Embedded Systems -- Focus: Embedded Internet
Internet protocols manage remote TV
Mike Chartier, Staff Engineer, DiviCom Inc., Milpitas, Calif.
03/01/99
Electronic Engineering Times
Page 82
Copyright 1999 CMP Publications Inc.
The shift to digital-TV technology is driving broadcasters to build networks of equipment that include embedded microprocessor-based encoders, video servers and smart multiplexers with embedded microprocessor intelligence. This requires sophisticated network-management support across multiple sites.
A well-known and -understood Internet protocol-SNMP (simple network management protocol) was chosen by DiviCom because it provides a robust structure for standards-based remote management over the Internet, making it possible for digital network equipment from a variety of vendors to easily communicate with one another.
Since broadcasting operations can range from a single crew in a truck to a complex station serving millions of customers, DiviCom had to design an encoder, the MV400, that was both portable and that could function as part of a sophisticated network with multiple channels. Footprint, power consumption and space were important issues for all potential users, leading the design team to aim for an encoder the size of a pizza box.
Compression of high-definition broadcasting presents an especially tough challenge since far more data must be compressed in the same amount of time as is required for standard-definition video. To support the U.S. terrestrial broadcast market, 1.5 Gbits of data must be compressed to less than 19.4 Mbits in accordance with the MPEG-2 standard for the compression and transmission of video, audio and caption data.
In addition to the MPEG-2 networking support, DiviCom decided to incorporate an Internet interface to support remote management through SNMP, which offers a standards-based approach for network management that is already well established in Internet devices such as routers. An alternative Internet-based approach for remote management is to embed a Web server in the device and use a browser-based host interface. DiviCom initially considered that approach but decided it would be very difficult to individually control and configure dozens of encoders this way. Since SNMP is specifically designed to simplify management of devices that are part of complex networks, it fits well with digital broadcast market requirements. SNMP also enables the MV400 to support configuration commands originating from third-party equipment; another important requirement.
To understand how DiviCom implemented SNMP in its encoder design, it is important to have a basic knowledge of the encoder's architecture. To perform the compression of high-definition video data, the MV400's video compression engine employs a parallel architecture with multiple compression chips. By having several compression chips processing the video stream at once, it is possible to compress high-definition data as quickly as standard-definition data. For performance reasons, synchronization within the parallel video compression engine is handled entirely in hardware.
A microprocessor controls the video compression engine, an audio compression engine and text data feeds, as well as the Internet and MPEG-2 networking. It also faces complex real-time requirements driven by the MPEG-2 packetization and networking requirements. In addition, the microprocessor manages user control functions. User control is provided both remotely through SNMP and via a control panel. The system provides 16 Mbytes of RAM for executing the application software and uses an 8-Mbyte flash system for storage. The MV400's power-up sequence was designed to support booting from either the network or from ROM/flash. That booting ability helps minimize traffic in large networks of encoders and has advantages in portable situations, such as when broadcasting from a vehicle.
MPEG-2 networking has hard real-time deadlines driven by the need to synchronize video, audio and text data packets for a particular frame since all the packets must arrive simultaneously at the set-top decoder to avoid the loss or scrambling of information. Not only must the microprocessor perform flow control to prevent the video frame buffer from overflowing, but the compression engine must keep the microprocessor informed of the frame rate it is seeing. This requires the microprocessor to communicate in real-time with the compression engine.
For example, the frame rate will vary depending on whether a film or an advertisement is being encoded. Without knowing the frame rate, it is impossible for the processor to correctly synchronize audio and caption data with the video frame. A further source of real-time constraints during MPEG-2 networking is the communication with the multiplexer that receives packets from the encoder to transmit on to the network. To smoothly transmit data from multiple encoders at the same time, the multiplexer communicates with each encoder to vary the bit rate.
In search of an implementation platform for the encoder software, DiviCom chose a real-time operating system, VRTX from Mentor Graphics' Microtec division. VRTX offers real-time programming services such as deterministic system calls, which enable the encoder software to meet its stringent real-time performance requirements. VRTX also allowed the design team to easily embed Internet protocols in the encoder using the integrated TCP/IP stack. Another important consideration in the selection of the real-time operating system was the availability of an integrated set of C and C++ cross-development tools to develop applications.
To perform remote management, an SNMP manager on a host computer communicates via the network to the SNMP agent resident in the MV400. The available remote-management functions are determined by the management-information base (MIB) resident in the encoder. The MV400 MIB consists of a list of tables, each of which is, in effect, an MIB.
The Component and Module MIBs contain information on the configuration of a particular encoder. This includes the serial number, software and hardware version numbers and the type of each board installed in the encoder. To provide more fine-grained configuration of a specific function, there are also Circuit MIBs for video and audio compression, ATM, Ethernet and serial communication.
Multiple configurations
And, to support easy reconfiguration of the encoder network for a particular requirement, the SNMP manager makes it possible for the operator to store multiple configurations and recall them if necessary. The network controller can then scan all encoders on the network, compare the MIB settings to those selected by the operator and reconfigure the equipment as needed.
The MIBs are also helpful when troubleshooting network problems. For example, an operator might use an incorrect IP address when setting up a network "map," which defines all components on a network and their interfaces. The Component MIB can be used to confirm that the IP address was set correctly.
In combination with the File MIB, the configuration information in the Component and Module MIBs helps determine how the encoder network might be upgraded. The File MIB records the exact storage location of the files. Each entry in the MIB contains a destination tag that indicates whether the file is stored in NVRAM, flash or on the network controller.
Whenever a software update is required, the file name entries in the File MIB are replaced with the new files. The encoder's file system then deletes the old files from flash and downloads the new files to flash.
March 01, 1999
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