A Brief History of IP - Audio Networks
IP networking is taking the radio and broadcast industry by storm, but as a method of distributing data, it has been available since the 1970’s. So, what are IP Networks? And why have they become so popular recently?
The history of IP is closely linked to that of the world wide web. Back in 1969 the U.S. Defense Department’s Advanced Research Projects Agency Network (ARPANET) funded research into much of the suite of protocols that make up todays internet, specifically IP and TCP.
In 1972 the Internetworking Working Group (INWG) was formed to standardize protocols and by 1973 the University College London (UK) and Royal Radar Establishment (Norway) connected to ARPANET and global connectivity was born, along with the first accepted use of the term “Internet”.
TCP is Born
1974 witnessed the birth of Telenet, a commercial version of ARPANET and Vincent Cerf and Bob Kahn publish “A protocol for Packet Network Interconnection”, the precursor to TCP. By 1982 TCP and IP emerged as the protocol for the internet and combined with public Domain Name Systems (DNS), established web domains formed to associate names such as .net and .com to IP addresses.
What soon became clear, and probably the single most important reason that has made IP so ubiquitous and adopted throughout the world, is the protocol is independent of the underlying hardware it is being distributed over. For example, IP can reliably work on ethernets’ 10BASE-T as easily as it can work on fiber channels 8GFC. The line-speeds may be significantly different, but the IP protocol works over both equally.
Flexibility is Key
The ramifications of this flexibility soon become clear as we look at distribution between different service providers. A backbone between New York and London may consist of a high-speed fiber running under the Atlantic Ocean. But at either end two different telco’s may connect to the fiber using 100Base-T ethernet. If they both comply with the IP and IEEE 802.x ethernet protocols, they will seamlessly work together.
IP addressing takes this one stage further as one computer can communicate with another computer on the other side of the world without understanding or caring how the routing and connectivity takes place.
Contrast this with broadcast and radio stations using traditional communications. To send an audio circuit from London to New York, the broadcaster would need to understand the routing and connectivity used as well as the massive administrative burden in making it happen. Broadcast stations at each end need specialized connections to the local telco such as 2-wires, balanced pairs or SDI feeds. All this significantly adds to the cost and complexity.
IP Separates Data from Hardware
Inside the station, IP still offers many advantages by simplifying the operational aspects of the system and providing greater flexibility. IP abstracts the data from the underlying hardware meaning any audio, video and meta-data is also distanced from the hardware resulting in a much more flexible system. As new audio and video standards are released to keep up with ever increasing audiences, they can be more easily deployed on an IP system.
It’s worth remembering that ethernet is a distribution system that could be used to transfer audio and video, and to some extent is responsible for the success of IP. However, ethernet has some limitations that make use outside a station difficult. For example, there is no concept of masking, so routing becomes much more complex, and there is no flow control or packet re-send mechanism for lost and dropped packets.
Timing is Critical
One of the major challenges of distributing media over an IP network is that of timing. Broadcast and radio circuits are incredibly expensive due to the guaranteed levels of quality of service they provide. We know an audio balanced 2-wire will have a bandwidth of 20KHz and latency less than one millisecond, but we pay for this through cost and lack of flexibility.
Due to the intricacies of routing technology, IP packets do not always take the same path and can arrive at their destination out of sequence, and the strict limits of sampling clock used in the original analogue to digital conversion is lost.
Packets Must Be Sequenced
Resequencing is straightforward as packets have an incremental counter to identify each packet as it is sent. Therefore, the receiver can easily put the packets in the correct order before they are presented to the digital analogue converter or decoder.
Audio and video streams over IP are sent with User Datagram Packets (UDP) that are encapsulated in IP packets. UDP adds the concept of source and destination ports as well as checksums to the video and audio payload data.
Ports add further granularity to IP addressing to allow a single device with one IP address to receive or send multiple streams. For example, a sound console might only have one IP address, but would have sixty-four separate port numbers for each of its channels.
Buffers are used as a temporary store to re-sequence packets.
Unlike with MADI or ASI-MPEG, the piecewise timing relationship in IP is not respected and further eroded by the network itself. Real Time Protocol (RTP) overcomes this by sampling a clock in the encoder to give each payload of audio or video a unique timestamp. The receiver reconstructs the audio or video stream using these timestamps and rebuilds the audio or video stream.
Network latency is a new concept to television and radio. Routing switches direct packets between ports to transfer data around different networks. This is a form of multiplexing but on a much more complex scale. Consequently, blocking can occur within the switch as packets are delayed in buffers leading to variable latency.
Buffers in receivers are used to negate the blocking effect of switches but if used too much will result in increasing the end to end latency. Great care must be taken to keep RTP timestamps within the buffer size.
Watch the Backbone Speed
Some network latency is inevitable, but we work on the principle that the backbone speed of the network is sufficiently large to keep it small. But we must continue to monitor latency and make sure it keeps within tolerable parameters, especially when controlling remote equipment.
IP has brought unimaginable cost-effective flexibility and scalability to television and radio stations. And as adoption of the technology expands vendors will bring better solutions to make best use of the flexibility IP offers so we can continue to meet the ever-increasing demands of viewers and listeners.
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