It is wise to create network segments or virtual private networks (VPN) when stacking audio and video traffic on a traditional local area network (LAN). Network switches with high-speed data busses, or backplanes, allow multiple bandwidth-intensive users or pieces of AV gear to coexist on the same LAN segment. The cost of the switch is often determined by the backplane speed – the faster the system, the more expensive it becomes.
Are you Being Served?
The single most significant feature in an AV network is Quality of Service (QoS). QoS can define your throughput speeds and minimum performance specifications. It gives the network engineer the ability to prioritize different types of network traffic. In newer technologies like Voice-Over-IP (VoIP), this ability is crucial. For example, if a network has several voice calls routing through it at the same time a mass email campaign is underway, the users on the phone may see the quality of their phone calls reduced. QoS will solve the issue by assigning voice traffic top priority.
In effect, QoS acts as a traffic cop, allowing the VIPs (very important packets) to pass at the front of the line.
For videoconferencing, “lookup tables” or “access control lists” and gatekeepers are used to prioritize multiple types of network traffic sent from point to point. Video packets need high priority to minimize delay in the transmission. Audio packets are very important too, particularly in a videoconference, where lip-syncing delays of more than half a second strain the human interaction level. Many organizations find themselves forced to upgrade their network gear because of new demands for audio and video. The telecommunications industry has used QoS several years, but it is only recently being well utilized for LANs in corporate settings.
Once network traffic leaves a LAN, QoS and transport control of content may be lost. This can make your local network QoS efforts worthless. When choosing data services, organizations must select one that suits their needs by determining whether an Internet provider offers guaranteed bandwidth to support functions such as videoconferencing. Frame Relay network customers can specify a Committed Information Rate (CIR). This is the minimum connection speed when the ISP is busy. Sometimes, ISPs offer “Zero CIR” at very attractive rates. However, zero bandwidth means no transmission, and you can’t schedule the downtime with this type of service. When planning a system, it’s a good idea to build it with capacity for twice the peak bandwidth needed to run audio or video traffic in order to cover a worst-case scenario.
Symmetry of the system is important for 2-way audio and video, so be sure to review both downloading and uploading speeds. Technologies like DSL (digital subscriber line) usually download at 1500 kbps and upload only at 256 kbps. The recommended minimum bandwidth for quality videoconferencing is 384 kbps.
Internet and AV – A match not made in heaven
You may be wondering where the Internet fits into this equation. The Internet offers great promise for connecting remote sites with interactive video, audio and data, however, the current state of the Internet works more like a leaky sieve than a funnel. Internet traffic routes can be random, with video and audio information bouncing from router to router before eventually reaching the right endpoint. The information usually gets there, but there is no guarantee that it won’t take the scenic route instead of the shortest distance.
Since there are potentially millions of packets to send for AV files, many of these arrive out-of-step. In the world of live two-way audio and video, the late arrivers must be thrown away, reducing the overall quality of the received signals. It’s like trying to drive hundreds of people across a busy city and have them reach the destination in order; some take the fastest route and others are driven through rush hour by a befuddled cab driver new to the area. Our quick research showed that data packets leaving our office pass through a minimum of 10 network routers before reaching their destination. The inherent latency in the relay of information varied from 50 milliseconds to more than 300 milliseconds.
Vendors continue to advocate new technologies to help transport and share interactive audio and video. In essence, network gear is being purpose-built for the needs of voice, video and audio. This is the next frontier in the new-wired world. The Internet is not the panacea, but it may be if the current technology platforms are harnessed correctly.
