Figure 1. The traditional crosspoint router is a matrix of electronic switches that can connect combinations of inputs to outputs. Click here to see an enlarged diagram.
For some time now, the broadcast industry has been exploring, and to a certain extent implementing, network technologies. However, in the routing field, where network technologies appear to offer considerable potential, there are still significant ongoing issues surrounding the subject that require explanation and discussion. Traditionally, broadcast and professional media signals have been distributed around a facility by a matrix crosspoint switch. While historically these switches have been the most economic solution to handling demanding television applications, the reduction in cost of network bandwidths means that a network may become a viable alternative.
The crosspoint router
The traditional crosspoint router is a matrix of electronic switches that can connect combinations of inputs to outputs. Typically, an input signal can be switched to multiple outputs making the router distributive. Figure 1 represents a typical cross-point router with four inputs and four outputs; the crosses represent active switches connecting an input to an output. In many television applications, it is important to work with uncompressed video. This means that switching solutions have to handle bandwidths of 270Mb/s (SDI video, SMPTE 259M) for a single video feed. With the introduction of high-definition television, which is gaining ground throughout the world, these rates increase up to 1.5Gb/s (ref SMPTE 292M).
For clean video switches, the devices handling the video have to operate synchronously with the video, switching between sources in a non-visible part of the video signal. This requires switches to be accurately timed to at least 40ms in the case of standard definition.
Figure 2. The cost of a crosspoint router increases approximately as a square law, and the percentage of actual bandwidth used at any one time in the router follows an inverse trend. Note that shown here is an approximation that only considers the core switching electronics and not the input and output stages. Click here to see an enlarged diagram.
Another key requirement that the crosspoint router satisfies is to have 100 percent availability for any switch. This is inherent in its design of having a separate electronic switch for every possible path through the router. However, this guaranteed availability comes at a price. The charts in Figure 2 illustrate how the cost of a crosspoint router increases approximately as a square law (note that this also applies to the physical size), and how the percentage of actual bandwidth used at any one time in the router follows an inverse trend.
A network solution A dictionary definition of a network is “a system of interconnected components or circuits” or in computing terms, a system that delivers a means of transferring data between devices. This is in essence what the crosspoint router is doing, so does a network offer us an alternative means of performing this task, and are there any benefits? Almost certainly there are no network topologies that follow the same cost and bandwidth curves shown in Figure 2. Therefore, intuitively there is a potential saving by better use of a system’s available bandwidth. The networks considered in this article all distribute information by switching packets of data over the network, thus avoiding the crosspoint router’s problem. However, the packetization and switching of individual packets requires data processing at the network nodes, which will introduce some form of delay within the network. What these delays might be and how they can be managed are key factors when considering real-time distribution of video. A network solution potentially offers additional benefits in two areas:
- Because a network distributes packets of data, this data can be coded representations of different signal formats allowing our switching infrastructure to support video, audio and other signals. With a crosspoint router, typically the electrical characteristics of different signal types have meant separate routers for each signal type.
- A crosspoint router is a single device, and all signals to be switched have to be wired directly to the router. Because a network is distributed, a network node can be located near a group of signals’ origin or destination, potentially saving large amounts of cabling in a video installation.
Modelling the crosspoint router on a network
The first area to be considered is how the functionality of a crosspoint router can be mapped onto a network. In this section, let’s assume that the network has sufficient bandwidth and insignificant delays. We will consider the network using the IP protocol as this is the dominant network protocol in use today.
Basic IP addressing allows us to send a packet of data to a destination defined by a familiar four field address, e.g. 192.168.0.1 defined in the packet header. When we consider the distributive nature of the crosspoint router, this point-to-point mechanism poses a problem. In a large installation, we may need to send the same source to many destinations. A system that transmitted a separate packet for each destination would require sufficient bandwidth and processing capabilities at the network input to cope with sending packets to as many destinations as required. When dealing with video this soon becomes unachievable, or at least very expensive. Input device bandwidth = 270Mb/s x number of outputs requiring video source
The multicast ability of IP offers a solution by allowing a network node to originate packets that can be sent to multiple destinations by the routers in the network. Multicast operates by a network source device originating packets with addresses in a reserved range (224.0.0.0 to 239.255.255.255). Network routers recognize these packets as a multicast and will forward them to all network devices that are members of the multicast. Membership of the multicast is initiated by a network node requesting membership via the Internet Group Management Protocol (IGMP); the network routers also use IGMP to communicate their requirement to receive (or not) multi-cast data.
