Internet Engineering Task Force INTERNET-DRAFT Authors Transport Working Group R. Dalias Category: Informational J. Matousek April 1998 L. Ong Expires: November 1998 Bay Networks Bay Networks SS7-Internet Gateway Architecture Status of this Memo This document is an Internet-Draft. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as ``work in progress.'' To view the entire list of current Internet-Drafts, please check the "1id-abstracts.txt" listing contained in the Internet-Drafts Shadow Directories on ftp.is.co.za (Africa), ftp.nordu.net (Northern Europe), ftp.nis.garr.it (Southern Europe), munnari.oz.au (Pacific Rim), ftp.ietf.org (US East Coast), or ftp.isi.edu (US West Coast). Distribution of this document is unlimited. Abstract This memo describes the Bay Networks Gateway architecture for interworking of PSTN SS7 with Internet. Signaling System 7 (SS7) networking is the primary means used in the PSTN for control of circuit-switched connections and value added PSTN services such as freephone (800/888) number translation, calling card validation and Intelligent Network services. The Gateway architecture provides a scalable method of supporting interworking between SS7 network elements and Internet elements such as a Remote Access Server (RAS). The Gateway architecture can support connection control and database access. Gateway design, functions and protocol are described. Dalias, et al Informational [Page 1] INTERNET-DRAFT draft-ong-ss7-internet-gateway-00.txt March 1998 Table of Contents 1. Introduction.................................................2 2. Applications.................................................3 2.1 Call Control................................................3 2.2 Data Base Applications.....................................4 2.3 VOIP (Voice over IP).......................................5 3. Gateway Architecture.........................................5 3.1 Gateway Design.............................................5 3.2 Gateway Functions..........................................6 3.3 Gateway Protocol...........................................7 3.4 Advantages.................................................8 Acronyms.......................................................11 Contact Information............................................11 1. Introduction Signaling System 7 (SS7) is the protocol that supports signaling between telecom network elements, such as switches and service control points. SS7 is in operation throughout the world linking the telecom switching infrastructure. SS7 is used to support many functions, including basic call control, for which it provides essential functions, and call supplementary services such as number translation and calling card validation. .............. ........................ . . . +------+ . . SS7 | / | . . Network | STP | . . | / | . . +------+ . .............../........\.............. / \ / A-link \ A-link / \ / \ +------+ +------+ | PSTN | TDM | PSTN | ----|Switch|--------------|Switch|---- +------+ Circuits +------+ Figure 1: SS7 Architecture for PSTN A gateway to the SS7 network is an essential element to the integration of telecom networks and the Internet that will allow Dalias, et al Informational [Page 2] INTERNET-DRAFT draft-ong-ss7-internet-gateway-00.txt March 1998 users to operate in a seamless environment for voice and data services. By accessing the telecom network with SS7, data network elements fit cleanly into the telecom network infrastructure as peer switches and control points and can exchange information with telecom network elements for cleaner routing and treatment of connections. This memo describes the architecture for an SS7 to Internet gateway, as implemented for a Bay Networks remote access server (RAS). The memo discusses the gateway design and functions, the protocol used between the gateway and the RAS, and the advantages of the design. Protocol functions include connection setup between telecom switch and RAS, registration and status information exchange for the RAS, and management functions for the channels between switch and RAS. The initial application of SS7 interconnection is to allow Internet access points such as a remote access server to appear to the telecom network as a peer telecom switch, for purposes of terminating calls for Internet access. Future applications include allowing exchange of information between more general nodes within PSTN and Internet, such as a PSTN SCP and an Internet telephony service, or a PSTN switch and an Internet information server, such as a directory. 