>THIS IS
THE WAY
>THIS IS
White Paper
The packet interconnect opportunity for MSOs
Author: Peter Krautle, Senior Advisor, Voice and Multimedia Global MSO Solutions, Nortel
Abstract
Interconnection of the Multiple Service Operator (MSO) Voice over Internet Protocol (VoIP) network to other carriers’ networks must be well planned and executed to ensure both that service revenue grows as quickly as possible and that extraneous costs are driven out of the network.
Most cable operators are implementing Time Division Multiplexing (TDM) interconnections from their softswitches to the Public Switched Telephone Network (PSTN). This approach is widely deployed and reliable, but has two shortcomings:
>Each Packet to TDM hop introduces a minimum of 20 milliseconds of delay due to encoding, echo cancella-tion, voice activity, de-jitter and IP delay overhead. The delay budget for a local or long-distance call can accom-modate this; however, it can be a problem for cellular or international calls which need a larger delay budget.
>Every off-network VoIP call drives the need for capacity on media gateways, representing a significant cost element for the MSO.
Some MSOs are planning packet inter-connections into carrier networks for off-network traffic to the PSTN, deliv-ering both technology advantages as well as enabling new, cost-effective busi-ness arrangements. Specifically, packet-to-packet interconnection introduces minimal delay to the voice bearer path connection and does not require media gateway capacity. The added advantage
of packet interconnection is the poten-tial to bypass the Inter-eXChange (IXC) network by leveraging the carrier’s IP backbone to connect directly to local tandems. For some off-network calls, the PSTN can be completely avoided when both an MSO and broadband service provider are using packet inter-connections into the same carrier network. In this scenario, the bearer path from a Media Terminal Adapter (MTA) in an MSO network flows directly to an Analog Terminal Adapter (ATA) in a broadband service provider network.
To be successful with packet intercon-nection, cable operators need solutions where:
>The MSO softswitches are capable of Session Initiation Protocol (SIP) or SIP for Telephony (SIP-T) inter-working with the carrier network
>The SIP/SIP-T standards on MSO softswitches aligns with the SIP/SIP-T implementation on the selected carrier’s softswitch
>The selected carrier has good PSTN connectivity into Rate Center and IXC tandems where cable systems targeted for VoIP deployment are located This paper will discuss all these elements in greater depth and describe a method-ology for planning packet interconnec-tions between MSO and carrier networks. About the author
Peter Krautle is currently Senior Advisor for Nortel in New York, providing network architecture support to MSOs globally for voice and multimedia solu-tions. His 23-year technical and manage-ment career at Nortel and General Motors covers numerous areas of Information Technology, Product Development, Systems Architectures and Customer Technology Solutions planning. His first exposure to MSOs began in 1995 when managing the development of a new cable modem for Nortel’s Cornerstone data products. Peter holds a Bachelors and Masters of Computer Science from the University of New Brunswick, and an executive MBA from Queen’s University in Kingston, Ontario. Acknowledgements
Many thanks to John Atkinson, Xuewen Li, Brian Lindsay, Rob Wood, Elaine Smiles, Parviz Rashidi, Ahsanuddin Mohammed, Emily Fung, Maurice Romaniuk, Chuck Coleman, Jim
Cermak, Rick Walczak, Graeme Currie, Bob Cirillo and Jim Gelsomini for your support. Special thanks to Roy Perry and Tom Buttermore for your commitment. Introduction
The traditional way to connect from the public PSTN to PacketCable-compliant softswitches is via TDM-based inter-machine trunks (IMTs) and TDM signaling through the SS7 network. TDM handoffs to local and long-distance carriers for voice signaling and bearer path traffic are a mature approach, with well-defined interfaces and hand-offs between softswitches as well as legacy TDM switches. However, there are potential shortcomings to the approach, including:
>Each packet to TDM conversion through a media gateway adds approx-imately 20 milliseconds1of delay. The additive effort of these conversions can lead to voice quality issues on some types of calls (e.g., cellular, interna-tional long distance).
>Every TDM circuit outbound from a packet network drives the need for capacity on a media gateway — a significant element of cost in a soft-switching solution.
>TDM facilities for trunks and SS7 signaling must be dedicated, and cannot be shared with other applica-tions. With packet interconnections for voice, IP bandwidth not used by voice can be leveraged by data applica-tions and improves the facilities utilization between MSO and carrier networks.
>Packet inter-connectivity makes it easier from a technical and regulatory perspective for MSOs to hand off voice calls between each other, bypassing the PSTN and changing the economics for interconnection costs.
Today’s early adopters of packet inter-connections are using SIP or SIP-T for call signaling connectivity between their own softswitches and application servers (e.g., voice mail server) as well as soft-switches located in carrier networks. The standards for SIP and SIP-T are still rela-tively new, and subject to interpretation by design teams. Furthermore, because some services do not map transparently today with packet interconnections, there can be some minor behavioral differences between VoIP features using TDM versus SIP signaling.
