Solving Cable Telephony Bandwidth Blues
BY LINDEN DeCARMO
Contrary to public perception, cable networks do not have infinite
bandwidth. In reality, they contain serious bottlenecks that can cripple
telephony applications. In this article, we'll explore the architecture of
cable networks, reveal their limitations for cable telephony, and explore
how the PacketCable standard proposes to solve these bandwidth issues.
Cable companies, also known as Multi-Service Operators (MSOs),
have invested billions of dollars in deploying coaxial (or coax) networks.
Unfortunately, coax is overwhelmed by the transportation demands of
packet-based multimedia content. Therefore, cable companies have been migrating to
Hybrid Fiber Coaxial (HFC). HFC is called a hybrid since portions of
the network remain coax, while performance-critical sections use higher-capacity fiber.
MSOs chose HFC because it has greater bandwidth, is more reliable than
coax, and most importantly, does not require that consumers' homes be
rewired. While HFC has significant advantages for multimedia transport,
the way it is deployed in cable environments can be problematic for
To reduce cost and complexity, MSOs use shared bandwidth for their HFC
networks. Shared bandwidth networks transport a fixed quantity of data
that cannot be adjusted as more devices are added to the network.
Therefore, the performance of these networks is inversely related to the
number of network users (a greater number of users must battle over fixed bandwidth).
Cable networks also operate in broadcast mode:
packets transmitted on HFC are broadcast (or simultaneously made
visible) to all computers on the network. Consequently, broadcast networks
require an arbitration scheme since only one device should broadcast at a
time. If there is no arbitration, two or more devices may try to
simultaneously transmit a packet (this is known as a collision). Network
administrators dread collisions because they degrade performance by
requiring retransmission of the affected packets.
Figure 1. HFC uses
broadcasts to transport data.
Only one device at a time can broadcast a packet.
The Ultimate Supervisor
The combination of shared bandwidth and broadcast transportation makes
cable HFC networks vulnerable to hackers. Therefore, access to the network
is controlled by a Cable Modem Termination System (CMTS).
Figure 2. The CMTS
controls access to the HFC network.
The CMTS's responsibilities include time slot enforcement, bandwidth
allocation, and Quality of Service (QoS) management. To prevent transmission
collisions, the CMTS allocates a time slot to each network device. During
this time slot, only the assigned network device can transmit data.
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
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.
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.
Figure 4. The MTA is
responsible for reserving and
committing bandwidth on the CMTS.
So far, we've concentrated on cable telephony QoS. However, these QoS
principles are equally applicable to video conferencing, gaming, or other
multimedia services. Unfortunately, these services may degrade network
throughput since they siphon bandwidth from lower-priority applications.
Therefore, MSOs should quantify the performance impact of a new QoS-based
feature before it is deployed.
Although cable networks are capable of transporting large quantities of
data, they have weaknesses when used for telephony. Two of the most
notable limitations are shared bandwidth and broadcast communication. The
CMTS circumvents these limitations by allocating bandwidth, enforcing
bandwidth caps, and providing QoS to high priority applications.
The importance of the CMTS will increase as it expands its role from
data monitor to QoS enforcer. After this transition is complete, HFC
networks will be a robust platform for cable telephony.
Linden deCarmo is a senior software engineer at NetSpeak
Corporation where he develops advanced call agent software. He can be
reached via e-mail at firstname.lastname@example.org.
A pioneer in Voice over IP (VoIP) network and infrastructure management
solutions, NetSpeak is a leading developer and marketer of advanced
telephony software for IP networks. NetSpeak's protocol-independent iTEL
software architecture, designed to meet the rapidly evolving business
needs of its Service Provider and Enterprise customers, delivers efficient
management of network resources and enables cost-effective introduction of
new VoIP applications.