2. Applications 2.1 Call Control Because the SS7 signaling is done out of band on a separate network, the end user can obtain 64KBPS clear channel TDM circuits between the switch and the RAS without incurring the cost of PRI. The SS7 signaling (call control) is done between the STP and the RAS, which is now classified as a SSP. All call control messages will be sent over the SS7 network and the payload will be sent on the TDM circuits between the switch and the RAS. A simplified diagram below shows the relationship of the STP, PSTN Switch, and the RAS. The diagram shows a dedicated "signaling Internet" used between the RAS and Gateway to ensure physical separation of signaling and data traffic, however other arrangements are possible. Dalias, et al Informational [Page 3] INTERNET-DRAFT draft-ong-ss7-internet-gateway-00.txt March 1998 ........................ ...... . . . . +------+ . . . SS7 | / | . SS7 . +-------+ . Network | STP |------------|Gateway| . /| / | . . +-------+ . / +------+ . . |S .........../.../........ . |i / / . A|g I __________ / / A-link . S|n n / / . P|a t / / . |ling e +------+ +------+ . +-----+ r | PSTN | | PSTN |-----TDM---------------| RAS |-------n | SCP | |Switch|-----------------------| |Data e +------+ +------+ Circuits . +-----+ t . . Figure 2: SS7-Internet Interworking for Call Setup 2.2 Data Base Applications SS7 has the ability for end-to-end routing of messages across the PSTN, using STPs for message routing, and SS7 Message Transfer Part (MTP), Signaling Connection Control Part (SCCP) and Transaction Capabilities Part (TCAP). This supports PSTN database applications such as 800 or freephone number translation, calling card validation, and calling name identification. It may be possible to take advantage of SS7 for data communications as well, as a reliable transport network for highly sensitive traffic, and as a supporting environment for equivalent database applications for data communications, such as billing applications, maintenance and configuration processes, etc. Another application of database capabilities in SS7 could be for trunk group selection to the RAS. Different standards have developed for modem termination that require connections to be terminated on a RAS equipped for a specific modem standard, depending on the caller's modem. Selection of the trunk group corresponding to a particular modem type could be enabled by triggering a query from the telecom switch to the Gateway (which may pass this on to another node) to ask for trunk group selection based on, e.g., called number, calling number, or some other classmark. Dalias, et al Informational [Page 4] INTERNET-DRAFT draft-ong-ss7-internet-gateway-00.txt March 1998 2.3 VOIP (Voice over IP) Allowing for call termination and origination through the public telephone networks, with direct control over message delivery, is expected to reduce the cost of delivering toll by-pass services. VOIP may require additional features in the future to make it comparable with standard telephony service, including features that are currently implemented using the SS7 network. Remote Access Servers containing both SS7 and VOIP functionality will provide Internet providers with improved ability to launch new voice offerings. In the long run, Service Providers will need to integrate SS7 and IP control capabilities to provide transparency of service to users on PSTN and VOIP networks. Projections that some significant fraction of voice traffic will utilize IP networks in the future suggest that the ability for PSTN users and VOIP users to locate and talk to each other and access similar services will be essential in the future. Transparency of routing and services will be enabled by the connection of PSTN SS7 signaling with directory and service information in IP networks to support number translation, routing and calling card services for calls transiting from PSTN to IP and vice versa. 3. Gateway Architecture 3.1 Gateway Design The SS7-Internet Gateway needs to take into account a number of factors in its design: - SS7 links are designed to carry signaling for large telecom switches, which handle many more terminations than a single remote access server. A single 56 Kbps SS7 signaling link can support 50,000 busy hour call attempts. - the SS7 network addressing scheme is also designed to handle a limited set of signaling points. The ITU version has an address field of 14 bits to identify all signaling points belonging to the international network, while the U.S. national version uses 24 bits to identify signaling points belonging to North American networks. - SS7 protocol layers come in a number of versions, including ITU and various national versions. An SS7-Internet Gateway needs to be able to support these different versions. Dalias, et al Informational [Page 5] INTERNET-DRAFT draft-ong-ss7-internet-gateway-00.txt March 1998 Taking these factors into account, the Gateway is designed to be a separate entity providing gateway service to a community of RAS devices. This allows consistency with the scaling assumptions for SS7 links and addressing, and also allows the SS7 protocol handling function to be modularized, so that Gateways can be designed to support different SS7 versions without affecting the RAS. Modularizing the Gateway also opens the arrangements for