The needs of a broadcaster are diverse. The list of must-haves includes voice and data circuits between different sites and to the public switched network, as well as live video and audio circuits for contribution and distribution.
Unlike a typical data circuit, video transport is unidrectional. Broadcasters simultaneously deliver the same signal to many sites for regional distribution or to feed transmitter networks. This necessitates temporary high-bandwidth circuits for outside broadcasts.
A WAN for broadcast applications must carry some or all of the following as live streams:
- analog composite video;
- analog audio;
- 270Mb/s for uncompressed digital video or MPEG DVB-ASI; and
- MPEG over IP.
They must also deliver large media files for non-real-time applications. An hour-long show recorded at 50Mb/s is a 22GB file. Even a 10-minute news clip at DV25 is nearly 2GB.
Whereas data can use opportunistic bandwidth, live video needs a high quality of service with low latency. This has ruled out the packet-switched networks, such as frame relay for video, but later technologies, such as SONET/SDH and ATM, offer the desired performance.
Most broadcast networks have a system of contribution links. Some connect regional production centers; others connect local newsrooms. Many are linked with existing analog circuits or with more recent digital connections, often using 140Mb/s circuits. As broadcasters offer more themed channels and newscasts expect live links, there is a greater need for these contribution links.
Distribution and STLs
Many broadcasters are rolling out DVB-T networks to replace their old analog transmissions. The capability to offer four TV channels in one analog 8MHz frequency slot and the advantages of MPEG transmission is driving the changeover. However, the additional channels mean more circuits from master control to the transmitters. The introduction of terrestrial HD services further adds to the bandwidth requirements.
Broadcast Services Denmark
Broadcast Services Denmark (BSD) distributes TV and radio in Denmark and is a subsidiary of the public service broadcasters DR and TV2. Recently, the broadcaster wanted to upgrade an existing analog infrastructure to digital terrestrial transmission. This upgrade required a cost-effective solution that could transport DVB-T signals from the production facilities of the two broadcasters to all the transmitter sites throughout the country.
BSD also wanted to build production and contribution networks to link broadcasters and their studio facilities with real-time video connections. These networks would be used specifically for live broadcasts and the creation and editing of video for these productions. The production and contribution networks also needed to transmit high-speed data and IP services.
To meet all of the above requirements, the broadcaster looked for an underlying infrastructure that would include traditional radio links, a leased line SDH network or a new fibre network. This would include any mix of fibre, optical wavelengths, SDH leased lines and radio links. A combination of transport services would give the broadcaster the maximum flexibility and the lowest cost per link.
BSD also implemented a new type of network management. (See Figure 1.) The new workflow allows users to manage these disparate services and maintain the low jitter and low latency needed for live television. The solution involved a new type of network management.
Temporary circuits for OBs
Broadcast news can make use of satellite or microwave for temporary connections to backhaul stories. Outside broadcasts generally need more bandwidth to create better video quality and to transport HD signals. Large events will require many feeds back to the broadcast center. A satellite rarely has sufficient capacity for such applications, so the alternative is to use fibre wherever possible.
A recent example of this is the facility at the 2006 FIFA in Germany. As the first World Cup to be covered in HD, the bandwidth requirements were larger than previous events. Host broadcaster HBS used T-Systems' fibre networks to link the many venues with the International Broadcast Center in Munich. The network infrastructure featured a high-speed and broadband fibre backbone that provided bandwidth for multiple HDTV services.
The broadcasters needed links for SDI 625/50 at 270Mb/s with four embedded mono audio channels (using SD140 transmission coding format at 140Mb/s), as well as HD-SDI 1080i/50 at 1.485Gb/s with four embedded mono audio channels. The HD was coded as MPEG-2 HQ for transmission.
Because it was such an important event, satellite backup was used for fallback services. Dual 20Gb/s circuits connected each venue via protected dense wave division multiplex (DWDM) fibre connections. The DWDM distribution network could handle compressed (MPEG-2 at 4:2:0 and 4:2:2) and uncompressed transmissions with multiplexers and demultiplexers. HDTV transmission as a non-real-time file transfer was also provided for the broadcasters.
Telcos offer a variety of services, including PDH/SDH, ATM, IP and occasionally dark fibre. None of these are entirely suited to video transmission, especially live video. The packet- or cell-switched networks are designed for data and suffer from variable latency and packet loss. ATM offers a way around this by traffic classification: constant bit rate, variable bit rate, available bit rate and best effort (with the last two being opportunistic).
PDH and SDH use time division multiplexing. Many channels are allocated time slots in a serial data transmission. These systems were designed to multiplex 64Kb/s phone circuits. A 2Mb/s E-1 circuit carries 30 voice circuits, and a 144Mb/s E-4 multiplexes 62 E-1s. The original PDH (nearly synchronous) is being replaced with SDH, where the send and receive ends are synchronized. This removes the need for data buffering.
A typical optical carrier, such as OC-192, with a bit rate of nearly 10Gb/s, has a capacity of 256 E-4 multiplexes. The rigid hierarchy of PDH/SDH was not directly suited to data transmission. The requirement for bandwidth can be in occasional bursts rather than the fixed allocation of a phone call.
ATM offers a more flexible allocation of bandwidth. It splits data into small fixed-sized cells rather than the variable-sized packets of frame relay. It was designed to handle streams or data packets. The small cells (48-byte payload) are especially suited to low latency traffic, such as voice and multimedia. Although ATM had much promise, it has not proved to be widely deployed. Other technologies have taken over, with IP being the ubiquitous one. The development of VoIP allows voice and data to be carried over a single IP transport layer.
Multi-Protocol Label Switching (MPLS) technology is gaining traction. It can build IP networks over frame relay, ATM or Ethernet with managed quality of service and traffic engineering. The latter allows bandwidths to be specified for the connections, a prerequisite for many broadcast applications.
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