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Feature Article
July 2001


QoS Solutions For VoIP-Enabled Cable Modems


Quality of Service (QoS) is essential to successful cable telephony deployment. While consumers may choose to accept less than toll-quality services to obtain other features (as with mobile phones, for example), they are not expected to tolerate inferior quality for home and small business phone calls. Though circuit-switched services based over cable networks are deployed today, the use of IP-based solutions opens the door for QoS applications, in addition to telephony services. The cable network presents additional challenges unique to IP telephony services deployment, however. To maintain appropriate QoS, the limited, asymmetrical bandwidth of the cable network resources requires a prioritization and permission scheme for the customer premise equipment (CPE) transmission of data and voice packets. This important factor has been addressed in the underlying standards and technology for IP-based cable telephony.

Of course, quality of service mechanisms for IP networks are already established and deployed. Techniques such Multiprotocol Label Switching (MPLS) and Resource Reservation Setup Protocol (RSVP) can be employed at higher layers and in the backbone network to assure end-to-end quality and priority services for various classes of applications. Additional QoS features are necessary at the network's cable portion (from the network access to the user interface), to provide toll-quality voice and varying levels of service for provisioned applications. Standards for these QoS mechanisms are established primarily by CableLabs activities. As additional cable-based services such as voice and video gaming extend into the home, additional quality and class of service features are addressed by various in-home network technologies including 802.11 and Home Phoneline Networking Alliance (HPNA).

QoS At The DOCSIS layer
The cable industry has migrated to the Data Over Cable Service Interface Specification (DOCSIS) standard as the de facto protocol standard for Internet over cable. The first version of the specification (DOCSIS 1.0) was targeted at basic data transmission, providing a best-effort level of service. Version 1.1 builds on the previous 1.0 specification and enables operators to provide consistent and reliable digital services (such as voice and video) through the use of sophisticated QoS and network management mechanisms. The foundation for QoS for all cable-based services is provided within the physical and transport layer of the Cable IP infrastructure, as defined by DOCSIS 1.1. Building upon the DOCSIS 1.1 QoS foundation, the PacketCable Dynamic Quality of Service (DQoS) specification addresses additional features necessary for real-time voice applications.

DOCSIS1.1 supports four primary service categories: Unsolicited grant service (UGS); real-time polling service (rtPS); non-real-time polling service (nrtPS); and best effort (BE) service.

In a UGS flow, the cable modem (CM) is guaranteed to receive from the cable modem termination system (CMTS) fixed-size grants at periodic intervals, without the need to explicitly send requests. The tolerated grant jitter is negotiated at service setup, in addition to the grant size and the period. The use of a UGS reduces latency by eliminating the request-grant cycle for every packet. However, using a UGS is inefficient for applications that don't require a constant data rate over time, such as voice with silence detection. A variant of UGS -- unsolicited grant service with activity detection (UGS-AD) -- enables the CMTS to detect flow inactivity (through non usage of grants by the CM), by sending unicast request opportunities (polls) at periodic time intervals. The CM can use the unicast request opportunities to send requests to resume voice transmission avoiding latency incurred by contention.

In a nrtPS flow, the CM is guaranteed to receive from the CMTS unicast request opportunities at periodic intervals. If the CM does not use the request opportunities, the CMTS allocates the reserved bandwidth to other flows, overcoming the inefficiency of UGS. In a nrtPS flow, the bandwidth is not guaranteed to the flow. The CM, however, is allowed to use multicast request opportunities for the flow as well. The last QoS category, best effort, defines the minimum traffic rate (which the CMTS must reserve) and the maximum allowed rate as the main service parameters.

The flow in which a packet is transmitted is based on the content of the Ethernet and IP header fields, allowing every application to receive a different service flow. Multiple data flows (each flow corresponding to a service and identified by a service ID [SID]) concurrently exist in a CM. A transmission request in the upstream and the corresponding grant includes the SID as the flow identifier. The CM and the CMTS negotiate the QoS for each flow upon allocation and dynamically as the service requirement changes. QoS is then achieved by the implementation of sophisticated scheduling mechanisms in the CMTS. A classification function is applied to every packet.

