Telephony over Cable: Using IP

High
speed Internet and remote data access services are growing at a
phenomenal rate that has spurred considerable interest in
broadband technologies. Major efforts are currently underway to
provide greater bandwidth to users by a multitude of access
technologies. While the debate over which architecture is best
to support such services will continue, the only confident
prediction is that the requirement for capacity and bandwidth
will grow. In the longer run, the voice, video and data networks
will converge and become digital (including current analog
entertainment networks). The hybrid fibre coax (HFC)
technology promises to meet this requirement of high capacity in
last mile access sub-network. It also brings a tremendous
opportunity for cable operators to offer bundled
telecommunication services covering all forms of entertainment,
e-commerce, and communications.

Realizing the huge revenue
potential that exists in cable services from a large base of
cable-wired homes, several industry efforts have been underway
which has developed cable data network standards and
specifications such as Data Over Cable Service Interface
Specification (DOCSIS) and PacketCable. CableLabs (Cable
Television Laboratories), a R&D consortium formed by cable
operators to fund, plan, research, and develop technologies for
CATV industry, and to distribute that technology to its members
and industry manufacturers has been very active in this effort.
One of the major objectives of these specifications is to be
vendor-independent and remain interoperable. In the following
section, a detailed explanation is provided for the functional
architecture of telephony over cable as per PacketCable
specifications.

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For telephony service, consumers do not know the difference
between circuit switched or IP-based packet networks; nor do
they care. They simply expect reliable "lifeline"
service along with Custom Local Area Signaling Services (CLASS)
features such as call waiting, call forwarding, caller
identification, etc. To achieve such reliable voice
communication on IP-based network, there are two call signaling
architectures being defined by PacketCable at CableLabs. These
are NCS (Network-based Call Signaling) and DCS (Distributed Call
Signaling). The NCS architecture is based on centralized call
control between network elements and endpoints. Whereas DCS
architecture is based on distributed call control. This is
explained below in detail with network architecture and
telephone call flow.

Network
Architecture

The
figure shows end-to-end network architecture for telephony over
cable using IP technology. It can be broadly divided into three
sub-networks: subscriber premises network, HFC plant, and
packet-based cable network. Other sub-networks with which it
interacts are Operations Support System (OSS) network, Internet
network, Signaling System # 7 (SS7) network and Public Switched
Telephone Network (PSTN). Subscriber premise network

This
sub network consists of the home network and subscriber
equipment. Subscriber premise equipment includes Multimedia
Terminal Adapters (MTAs), Cable Modems (CMs), analog and digital
phones, one or more PCs, televisions. The MTA provides
interfaces to basic telephone units. In the downstream
direction, the MTA converts the voice IP packets, received from
the CM, into analog circuit lines and delivers them to telephone
sets via a normal tip/ring metallic interface. In the upstream
direction, the MTA converts the analog signal coming from the
analog phones into baseband IP packets; these packets are sent
to the CM. The CM is responsible for converting the baseband IP
packets into digital radio frequency (RF) form for transmission
over the cable system. Typical cable modem systems have several
hundred Kbps of upstream bandwidth modulated using QAM
techniques and several Mbps of downstream bandwidth modulated
using 64QAM or 256QAM. The multimedia architecture built upon
cable modems may also have Ethernet digital port for interfacing
with personal computers to provide the high speed Internet
service and with other IP packet sources to provide services
such as videoconferencing. In addition, the CM may also be
connected directly to IP telephones.

HFC sub-network

This sub-network is an
important outside plant network, which connects the core network
to customer premises. At the edge of the HFC plant there are two
elements: the Cable Modem (CM) and the Cable Modem Termination
System (CMTS). The CM is located at the edge of the subscriber
premises and the HFC plant. The CMTS is located between the
packet-based cable network and the HFC plant. The CMTS is
physically located at the distribution hubs or headends. The CM
provides the termination point and the conversion point between
IP over RF. CMTS and CMs communicate to each other using the
CableLabs DOCSIS standards. The CMTS interfaces with routers to
provide IP connectivity to data networks and to call agents in
the packet-base cable network. It can be located in distribution
hubs, headends or elsewhere within the network.

