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.