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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