The packet-base cable network

It houses a number of
network elements that are critical in the implementation of IP
telephony. These network element are made of a number of
functional components including: Call Management System (CMS),
Trunking Gateways (TGW), Announcement Servers (ANS), Domain Name
Servers (DNS), Media Gateway (MG), Media Gateway Controller (MGC),
PSTN Gateway, Edge Routers, QoS Policy Servers, DHCP Server,
Element Management System (NMS). NCS Call Flow

In
case of NCS-based IP telephony, the endpoint devices minimally
participate in call control signaling. Most of the
intelligence of the network resides in network operator’s
central devices (call management systems). The call signaling
uses the path between CMTS (Router) and the CMS, and the bearer
channel uses the direct path between the Router and the MG. One
advantage of such approach is that call features can be added or
modified by introducing changes in a few call control devices
instead of being added to the many endpoint devices.

The telephone call from
the PacketCable phone to the PSTN takes the following steps in
the NCS protocol. All messages are acknowledged, but the
acknowledgements are not listed.

• When the
  telephone goes off-hook, the MTA plays dial tone, collects
  the digits as per the specified digit map, and then send a
  notify message (NTFY) to the CMS indicating the user has
  gone off-hook and dialled the included digits.
  
  

• The CMS uses
  the digits dialled to determine the route and destination
  for the call. After the authorization is performed and it is
  determined that the call request is valid and routable, the
  CMS sends a message to the MTAo requesting that it create a
  connection entry. In the same message the CMS indicates that
  it wants to be notified if the user places the phone back
  on-hook. The response to the created connection request
  includes information about the Internet Protocol (IP)
  address and the Real Time Protocol (RTP) port numbers and
  parameters to be used by the MTAo. 4. The CMS sends the
  appropriate messaging to deliver the call to the PSTN
  gateway call agent (CMST).
  
  

• Meanwhile,
  the MTAo signals a request (CREATE_SERVICE_FLOW request) to
  the cable modem. This causes the CMo to interact with CMTS
  to reserve bandwidth on the local access cable.
  
  

• The MTAo
  issues a request (ACTIVATE_SERVICE_FLOW.request) to the CMo
  seeking to activate the service flow.
  
  

• When the MGC
  receives the request to route a call to the PSTN, it signals
  to the Signaling Gateway (SG) requesting an SS7-ISUP Initial
  Address Message (IAM) be sent to the PSTN.
  
  

• Simultaneously,
  the MGC signals to the MG requesting that a particular
  channel be allocated, and indicating the IP address and RTP
  port of the MTAo to be communicated with. The MG is
  instructed to make this connection operational (SendReceive)
  immediately. The response from the MG to the MGC includes
  information about the IP address and the RTP port numbers
  and parameters to be used by the MG.
  
  

• The MGC then
  communicates the IP address and RTP information back to the
  CMS.

The CMS sends
a message to the MTAo providing it with the IP and RTP
information that is needed to establish communication.



• When the
  SS7-ISUP Address Complete Message (ACM) message arrives from
  the PSTN, it is passed from the SG to the MGC, and the
  information is relayed to CMS.
  
  

• CMS sends a
  message to the MTAo repeating its request to be notified if
  the user goes on-hook and asks the MTAo to play ring-back to
  the user.
  
  

• When the
  SS7-ISUP answer message (ANM) arrives from the PSTN, it is
  passed from the SG to the MGC and the information is relayed
  to the CMS.
  
  

• CMS sends a
  message to the MTAo telling it to cease playing ring-back
  and to change to SendReceive mode. This message includes the
  request to be notified when the user hangs up.