Although the benefits of packet inter-connection between softswitches and carrier networks are substantial, imple-menting the solution requires a moti-vated MSO, recognition that upfront planning is required to minimize inter-operability issues, flexibility to adjust service offerings based on current SIP implementations, and strong collabora-tion between the MSO and vendors. The planning of packet interconnections between MSO softswitches and carrier networks requires the following: >A motivated MSO who can bring the
carrier and vendors together to work through the interoperability problems and inevitable roadblocks in a timely fashion
>A certification/test environment to resolve standards interpretation and implementation issues
This paper first begins by outlining general TDM and packet interconnec-tion models between MSO and carrier networks for softswitches, and then addresses the steps needed to plan for successful packet interconnections.
1 Packet delay is dependent on packetization. If a 10-millisecond packetization rate is used, a minimum of 10-milliseconds encoding delay is introduced. However,
other factors contributing to packetization delay such as echo cancellation, voice activity, de-jitter buffers and IP network delay also play a role. For the purposes of this paper, a 20-millisecond average packetization delay will be used for packet-to-TDM conversion.
Traditional TDM intercon-nections into the public PSTN network
Figure 1 provides a simplified view of traditional TDM connectivity for MSO PacketCable-compliant softswitches communicating with the Public Switched Telephone Network (PSTN). MSO planning engineers must first work with their marketing groups to identify the cable systems where VoIP service will be offered. These cable systems are then correlated to PSTN rate centers for determining local, intra-lata, inter-lata, 911 PSAP and operator services tandems. The identified tandems may also be serviced by multiple carriers with differing pricing strategies for facilities and interconnections — therefore, an analysis of circuit costs and the avail-ability of facilities is normally undertaken to decide on final carriers to be used. Based on subscriber forecasts provided by the MSO’s marketing organization on a per-cable system basis, trunk circuits are then engineered and ordered for carrying off-net calls between the soft-switch media gateways and the PSTN. The SS7 network is used between softswitches and carrier voice switches to signal incoming/outgoing call informa-tion such as the calling number of the
subscriber and the trunk that the calling switch has used to reserve the telephony call. The receiving switch also uses the SS7 network to request services infor-mation such as calling name (CNAM), local number portability (LNP) and toll-free (800/900) data from the PSTN. At the early stages of VoIP network deploy-ment, MSOs typically order a pair of SS7 Access links (A-links) from their softswitch to geographically diverse Signal Transfer Points (STPs) provided by an SS7 carrier.
Key advantages for MSOs using tradi-tional TDM interconnections into carrier PSTN switches include:
>The TDM interfaces and connectivity into the PSTN are well-defined from both a technical and regulatory perspective. For example, all major carriers in the United States and Canada today have wholesale divisions servicing facilities-based Competitive Local Exchange Carriers (CLECs). Provided an MSO files as a CLEC, the process of ordering interconnec-tion circuits is straight-forward. >Telecommunications engineers and
operations personnel are familiar with engineering and operating these types of interconnections.
>Traditional TDM interconnections offer the best opportunity for seamless feature transparency to MSO VoIP subscribers. Packet interconnections between networks can introduce minor differences to the behavior of some features when compared to their usage on TDM switches.
There are also some shortcomings with using traditional TDM interconnections into carrier switches. From a technical perspective, every analog-digital conver-sion of the bearer path by media gateways adds approximately 20 milliseconds to the end-end one-way delay budget required for toll-quality VoIP conversations (typi-cally in the 150 to 180-millisecond range). For North American calls that originate and terminate on wireline TDM telephony endpoints, this added IP/PSTN conversion delay is normally not an issue. However, the delay budget becomes more problematic with a call forwarded multiple times as well as some cellular and international calls. From a commercial perspective, there are PSTN trunk and SS7 facility costs, reciprocal costs for terminating calls, and the cost of intra- and inter-LATA long distance costs associated with MSOs connecting their softswitches using classical PSTN TDM approaches. TDM trunks to the PSTN are also dedicated circuits, and cannot be shared with other network applications (e.g., high-speed Internet) when the trunks are under-utilized. There are also network service costs such as per-call query charges to the Local Number Portability (LNP) and Calling Name (CNAM) databases. With many MSOs entering the market with a bundled VoIP offering that includes calling name display and North American long distance for a fixed monthly charge, long distance and CNAM query charges increase significantly as the MSO VoIP subscriber base grows. The success of MSOs who have launched residential VoIP service in attracting subscribers is driving the need for alter-native connections to PSTN softswitches.
