Network distribution concepts are finding their way into all facets of the digital video server revolution and wide area media content distribution systems. Corporate backbones are extending to broader geographic environments, centralized broadcast operations are changing how content management is handled, and the Internet is offering greater opportunities for distribution. In 2002, many are deploying live video, video-on-demand and other forms of streaming media. Much of the content is distributed over corporate LANs, wide area networks, the Internet, and via wireless applications. Even DTV has its promises for applications related to media delivery aimed at "other-than-television" or AV media content.
The hardware required for the storage and distribution of multiformat media as data is no longer rocket science. Today's large-scale media server systems in broadcast, cable and news facilities already employ multipurpose, multiformat encoding schemes for various uses and means of delivery. The most familiar -- proxy encoding -- can store hi-resolution content and make that same content available for browse and streaming media applications at lower resolution. Getting that content -- as files or streams, to the users in a reliable, cost-effective method, is the next challenge for this decade.
Closed systems, those integrated within the facility for specific purposes, are relatively straightforward in their approach to internal distribution. Typically, the accepted physical transport media consists of a mixture of coaxial, fiber optic or unshielded twisted pair (UTP), as is the case for Ethernet or IP LANs. Interface hardware, the servers and encoder and decoder combinations make the conversions into and out of user interfaces, presentation platforms and display technologies. Complexities begin when the mixtures of content must be sent to other facilities over corporate LANs or other transport mediums, or when "nonbroadcast-centric" servers are used to store, forward or play out the content, as well as handle the lower-resolution proxies, decision lists and asset management programs that contain data, AV, graphics and other forms of information.
ONE TO MANY
With the increased dependence upon visual media for communications, many are seeking alternatives.
Today, along with traditional video ingest and playout, video servers can push products to other servers over a variety of means. Delivering files "one-at-a-time" to one recipient at a time, using gateway interfaces that use ftp is quite adequate when only a few recipients need that information, such as in a small group of four or five stations, but begins to suffer from system performance issues when the same file must be delivered to 10 or 20 stations or more. Multiply the individual files that must be sent daily and the ftp model fails as a general solution for the entire network's group of facilities.
End users are looking for content distribution solutions other than ftp. The Internet has spurred new methods for content distribution using IP (Internet protocol). One of the solutions for moving IP content from a central production facility to affiliates or group stations is called multicast. Developed by the Internet Engineering Task Force (IETF), IP Multicast is a suite of standards that sends information to multiple destinations simultaneously.
Once the media is in the data domain, other choices become available. To transmit data to a host or host group, those file transfers are grouped into prescribed categories. Defined as unicast, multicast or broadcast, these categories are described in Table 1.
In a unicast mode, a packet or frame will be routed to only a single device on a network or the Internet; essentially the file or package of files are transmitted for only a single receiver whose address is known in advance of the transmission. This is the simplest of network communication functions, but it becomes inefficient when you need to send the same file to multiple hosts.
Since Ethernet/802.3 is a broadcast technology, you can achieve simultaneous communications with a group of devices on the network. This is called multicasting.
For multicast, files are transferred to a group of receivers defined for that particular transmission. Special registration by the recipient and or forms of routing may be needed to use multicasting.
When you want to communicate with all the devices on the network, broadcast mode is chosen. Here, files are sent using a generally specified broadcast address. The destination field of the Ethernet frame is set to an address of six; all "FF" hexadecimal numbers. Under this operation, all stations must be able to act on frames sent in broadcast. Broadcasting brings a host of problems to larger networks and the Internet. Only certain instances for broadcast are generally accepted, because many routers may block broadcast identifiers as a preventative measure for network integrity.
Multicast, a limited form of broadcast networking, efficiently sends text, audio and video on the Internet or to a specific group of targets on an internal network. Multicast is a one-to-many transmission in which a sender multicasts a message to a predetermined group of targets (recipients), notifying them that a message is coming. Each target recipient then registers to receive the information prior to transmission. Each group of files, broken into blocks and frames, is sent simultaneously to the recipients. Bad or missing frames are tallied by the receiver. The initiator of the files then requests a listing of each receiver's bad or missing frames, based upon each block. Only those frames requested by the receiver's notification to the initiator are re-sent.
'BEST EFFORT' DELIVERY
IP Multicast is an open standard for distributing data to multiple recipients. Here the multicast recipient group can change dynamically. Hosts may join or leave a group at any time, and a host may be a member of more than one multicast group. IP Multicast uses "class D" addressing, a specific form of the IP address designed for multicasting. Its address is 32 bits long, with the first 4 bits identifying the class D addressing and the balance identifying the multicast groups.
IP Multicast is a "best-effort" delivery service, assuming that all the receivers in its group can accept and handle the data. To improve reliability and stability, an additional protocol, MTP (Multicast Transport Protocol), can operate on top of any network protocol, provided the data link layer includes multicast. MTP (RFC 1301) provides flow control and service improvements, including the ability to retransmit replacement packets to a specific branch in the multicast group.
Certain considerations must be taken when dealing with multicast. For Internet working applications, IP routers must be configured to route IP Multicast traffic. For the LAN, most NICs (network interface cards) support IP Multicast, so configuration changes are not an issue. IP Multicast using frame-relay services is not inherently supported, although third party products are available that reconfigure IP Multicasting for frame relay.
The latest IP Multicast products on the market compress and group multiple files for transfer as a single package file. Incorporated in the messaging are instructions on how the file package is to be processed at the host recipient. File management, logging, automation and monitoring of the file transfer activity is handled by the initiator. Installation and software upgrades are also part of the system software, so maintaining common applications, drivers and the like is all managed remotely.
Finally, in cases where unidirectional transmissions are all that are available (such as in satellite systems) or where no backchannel is available for return status, forward error correction (FEC) is employed. Where space and time allows, files could be sent repetitively and then, once at the receiver, checked and replaced as needed. A further methodology for one-way terrestrial or satellite distribution is to include "repair packets" that are used to reconstruct corrupted or missing data without re-transmission or multiple transmissions of the same files.
Streaming media and multisite/multicast networking are shaping how content distribution will be achieved. On-demand serving is already in use for news, with other services addressing commercial content delivery and programming. Broadcast station groups are close to deploying this technology for centralized broadcast operations at various levels, with some either deploying it internally (themselves) or through service carriers.