The users are now talking.
Call termination requires steps to delete the connections at the
MTAo and at the MG, to signal call termination through the SG,
and to release the service flow between the cable modem and the
CMTS. DCS Call Flow

In
case of IP telephony under the DCS signaling, the endpoint
devices have an important roll in call control signaling. The
intelligence of these devices becomes significant. A part of the
intelligence of the network will reside in the MTAs. One
advantage of such approach is that Gate Controller (GC) has much
less responsibility than its counterpart in NCS (i.e., CMS).
This allows for the GC to be able to manage more MTAs than the
CMS. One disadvantage of this architecture is that when call
features are added or modified, all MTA will need to be upgraded
(via software download).

As in NCS, the current DCS
specification supports both embedded client model (a model in
which the MTA and the cable modem are built into the same box)
and the stand-alone MTA.

The figure shows main
functional components that support IP telephony in this
architecture. Note that GC is the DCS name for the CMS (in NCS),
but the responsibilities of the former are minimal with respect
to the latter’s. The major functional components are MTA,
Media Server (MS), and the GC.

• The telephone
  call from the PacketCable phone to the PSTN takes the
  following steps in the DCS protocol.
  
  

• MTAo detects
  off-hook condition, gives dialtone, collects digits until
  complete number has been entered, and sends an INVITE
  message to GCo.
  
  

• GCo
  authenticates MTAo and verifies the calling number belongs
  to MTAo that is authorized for originating service. GCo
  consults a directory server and determines that it must pass
  the call to GCT.
  
  

• GCT.
  authenticates the message from GCo, consults a directory
  server and determinates the call should be handled by MGC.
  GCT sends an enhanced version of INVITE (no ring) to MGC.
  
  

• MGC
  authenticates that the sender was GCT. It examines the
  destination number, and determines the proper MG to receive
  this call. MGC send a Create_Connection message to MG.
  
  

• MG reserves
  the PSTN circuit and IP port.
  
  

• MGC receives
  the acknowledgement from MG and sends a status message to
  GCT.
  
  

• GCT. then
  receives the message from MGC and sends it to Gco.

GCo sends a
message to MTAo.



• MTAo sends
  acknowledgement message directly to MGC at the address given
  in the contact header.
  
  

• The MGC, upon
  receipt of the acknowledgement message from MTAo, attempts
  to reserve the network resources necessary for the call. The
  MGC sends a command the MG to perform the resource
  reservation. At the same time, MTAo attempts to reserve the
  network resources. If resource reservation is successful,
  MTAo sends an INVITE (ring) message directly to MGC.
  
  

• Once MGC
  receives the INVITE (ring) and has successfully reserved
  network resources. MGC signals call to the PSTN over the SS7
  network (via the SG).
  
  

• PSTN responds
  to the IAM with ACM. SG forwards ACM to MGC. MGC sends
  Ringing to MTAo.
  
  

• PSTN confirms
  connection. SG sends confirmation to MGC. MGC commits to
  resources.
  
  

• MGC send
  acknowledgement directly to MTAo. MTAo commits to network
  resources and sends acknowledge to MGC. Simultaneously, MGC
  send command to MG to open two-way RTP channel.

At this point the call is
in progress. Call termination requires steps to delete the
connections at the MTA and at the MG, to signal call termination
through the SG, and to release the service flow between the
cable modem and the CMTS.

PacketCable standard
enables IP-based delivery and brings the greatest opportunity to
combine voice, video and data on HFC networks. Trials of IP
telephony using PacketCable standard over HFC have been
successful. As the communication services converge, HFC becomes
the preferred access technology to deliver bundled broadband
services for all types of networks, either built on Greenfield
or expanded/ converted from existing infrastructure. By offering
new two-way digital services, cable operators can also make
inroads into the voice telephony and data market while
preserving core entertainment businesses. However, the IP-based
telephony on cable must mirror availability and reliability of
circuit phone. With this reliability, consumers are likely to
accept HFC-based telephony as their primary phone service.