Figure 1. Traditional connectivity to public voice carriers from softswitches
LIDB Tollfree LNP CNAM SS7 SS7 SS7 SS7 SS7
IP voice bearer path TDM voice bearer path SS7 inter-switch signaling
MSO network Carrier network
Local tandems (rate centers) 911 LD/IXC OS SS7 IP TGCP ISUP ISUP ISUP/ MF ISUP/ MF CMS MG NCS NCS MTA MTA CMTS CMTS HFC
In a rapidly-growing VoIP network using TDM trunk interconnects, some MSOs have deployed multiple softswitches, each with traditional TDM interconnec-tions into the PSTN (see Figure 2). Because of the lack of early availability of inter-call server signaling via SIP by some manufacturers, an MSO ‘on-network’ call from an MTA on one softswitch (CMSA) to another softswitch (CMSB) must be hair-pinned through the media gateway. Depending on the sophistication and feature richness of the translations and routing algorithms within the soft-switch, the IP bearer path for this call may actually traverse a PSTN tandem switch, incurring potential reciprocal or long-distance charges. Because the bearer path between this MSO on-net to on-net call traverses two media gate-ways, the end-to-end delay budget for this call has increased by 40 milliseconds, and may present voice quality issues if one of the MTAs has forwarded their VoIP phone to a cell phone for incoming out-of-region cellular or international calls.
Finally, without SIP signaling between MSO softswitches enabled, call signaling must occur through the SS7 network. As the VoIP subscriber base increases, multiple softswitches will be deployed,
with each softswitch requiring redundant SS7 links and linksets. As these SS7 costs continue to increase, the MSO must either move to SIP signaling between softswitches, or explore use of deploying a standalone Signaling Transfer Point (STP) in their network. The STP aggregates SS7 traffic from MSO softswitches onto an external set of T1s with multiple linksets to the public SS7 network.
The emergence of SIP-based packet interconnections on softswitches and deployment models
SIP-based packet interconnections have been introduced on most manufacturers’ softswitches for signaling calls between call servers — some as early as 2001. Early adopters of SIP packet intercon-nections were implemented using draft versions of the IETF SIP standards. The initial requirement placed upon early adopters of SIP packet interconnections was to solve the problem of signaling calls to other softswitches or application servers within the MSO network without having the IP bearer path trombone through an IP/TDM Media Gateway. As SIP trunk signaling began to mature for voice, manufacturers with
pre-stan-dards versions of SIP implemented programs to align with the standardized versions of SIP, starting with RFC 3261 in late 2002. Other softswitch manufac-turers who implemented SIP packet interconnections later worked directly from the standards-track IETF RFCs. Public carriers who were early to market with SIP line and trunk services also implemented parts of their network using pre-standards versions of SIP, and have been upgrading their solution through a combination of planned programs and customer opportunities. The advantages of SIP-based packet interconnections on MSO softswitches are significant:
>Ability for a softswitch to signal calls to another softswitch without requiring the public SS7 network or using a Signaling Transfer Point (STP). Using Figure 3, if CMS(A) and CMS(B) are using SIP signaling between each other, the bearer path can be kept on-network with an RTP stream being established directly between the two MTAs without going through the media gateways and the PSTN. >Greater flexibility in forming
inter-connection agreements with carriers and other MSOs that allows PSTN media gateway connectivity to occur outside the MSO’s initial network. This also facilitates VoIP bearer path exchange between MSOs, and reduces the purchase of PSTN services from competitors.
>Ability to manage the end-end delay budget more effectively as more traffic is kept on-network and not forwarded to the PSTN.
>As MSO VoIP networks grow, SIP-based packet interconnections can help reduce the expense in media gateway ports.
Figure 2. Bearer-path flow for on-net to on-net calls for some MSOs with multiple softswitches
SS7
IP voice bearer path TDM voice bearer path SS7 inter-switch signaling
MSO network Carrier network Local tandem IP CMS(A) NCS NCS MTA MTA CMTS CMTS HFC MTA MTA NCS NCS HFC CMTS CMTS CMS(B) MG-A MG-B
Figure 3 represents the simplest form of SIP packet interconnect programs between MSO softswitches which lever-ages the IP core backbone used for high-speed data services. Dedicated bandwidth for TDM voice trunks between Class 5 switches or softswitches that use the SS7 network and media gateways to hairpin calls is replaced by an RTP/IP bearer path stream between MTAs or media gateways that are controlled by separate softswitches. The IP network is engi-neered to prioritize VoIP traffic over high-speed data traffic, and unused VoIP bandwidth can be used by other lower priority applications.