QoS At The Voice Telephony Services Layer
The PacketCable DQoS specification applies to the voice and other applications layers. DQoS utilizes various protocols, and leverages DOCSIS 1.1 to allow for provisioning, transport, and billing of varying levels of service classifications. A bi-directional data flow session is established between two clients. By assuring that CM and CMTS adheres to the QoS rules, the DQoS spec provides for the necessary resources to be reserved and then subsequently committed for each data (voice) flow session. Two-phase (reserve and commit later) and single-phase (commit) QoS activation models are required, as well as resource changes during a session and dynamic binding of resources (e.g., transferring call session parameters for call waiting). Additionally, the PacketCable Codec Specification includes the use of a low-delay vocoder and a high-fidelity vocoder when using compression of voice, further improving on the voice quality.

Leveraging existing design implementations of successfully deployed VoIP solutions plays an important role in cable IP telephony quality as well. Efficient design of the DSP and PacketCable software, as well as tightly coupling the UDP (voice packet) processing with the cable modem MAC, helps reduce latency. Adaptive jitter-buffer and voice playout management schemes also help reduce latency and help mitigate the effects of packet loss. Line echo cancellation is necessary in voice-enabled cable modems, and echo cancellers must include processing for double talk and high background noise.

QoS At The Home Networking Implementation Layer
Although there are numerous home networking technologies, 802.11b and e, and HPNA have achieved some prominence and a higher level of deployment.

The 802.11b standard in particular is becoming a dominant force in wireless home networking. The standard extends data transfer capability to 11Mbps, thus providing the ability to transfer high-quality audio and video content over the network. Components such as 802.11b PC cards, PCI cards, and external base stations are being sold today that enable multiple PCs to connect with the home or office as well as other Internet appliances such as PDAs. The 802.11a standard provides even higher data rates (>50Mbps) using the 5.7Ghz band. The 802.11g specification will provide a direct extension to the 802.11b using the same 2.4Ghz band and providing data rates of 22Mbs.

The original 802.11 specification used a basic contention-based MAC and as such did not provide real mechanisms for QoS (as much as plain old Ethernet did not). Ongoing work on the 802.11e specification is intended to provide the necessary extensions to the 802.11 MAC in order to provide real QoS. In determining the spec, care has been taken so that older 802.11 devices will not degrade the QoS capabilities of newer 802.11e devices.

The 802.11e specification includes multiple levels of QoS. The basic level provides for a simple prioritization mechanism; higher levels are targeted to provide real QoS with guaranteed bandwidth and delay on a per-stream basis. These capabilities are similar to, and in some cases exceed, those that exist in the DOCSIS 1.1 specification. The 802.11e specification includes multiple bandwidth management schemes that can coexist on the same physical channel. The coexistence of different level QoS devices is similar to the co-existence capability of DOCSIS 1.0 and DOCSIS 1.1 modems on the DOCSIS network.

An HPNA-based network enables devices to transmit packets over residential phone lines. Two specifications have been developed: HPNA 1.0, which offers a data rate of 1 Mbps using an Ethernet-like carrier sense multiple access (CSMA) protocol without any QoS mechanisms, and HPNA 2.0, which offers a data rate of 10 Mbps using a prioritized CSMA protocol.

An HPNA 2.0 network supports eight priority levels from zero (lowest) to seven (highest). A time-division scheme is used to enable prioritized access. For every priority level, a designated time slot is established. Devices are allowed to start the transmission of a packet only in (or after) the time slot that corresponds to the packet's priority as determined by the device. Since collisions can still occur -- usually between packets of the same priority -- a contention resolution algorithm (CRA) based on a random back-off mechanism is implemented. This scheme
possesses characteristics that inherently impact the usability of the HPNA network for services requiring guaranteed QoS. Network collisions and the lack of centralized prioritization of packets means that although a mapping of IEEE 802.1D priority levels to HPNA priorities is provided, the resulting
network latency can't be guaranteed (higher priority "preempting" packets may affect the CRA completion time).

These limitations call for higher-level network synchronization -- a higher layer management entity that controls both the priority and timing of packet transmission at the MAC and PHY layers. Such a scheme enables the provision of quality-demanding services such as voice and video over phone lines.

The various layers of a cable IP telephony solution each play a role in providing the overall quality and class of service levels for voice calls. Though solutions for cable IP telephony QoS are multi-faceted, they are achievable.

Debbie Greenstreet is product management director at Telogy Networks, A Texas Instruments Company, and is responsible for TI's Voice over Cable Modem solution and has been managing Telogy's VoIP products for over three years.

Itay Sherman has worked for Texas Instruments for the last two years in the Cable Broadband Department in the capacity of Manager of TI activities in CableLabs. As part of this work he is deeply involved in the development of the recent specification for packet-based services over the cable system as well as the emerging home networking specification.

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