Figure 3. Shown here is an example representation of a 4x4 crosspoint router with equivalent routing on a multicast network. Click here to see an enlarged diagram.
Figure 3 shows an example representation of a 4x4 cross-point router with equivalent routing on a multicast network. Here, each video source is transmitted on the network with a unique multicast address. The four destination devices are each subscribed to one of the multicasts to achieve the equivalent end-to-end connection as the cross-point router.
IP over Ethernet
Ethernet has become the dominant network technology in the IT computing world, and its widespread adoption means that costs are continually falling. With Gigabit Ethernet, it also provides the bandwidth we require for handling video. Ethernet does not define any device-to-device timing or flow control mechanisms. Network devices transmit data packets asynchronously to one another. The delays between network nodes are a function of the network loading and the devices in the network, such as switches and hubs. Where network traffic exceeds the bandwidth of a network device and data is lost, Ethernet does not provide retry mechanisms.
The variable network latency and lack of guaranteed data delivery would seem to suggest that Ethernet cannot deliver the quality of service required for video distribution. However, solutions are provided by protocols higher in the Open Systems Interconnect (OSI) model, Ethernet defining layers one and two.
Examples include:
- The Transmission Control Protocol (TCP) provides acknowledge and retry mechanisms to ensure successful transmission of data.
- The Real-Time Transfer Protocol (RTP), a member of the IP protocol set, provides packet ordering information and timing information relating to the source data being carried in the packet. This can allow a solution to be engineered around the limitations of Ethernet.
ATM
Asynchronous Transfer Mode (ATM) is a network technology based on high-speed packet switching with inherent quality of service features such as bandwidth management. ATM uses a virtual circuit concept where quality of service parameters are negotiated when a connection between two network devices is established. Data is carried in small fixed length packets, or cells. The cell has a 5-byte header, containing the virtual circuit identification, and a 48-byte payload. The quality of service offered by ATM falls into four main categories:
- Constant bit rate (CBR) specifies a fixed bit rate so that data is sent in a steady stream. This is analogous to a leased line.
- Variable bit rate (VBR) provides a specified throughput capacity, but data is not sent evenly. This is a popular choice for voice and videoconferencing data.
- Unspecified bit rate (UBR) does not guarantee any throughput levels. This is used for applications, such as file transfer, that can tolerate delays.
- Available bit rate (ABR) provides a guaranteed minimum capacity but allows data to be bursted at higher capacities when the network is free.
Because ATM can emulate a point-to-point circuit capable of delivering CBRs it has been used extensively for wide area distribution of video as it supports the characteristics required to ensure continuous video transfer.
DTM
Dynamic Synchronous Transfer Mode (DTM) is a reletively recent technology specifically designed around the requirements of media applications. It provides guaranteed quality of service with low network latencies. DTM operates on a time division multiplexing principal where network bandwidth is divided into time slots.
Channels are provisioned between network nodes where a specified number of timeslots are allocated to a channel. By having a predetermined number of timeslots, the channel will be guaranteed a constant bandwidth, and the latency between network nodes will be constant. Another key feature of DTM is that it supports multicast, an additional benefit when looking at the application as covered previously in this article.
Conclusions
Networks are being used for the distribution of video, and there are technologies that can allow us to provide an equivalent solution to the crosspoint router for video with a network. Today, we are still at a point where crosspoint routers are significantly cheaper than a network equivalent, but already applications exist where a hybrid of the two approaches are being used. Typically, the network will provide connectivity over the wide area with crosspoint routers, providing local switching at the network nodes.
With the ever increasing performance of networks and decreasing costs of the technology, it is almost certain that networks will become a viable alternative to the crosspoint router in some media distribution applications. Neil Maycock is chief technical officer of Pro-Bel.