Softswitches should support both SIP signaling for packet interconnections between MSO softswitches as well as SS7 connectivity for calls destined for the PSTN. In Figure 3, both softswitches CMS(A) and CMS(B) have SS7 links to the PSTN as well as SIP signaling to support packet interconnections. This flexibility allows an MSO to leverage its core IP network to direct off-net calls to the nearest media gateway supporting a subscriber within a specific rate center. For example, a subscriber controlled by CMS(B) and located in LATA2 placing an off-net call to a subscriber located in LATA1 can configure its routing and translations tables so that the voice bearer path from the subscriber’s MTA can be directed to media gateway A (MG-A) controlled by CMS(A). This approach allows an MSO to control its long-distance charges by leveraging its IP backbone network to bypass the interconnect exchange carriers (IXCs) and local carriers for a percentage of long-distance calls and direct these calls directly to the local tandems in each LATA or major rate center.
Figure 4 represents two MSO softswitches in a local ‘Class 5’ and long-distance ‘Class 4’ configuration. In this configu-ration, all local lines serviced from the HFC access network are connected to the Class 5 softswitch [CMS(A)]. As well, connectivity to tandems for local
off-network calls is also supported from the Class 5 softswitch. The Class 5 routing and translations tables are configured to understand which numbers are local or ported in, and which off-network calls can be forwarded to local tandems. All other calls traditionally considered as long-distance are forwarded to the Class 4 softswitch [via SIP signaling from CMS(A) to CMS(B)] for further treatment.
The MSO should design their VoIP network to place media gateways in major LATAs where long distance traffic is being generated and where there is affordable access to IP transport. For example, an MSO may determine that there is significant long-distance traffic to Chicago from its cable systems in Nashville and that the MSO already has reliable IP transport for Internet peering between Nashville and Chicago. By
Figure 3. Flow of bearer path for MSO on-net and off-net calls when SIP signaling is used between softswitches
Figure 4. Softswitches using SIP for inter-CMS communications in a Class 4/5 configuration
SS7
IP voice bearer path on-net to on-net IP voice bearer path on-net to off-net SIP inter-switch signaling SS7 inter-switch signaling MSO network Carrier network
Local tandem rate center LATA 1 IP CMS(A) NCS NCS MTA MTA CMTS CMTS HFC MTA MTA NCS NCS HFC CMTS CMTS CMS(B) MG-A MG-B SIP Local tandem rate center LATA 2 SS7
IP voice bearer path SIP inter-switch signaling SS7 inter-switch signaling
MSO network Carrier network Intra-LATA tandems IP CMS(A) HFC MTA MTA NCS NCS CMTS CMTS CMS(B) Local Class 5 SIP Long distance Class 4 LD/IXC tandems Local tandem 1 Local tandem 2
placing a media gateway in Chicago connected to the Class 4 softswitch, the network can be engineered so that off-net calls between Nashville VoIP subscribers and Chicago TDM voice subscribers are carried over its internal IP transport network, therefore bypassing the Inter-exchange Carrier (IXC) and reducing long-distance charges.
The MSO may also be legislated to offer their VoIP subscribers a choice of long-distance providers. In the Class 4/5 softswitch configuration, trunks to each IXC carrier would be connected to the Class 4 softswitch, with the Carrier Interexchange Code (CIC) for the subscriber specified as part of the line profile on the Class 5 softswitch. Both the Class 4 and 5 softswitches must also have translations and routing engines that can map CIC codes to lines and the correct long-distance trunks to the selected Interexchange Carrier. If the MSO deploys both Class 4 and 5 softswitches and develops several agree-ments with long distance providers to contain costs based on time-of-day use, the Class 4 softswitch with its least-cost routing functionality can be configured to use the least-cost long-distance trunks based on call attributes such as time-of-day and class of service.
Some MSOs have implemented or are considering a packet interconnect agree-ment with carriers that possess the following attributes:
>The MSO and carrier define a common meet-point for the exchange of all on-net to off-on-net traffic through facilities that are normally physically co-located. >The MSO softswitch is ordering
facili-ties to exactly one location — the carrier end of the meet-point. >The carrier is responsible for
engineer-ing all trunks from their side of the meet-point to the rate center tandems for each cable system serviced by the MSO softswitch. As well, appropriate trunk group connections to IXC, 911 and operator services tandems must also be engineered.
>The carrier is also responsible for assigning new telephone numbers (TNs) to the MSO softswitch, and managing local number portability requests on the MSO’s behalf. >In some cases, the MSO does not wish
to file CLEC status with the appro-priate regulatory bodies, but rather deliver VoIP services under the auspices of the carrier.
A common name given to the carrier for this type of interconnect model is the Voice Service Provider (VSP). Figure 5 illustrates the TDM-based Voice Service Provider for MSOs. The carrier and the MSO agree on the meet-point interface, and then based on the MSO’s VoIP expected growth in each serving cable system, the sizing of the meet-point facilities is engineered (e.g., DS-3, OC-3, OC-12, OC-48). These meet-point facilities are then connected to media gateways on the MSO side of the network, and either media gateways or TDM switches in the carrier’s network. The carrier then engineers trunk groups from the meet-point to the local tandems in each rate center where cable systems are located. Facilities are also engineered to IXC tandems for long-distance as well as operator services and 911 trunks. These facilities are mapped to trunks on the carrier side and then replicated on the MSO softswitch in its translation tables.
