By Phone Cards

Figure 3. Time slots help prevent cable network devices
from exceeding their bandwidth.

Time slots also enable the CMTS to perform bandwidth management. Each time slot defines the maximum amount of data that can be transmitted during a specific time period. Any attempt to broadcast data outside the authorized time slot will be rejected by the CMTS. Thus, by controlling the size and frequency of a device's time slots, the CMTS is able to constrain the maximum amount of data that device is able to transport.

A third task for a CMTS is enforcing network QoS. QoS describes the techniques used to ensure that packets reliably traverse a network. For instance, the CMTS may reduce the time slots allocated for lower-priority (or best effort) data to ensure that higher-priority QoS traffic (such as voice) arrives safely.

Telephony Is Demanding!
Cable companies are anxious to offer local telephone service over Internet Protocol (IP). To compete in this market, they must be able to match the reliability and audio quality offered by existing local phone companies. Consequently, dropped or lost audio packets are unacceptable. To be viable, cable telephony solutions must overcome the shared bandwidth limitations of IP-based HFC networks.

To avert these restrictions, PacketCable, the organization responsible for standardizing packet-based multimedia in cable networks, has enhanced the CMTS with additional multimedia-related QoS features.

For instance, a telephony-aware CMTS has APIs to control multimedia QoS attributes and to reserve and commit bandwidth. One QoS attribute is the number of sessions (or streams) dedicated to a single user. If a user exceeds the number of simultaneous streams that have been paid for, the CMTS may refuse to guarantee QoS for subsequent calls.

Another QoS attribute is the priority (or service level) associated with each multimedia stream. If the CMTS cannot provide the desired QoS priority, it notifies the requesting application before audio is transmitted. The program, in turn, can play a warning message and then a busy tone (the tone sequence may vary between MSOs). Without this feature, users would have to experience audio breakup before the application could detect problems.

Bandwidth reservation instructs the CMTS to reserve a specified number of time slots for audio transmission. However, these slots may be re-used for lower-priority data packets until the bandwidth commitment phase. Consequently, reservation does not influence network data flow. By contrast, bandwidth commitment affects HFC traffic since it only permits audio packets to flow through the committed portion of a CMTS.

Telephony Police
In a previous TMCnet.com article, "Fat Or Thin? Choosing The Best Client For Cable Telephony," I noted that PacketCable has chosen the Network-based Call Signaling (NCS) model for its initial cable telephony deployments. NCS places simple endpoints known as Multimedia Terminal Adapters (MTAs) in consumers' homes. These devices are controlled by an intelligent network entity called a Call Management Server (CMS).

The CMS uses the Common Open Policy Service (COPS) protocol to manipulate "gates" on the CMTS. A gate in PacketCable terminology represents a data roadblock -- these gates must be opened before data can flow through the CMTS.

By default, all gates are closed. When the user dials a phone number, the MTA transmits the digits to the CMS. The CMS analyzes these digits and determines the IP address of the MTA that the user wishes to call. It then creates a gate on the CMTS by requesting a specific QoS level from the CMTS. The CMS then causes the remote MTA to start ringing.

Once the gate is created, the calling MTA is responsible for reserving the bandwidth necessary to complete the call. When the calling party picks up the phone, the remote MTA requests that the CMTS commit the bandwidth (see below). After the conversation is over, the MTA issues a message to the CMTS to close the gate and free the time slots.

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