Supporting Real World Voice Rollouts with an EMTA–轉載

引言

本文轉載自TI的Supporting Real World Voice Rollouts with an EMTA的白皮書，詳參原文。

內文

In a PacketCable™ environment, an embedded multimedia terminal adapter (EMTA) is the appliance that connects legacy telephones to a cable operator’s advanced IP communications network.  Coupled with an embedded DOCSIS® cable modem, the EMTA performs voice compression, packetization, security and call signaling for IP communications services, whether telephony, fax services or narrowband data connectivity.  Therefore, the quality and reliability of a cable operator’s IP communications service offering is only as good as the EMTA installed in each customer’s home.

Before the first EMTA can be deployed on a commercial cable system, it must first successfully pass the rigorous certification testing performed by CableLabs® for North American deployment and tCom labs for European deployment. In addition to the battery of tests performed by the various industry labs, the EMTA is also subjected to a rigorous round of MSO specific tests. The importance of this extensive testing and validation is clear, but what other EMTA product attributes and features are important for large-scale consumer deployment into home environments with a wide range of legacy communication devices? Outlined below are four key items that are being addressed by EMTA technology providers that are especially critical to support a robust, scalable, high-volume voice rollout.

Fax Machines

Fax machine performance can be hindered by a packet network when packet loss conditions exist.  Recognizing this problem, the International Telecommunications Union (ITU) developed a fax relay specification known as T.38. This specification describes a mechanism of transmitting fax signals over IP networks by allowing the voice adapter, or EMTA in the case of a PacketCable network, to recognize facsimile calls, intercept the fax signaling and data, and re-transmit the data in data packet format.  In this case, the EMTA actually behaves partially like a peer fax machine in that it detects and generates control signals and communicates directly with the initiating facsimile device. The bottom line is that T.38 adds functionality to the EMTA that allows fax transmission to work.

Data Modem Calls

Data modems continue to exist in many different formats throughout the world. Although it may be reasonable to assume that few people will opt to use a dial-up modem instead of the broadband connection that is native with a voice-over-cable service, it is also reasonable to assume that there will be legacy devices with embedded voice band modems to support.  Alarm and monitoring systems offer a good example of modem devices that successful voice services must support.  Like fax transmissions, data modem calls can be hindered by packet loss in a network.  However, unlike the fax machine solution, complete data modem functionality is not practical to put into an EMTA.  Recognizing this, the ITU is working on a solution that allows an EMTA to detect a data modem call, set up the proper channel vocoder and provide extra protection to avoid packet loss situations.  This specification is called V.VBD, where VBD stands for Voice Band Data. V.VBD in a PacketCable network would rely on the already established G.711 PCM codec to be used for data calls, and it would use RFCs 2198 and 2733 for redundancy and Forward Error Correction (FEC).  Altogether, this provides a very robust mechanism to detect data calls and add the proper amount of care in the network so that the data calls are reliably handled.

Low Bit Rate Vocoder Support and Interoperability

Low bit rate vocoders, or LBR codecs for short, substantially reduce the bandwidth required for transmitting voice over a packet network while maintaining acceptable levels of voice quality.  LBR codecs are actually very sophisticated software algorithms that run on a digital signal processor (DSP).  The current PacketCable specification calls out one mandatory and two optional ITU codecs: G.711 (mandatory), G.728 and G.729E.  The G.711 codec utilizes 64 kbit/s for a voice call, G.728 utilizes 16 kbit/s, and G.729 utilizes 11.8 kbit/s.  Support for the optional codecs is beneficial as they provide operators with the opportunity to conserve network bandwidth and thus lower operational costs.  Interoperability of the codecs on different equipments is necessary to insure that LBR-coded voice quality is not degraded, distorted or garbled.  The solution to rigorous interoperability of LBR codecs starts at the software level.  Codecs such as the G. codecs from the ITU have gone through a formal process to ensure that bit accurate software models of the codecs are available to the technical community.  The availability of codec models allows the creation of test equipment and basic implementations that can be formally validated against one another.  As systems are constructed, interoperability testing is again performed at the system level to ensure that no hardware or other system artifacts have disturbed the basic interoperability.

Server Interoperability

Server interoperability, from an EMTA’s point of view, entails ensuring compliance with multiple implementations of the different types of servers in the PacketCable network, including the provisioning server and the call management server.  The provisioning server is responsible for communicating configuration and control information to the EMTA.  The call management server, sometimes called the call server, sends and receives messages to and from the EMTA to control call setup and tear down.  Robust interoperability with these two servers is critical to provide a user with the consistent quality of service that has come to be expected from the telephone.  There are no shortcuts here.  The solution to server interoperability is for vendors to spend significant time together testing interoperability to verify that no issues exist under a variety of network configurations.  As products are maturing, vendors are addressing these issues, exchanging equipment and working together to verify the interoperation of their products.

Summary

EMTAs are very important to the success of voice rollouts for MSOs.  They will be deployed in conjunction with a vast array of legacy phone equipment and contain protocols that communicate with a number of peers in the network.  Because the quality of the MSOs’ service deployment can only be as good as the quality of the EMTA, choosing an EMTA that elegantly supports a wide range of existing phone equipment and seamlessly interoperates in the network will allow for a pleasant user experience and a robust ramp of IP-based phone services.

參考文獻

原文網址 http://www.ti.com/lit/wp/spay017/spay017.pdf