The advantages of implementing a TDM-based VSP model for MSOs include:
>Faster time to market for turning up a softswitch into deployment, particu-larly if the selected carrier already has existing underutilized trunk facilities into tandem PSTN switches. Ordering and turning up trunk facilities into PSTN tandem facilities can take considerable time.
>Well-defined TDM interfaces between the MSO and carrier network, allowing carriers to re-use existing infrastructure (people, tools) to operate and trouble-shoot the network.
>The carrier can provide the MSO with additional interconnection services such as assistance with regulatory filings, allocation of new telephone numbers and Local Number Portability.
Figure 5. TDM-based Voice Service Provider model for MSOs — off-net call
SS7
IP voice bearer path TDM voice bearer path SS7 inter-switch signaling MSO network Carrier network IP MSO CMS HFC MTA MTA NCS NCS CMTS CMTS MG Meet-Point CNAM LIDB Local Carrier CMS IP HFC MTA MTA NCS NCS CMTS CMTS Tollfree LNP LD/IXC 911 OS
MSOs have also encountered shortcom-ings in using the TDM-based Voice Service Provider approach in connecting their softswitches to the PSTN. The repeated transition of converting VoIP packets to TDM and back again adds to the delay budget, and contributes to overall voice quality degradation. In the Figure 5 example, the two media gate-ways at either end of the meet-point can add 40 milliseconds to the delay budget. Re-engineering of trunk group sizes as the network grows is expensive and is statically engineered, unlike the IP network where the cost of adding band-width to the network and moving endpoints is lower. Finally, the cost of trunk ports on the IP/TDM media gateways is a significant component of the overall end-to-end VoIP solution cost. Figure 6 provides a high-level view of an IP-based Voice Service Provider model that is implemented by some MSOs. The major differences between the TDM and IP Voice Service Provider interconnect models are:
>The elimination of IP/PSTN gateways at either end of the interconnection meet-point between the carrier and MSO network
>The removal of the SS7 TDM link between the MSO and PSTN SS7 network
Signaling of calls between the MSO and carrier softswitches is now done via SIP, and allows the bearer path for VoIP calls destined for the PSTN to go directly between the MSO MTAs and the Media Gateways controlled by the carrier network. The potential signal degrada-tion caused by repeated transidegrada-tions of the voice bearer path between IP and TDM is reduced, and media gateway costs are eliminated at either end of the interconnection meet-point. Because the interconnection meet-point is IP, the
MSO and carrier can re-use their existing IP backbone infrastructure and manage bandwidth between voice and data more efficiently. Finally, with the SS7 link no longer required between the MSO softswitch and carrier network, the public filing requirements for an MSO offering VoIP service are diminished. In essence, the MSO’s softswitch may be viewed as an extension of the carrier’s network from a regulatory perspective, with PSTN entities such as telephone numbers, SS7 point codes and Common Language Location Identifier (CLLI) codes owned by the carrier.2
As carriers and MSOs begin working with IP-based Voice Service Provider interconnection models and SIP signaling, issues such as security, NAT/firewall traversal, topology hiding, call admission control handling and endpoint inter-operability must be addressed. Each MSO and carrier solves these issues in a slightly different fashion, including some of the following approaches:
>Introduction of border gateway func-tionality where the softswitch incorpo-rates NAT/firewall and topology hiding capability for the SIP signaling between softswitches to maintain call control. A separate media portal is deployed to do border control for the RTP bearer path stream. The softswitch also exchanges call state information with the RTP media portal and gathers call statistics.
>A session border controller, which integrates signaling and bearer path border gateway functionality into the same platform.
>Use of routers, firewalls, intrusion detection devices and network address translation solutions to protect the edge of the MSO network and resolve conflicts in private address spaces.
Figure 6. IP-based Voice Service Provider model for MSOs — off-net call
2 For completeness, some MSOs have chosen not to operate their own voice switch, but outsource this capability to a carrier — this model is sometimes called a
hosted Voice over Telephony solution. The TDM or VoIP traffic from the MSO network is passed to the carrier network, which then operates the voice switching infrastructure on behalf of the MSO. In the hosted Voice over Telephony model, a voice switch is either dedicated to an MSO, or shared with multiple MSOs.
SS7
IP voice bearer path TDM voice bearer path SS7 inter-switch signaling MSO network Carrier network
IP MSO CMS HFC MTA MTA NCS NCS CMTS CMTS IP Meet-Point CNAM LIDB Local Carrier CMS IP HFC MTA MTA NCS NCS CMTS CMTS Tollfree LNP LD/IXC 911 OS
Figure 7 illustrates the IP-based Voice Service Provider model with border control functionality that protects both the carrier and MSO network. Each of the methodologies listed above are appropriate in specific circumstances for securing the network edge, translating IP addresses, hiding network topology, etc. Figure 7 also illustrates the SIP signaling path from an MSO softswitch to a SIP proxy server in the carrier network. Some carriers only permit SIP signaling into their network via a SIP proxy server that acts as a back-to-back user agent, and forwards the signaling to the correct softswitch.
Planning for SIP interconnec-tions and IP peering between VoIP networks
Packet interconnections between MSO and carrier softswitches require upfront planning and inter-working characteri-zation for some of the following reasons: >The SIP implementation on
soft-switches for packet interconnections is still early in the technology adoption phase for most MSOs and carriers.
Interoperability testing is required to characterize services and call behaviors between softswitches to ascertain standards are being interpreted and implemented in a similar fashion. >Features work transparently between
MSO and carrier softswitches. In the late 1980s, a number of Informa-tion Technology organizaInforma-tions for corporations who were implementing LAN-based networks tightly restricted the use of Ethernet cards as well as TCP/IP drivers to models that had been tested and certified in their interoper-ability labs. Because Ethernet was still relatively new and TCP/IP vendors implemented slightly different versions of the standards3, network administra-tors discovered that minor card/protocol stack incompatibilities or behaviors created network outages (broadcast storms, babbling cards, etc.). To mini-mize the number of network trouble incidents, Ethernet cards and drivers were certified in a lab environment to guarantee reliability and availability. Today, the maturity of Ethernet hard-ware and TCP/IP drivers has reached
the point where consumers can purchase these components commercially, and plug them into public and enterprise networks with confidence that they will operate reliably.
Like the Ethernet market segment in the 1980s, performing IP peering between MSO and VoIP networks using SIP signaling is still an emerging intercon-nection model and inherits similar inter-operability issues:
>With a suite of SIP standards now defined, softswitch vendors are either implementing or upgrading their SIP signaling capability between softswitches and are beginning interoperability testing. During the due diligence and planning phase for interoperability execution, some softswitch vendors have implemented different subsets of the SIP standards, or are at different versions for the same standards. >For PacketCable VoIP functionality,
there are minor differences in feature behavior when using SIP signaling and IP peering between MSO and carrier networks for VoIP traffic. >Carrier VoIP softswitches do not adhere to the PacketCable CMSS standards today for inter-softswitch communications. Therefore, early adopters of IP peering between MSO and carrier softswitches are adhering to the carriers’ implementation of SIP, and leveraging their breadth of TDM interconnections into the PSTN. >Border control to secure MSO and
carrier networks when doing packet interconnections is also an emerging technology solution. Depending on the type of border control implemen-tation, interoperability testing is also needed to characterize how calls behave from a service, reliability and availability perspective.
Figure 7. IP-based Voice Service Provider model with border control — off-net call
3 A typical problem in the 1980s was one TCP/IP vendor implementing IETF standards A, B, C and D, while another TCP/IP vendor implemented standards B,
C, D and E.
SS7
IP voice bearer path SIP inter-switch signaling SS7 inter-switch signaling MSO network Carrier network
IP MSO CMS HFC MTA MTA NCS NCS CMTS CMTS IP Meet-Point CNAM LIDB Local Carrier CMS IP HFC MTA MTA NCS NCS CMTS CMTS Tollfree LNP LD/IXC 911 OS Carrier SIP proxy Border
The first step in planning for packet interconnections is for an MSO to define their scope of planned feature offerings, both today and 12 to 24 months out (Figure 8). This input is important to help carriers and softswitch manufacturers map these capabilities to call flows using packet interconnections, identify areas where PSTN services are required (e.g., Calling Name, Calling Cards, T.38 Fax), and map these call flows to the appro-priate SIP standards. As well, the call flows are also important to understand how specific features will be handled when a call traverses between the MSO and carrier softswitch (e.g. T.38). The MSO will then request that the appropriate softswitch vendors conduct an analysis of packet interconnect inter-operability based on the current and future features, and report back on possible feature transparency issues. Some of the questions that arise during this analysis include:
>Will native SIP (RFC 3261) or Trans-parent SIP (RFC 3372) be used for call signaling between softswitches? >How will operator and 911 services be
handled?
>Will the Layer 3 protocol for SIP signaling be based on a reliable Trans-port Control Protocol (TCP) or Unidirectional Datagram Protocol (UDP)?
The first question addresses the seamless behavior of features on an MSO soft-switch versus the same features on a TDM switch, and how advanced network services information gets exchanged between MSO and carrier softswitches. To understand this point, some brief background regarding SS7 high-level signaling principals in a TDM world is in order. There are two primary message types used by SS7 to exchange informa-tion between softswitches:
>ISDN supplemental part (ISUP) — Protocol used in the SS7 network to signal incoming and outgoing calls between switches as well as the trunk used to connect/disconnect the call.
>Transaction Capabilities Application Part (TCAP) — The protocol used in the SS7 network for sending database queries (e.g., CNAM) to a Service Control Point (SCP).
In the PacketCable TDM model, if a subscriber on a TDM switch places a call to a VoIP subscriber on an MSO softswitch with calling name display activated, SS7 ISUP messages will be passed between the TDM switch and softswitch. The SS7 message exchange signals that there is an incoming call as well as the trunk that has been reserved for the bearer path between the two voice switches. Before the VoIP subscriber’s phone is notified to activate ringing on the phone, the MSO softswitch must first issue a TCAP message that queries the CNAM database. When the calling name information is returned, the infor-mation is passed to the MTA along with the signal to activate ringing on the phone for the incoming call. The decision of whether to use SIP (RFC 3261) versus SIP for Telephony (RFC 3372) for packet interconnection between MSOs and carriers for VoIP is primarily based on two determinants — the ability of the softswitches to support one or both protocols, and the degree of feature transparency expected by the MSO (Figure 9). With SIP for Telephony (SIP-T) signaling, the SIP headers are
Figure 9. SIP (RFC 3261) versus SIP-T (RFC 3372) signaling between softswitches for packet interconnect
Phase 1 Basic residential
features
• Single line VoIP • Calling Number and Name • Call forward
• 3-way Calling • Call Return • Call Waiting
• Anonymous Call Rejection • Call Return
• Local Number Portability • Voicemail
Phase 2 Enhanced residential
features
• Multi-line MTA support • Teen service —Local —Out-of-area • Call blocking —International —1-900
• Selective Call Acceptance • Selective Call Rejection • Long-distance Equal Access • SIP hard/soft endpoints • Videophones • Integrated multimedia Phase 3 Commercial features • Calling cards • Prepaid cards • Very small business
—Serviced by cable modem or Multi-port MTA —Multi-line hunting —Centrex customer groups —T.38 fax support • Enterprise —Fiber-fed —Centrex/IP • Business multimedia SS7 CMS(A) SIP CMS(B) SS7 PacketCable TDM interconnect Packet interconnect using SIP IP TDM IP TDM ISUP ISUP TCAP TCAP Inter-switch signaling • Call connect/disconnect (ISUP) • SS7 database queries (TCAP) Packet HDR SIP Header Payload
Inter-switch signaling • Call connect/disconnect (ISUP) • SS7 database queries (TCAP)
Packet HDR SIP Header Payload
SIP — for Telephony (RFC 3372 SIP-T) SIP — (RFC 3261)
used only for establishing a signaling session between the MSO and carrier softswitch endpoints. The SS7 payload is encapsulated entirely within a SIP header when transported between soft-switches, with the receiving softswitch typically stripping off the SIP header and passing the SS7 payload untouched to its SS7 signaling stack. The SS7 payload can then be decomposed appro-priately by the receiving softswitch. The advantage of this approach is that seam-less feature transparency with traditional TDM switches can be achieved. If SIP signaling is used (RFC 3261), the SS7 payload is decomposed by each softswitch and mapped into SIP headers (RFC 3398). Some mappings for specialized services such as operator hold, operator ringback and busy line verification are still in draft status within the IETF, with slight differences in implementation between each softswitch vendor. These operator and 911 service features are typically supported over MF trunks — unless the SS7 message format is reconstructed at the carrier softswitch side exactly, the ability to support this functionality is problematic. Whether these operator or 911 features are required by today’s modern operator services or 911 response centers4is open to discussion, as the feature set was created over 25 years ago when few tele-phones had displays and there was little capability for hard or soft call logs. If feature transparency for operator services and 911 features is a significant issue, the MSO can deploy a media gateway and direct SS7 connectivity for emer-gency support traffic while using SIP packet interconnections for all other calls. The SIP versus SIP-T differences are also important to understand when a TCAP QUERY is issued to retrieve calling name information from the SS7 network. Using the network topology on the right side of Figure 9, when
CMS(A) receives an off-net call, a TCAP QUERY request is issued to CMS(B) that retrieves the calling name information for the call originator. If SIP-T signaling is used between softswitches, then the TCAP QUERY information from CMS(A) is inserted unchanged within the payload of the SIP signaling packet to CMS(B). CMS(B) receives the TCAP QUERY payload from CMS(A), forwards the request to the SS7 network, retrieves the response and copies the response unchanged into the payload of the SIP response packet back to CMS(A). CMS(A) then takes the calling name information from the packet payload and forwards to the called subscriber for display on their phone. In effect, the TCAP QUERY request from CMS(A) (which has no direct connectivity to the SS7 network) is tunneled to CMS(B), who then strips away the SIP headers and forwards to the SS7 network. Using the same Figure 9 example and SIP signaling (RFC 3261), the TCAP QUERY fields for calling name display are mapped (via RFC 3398) to the SIP header by CMS(A) before being
forwarded to CMS(B). CMS(B) must then rebuild the TCAP QUERY request and forward to the SS7 to obtain the calling name information for the origi-nating subscriber. CMS(B) then must proxy the calling name information via the appropriate field in the SIP header when signaling the response back to CMS(A).
As MSOs expand their VoIP product offerings to service business subscribers, there may also be some feature compati-bility issues when native SIP signaling for packet interconnections (e.g., Centrex/IP endpoint from an MSO softswitch communicating with a SIP endpoint serviced from a feature server connected to a carrier softswitch). By identifying the features and working the call flows, many of the potential incom-patibilities can be identified and addressed during interoperability testing.
Figure 10. PacketCable CMSS in early packet interconnect deployments
SS7
CMSS inter-switch signaling SIP inter-switch signaling SS7 signaling
MSO network Carrier network MSO A CMS(A) HFC MTA MTA NCS NCS CMTS CMTS CNAM LIDB Local Carrier CMS IP HFC MTA MTA NCS NCS CMTS CMTS Tollfree LNP LD/IXC 911 OS CMS(B) MSO B
Strong commitment by MSOs to imple-ment packet interconnections with carriers helps bring together all constituents (MSO, carrier, softswitch vendors) in working through the poten-tial incompatibility issues, building and executing a comprehensive interoper-ability test plan, and then deploying the solution in a production network. PacketCable CMS to CMS Signaling (CMSS) is a profile of SIP and has been designed for intra- and well as inter-MSO signaling between softswitches. CMSS also has awareness of event message and PacketCable Legal Intercept functionality. All carrier softswitches have their own SIP profiles, and do not support PacketCable event messages and Legal Intercept architectures today. Therefore, MSO softswitches will need to support individual carrier SIP imple-mentations short-term in order to implement packet interconnections with the PSTN. MSOs with multiple softswitches in their network or wishing to use packet interconnections with other MSOs is the expected area where early implementations of CMSS will be deployed (Figure 10).
Conclusions
Packet interconnection between MSO softswitches and carriers is an emerging market segment, but represents the future direction of where voice and multimedia networks are headed. Although TDM connections to the PSTN will remain in place for many years, the MSO movement to packet interconnections will continue to grow in popularity for the following reasons: >More efficient use of voice and data
applications that runs on a converged IP network infrastructure.
>Reducing the frequency of Packet to TDM bearer path conversions (~20 milliseconds) through media gateways relieves pressure on the VoIP delay budget, and improves overall voice quality.
>MSOs launching voice networks may be able to get to market earlier with packet interconnections into carrier networks. The facilities efforts and regulatory skillsets required to turn up a network may be born by the carrier, allowing MSOs to place greater attention on growing their VoIP subscriber base.
>Packet interconnection offers a greater opportunity to avoid PSTN access and long transit costs — e.g., bypassing the PSTN for VoIP calls between MSO networks.
Like the emerging Ethernet market segment of the late 1980s, many of the interoperability issues associated with packet interconnections today are temporal and will become less of a concern over the next several years as SIP signaling and interoperability with other SIP, NCS, Media Gateway, Integrated Access Devices and analog terminal adapters improves. Those MSOs that are committed to using packet interconnections, work collabora-tively with carriers and vendors, and drive for high (but not complete) service transparency will be the first to enjoy the benefits.
The short-term issues for early adopters of packet interconnections with carriers include the following:
>Security and IP address management of MSO networks at the edge needs to be well thought through. A number of options are available to solve this problem including firewalls, intrusion detection devices and other border control mechanisms (e.g., session border controller).
>Interoperability of MTAs and other devices in the MSO network needs to be characterized with media gateways resident in the carrier network. It should also be noted that these media gateways may not be PacketCable-compliant.
>Other broadband service providers may also be using packet interconnec-tions with carriers; therefore, the bearer path for their endpoints (e.g., ATAs) may be communicating directly with MTAs in the MSO network. This interoperability should be charac-terized.
>There may be minor feature behav-ioral differences versus the TDM switching environment when using packet interconnections between softswitches, particularly for operator and 911 services. Those MSOs who require complete feature transparency for operator and 911 services may elect to deploy a media gateway for these services while using packet inter-connection for all other traffic. Finally, the emergence of packet inter-connections between MSO and carrier softswitches does not eliminate the need for IP/PSTN media gateways — TDM switches will continue to operate in today’s public networks for the foresee-able future. However, the increased use of packet interconnections by MSOs will shift the ownership of media gate-ways from MSO networks and towards carriers.
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