Gateways and RAS devices to allow multiple Gateway and RAS vendors to provide products that interoperate based on a common Gateway-to-RAS protocol. The Gateway will need to support SNMP to support management by remote network management applications, and enable management visibility into the signaling plane performance and status. The Gateway can also serve as a point of security in the future, providing functions such as access to RADIUS servers for authentication, screening on calling party number, and automatic callback. The Gateway will provide open APIs for service development leveraging its basic call processing functions. The addition of Gateway functions will add to the ability of the service provider to support varieties of Service Level offerings to customers. 3.2 Gateway Functions ............................ . . . +-----------------+ . . |mapping functions| . . +-----------------+ . . | | . GATEWAY . +------+ +------+ . . | ISUP | | ASP | . . |------| |------| . . | MTP | |TCP/IP| . . +------+ +------+ . .......|...........|........ | | | | SS7 Internet | | | | Telcom Remote Switch Access Server Figure 3: SS7-Internet Gateway Functions The Gateway supports the following functions: Dalias, et al Informational [Page 6] INTERNET-DRAFT draft-ong-ss7-internet-gateway-00.txt March 1998 - termination of SS7 protocols on the SS7 side, including Message Transfer Part (MTP), ISDN User Part (ISUP), and potentially Signaling Connection Control Part (SCCP) and Transaction Capabilities (TCAP) for database access traffic. Telephony User Part (TUP) may also be supported for some networks. This includes MTP network management functions as required for any SS7 signaling point. - termination of IP and LAN protocols on the Internet side, including TCP, IP, Ethernet and other LAN protocols. - for connection control, termination of the Gateway-RAS protocol, here called the Access Signaling Protocol (ASP). This maps between SS7 ISUP messages and connection setup to the RAS. - mapping of the Point Code and Circuit Identification Code (CIC) on the SS7 side to an IP address and Channel ID associated with the corresponding RAS device on the Internet side. This mapping is created during configuration of the Gateway, and is a static mapping. More generic mapping of SS7 Point Codes and Subsystem Numbers to IP address and application information is needed for future database access features. - support for Gateway redundancy and security features, to ensure that the Gateway reliability and security is consistent with signaling requirements. 3.2.1 State Information Some limited state information needs to be maintained at the Gateway to support network management features, including state information for the attached RAS devices and some state information pertaining to the circuits connecting the telecom switch and RAS. 3.3 Gateway Protocol A new protocol, the Access Signaling Protocol (ASP) provides the signaling interface between the SS7 Gateway and the Remote Access Server (RAS). This protocol will be defined in detail in a future document. The functions of the protocol include call setup from the telecom switch to the RAS, registration and status management of the RAS-Gateway relationship, and management of the circuits. Dalias, et al Informational [Page 7] INTERNET-DRAFT draft-ong-ss7-internet-gateway-00.txt March 1998 3.3.1 Call Setup The protocol must support basic call setup and release and provide similar functions and information to the SS7 ISUP call setup and release messages (esp. IAM, ANM, REL and RLC). The messages and parameters will be a subset of the full ISUP protocol, since ISUP standards take into account many situations that are not needed for remote access. The Gateway provides a mapping from a specific interface and channel at the RAS to the equivalent Circuit Identification Code (CIC) used in SS7 to identify that termination at the telecom switch. 3.3.2 Registration and Status The protocol must support management of the relationship between the RAS and the Gateway, providing functions such as notification when the RAS is ready to receive or generate traffic, and status of the circuits interfacing to the RAS. 3.3.3 Management The Gateway protocol must support circuit network management functions such as the ability to declare circuits out of service in case of failure, and the ability to block circuits. Blocking in SS7 terminology prevents future call attempts by one side or the other for the circuit, and results in graceful shutdown of the circuit to allow maintenance actions to take place. During graceful shutdown of a T1 circuit, for example, all DS0 channels gradually revert to the idle state as existing calls are released. When all channels are idle, the T1 can be removed from service. 3.3.4 Security Security of control information exchanged with the SS7 network is important to avoid any danger of damage to the signaling and control information being passed between PSTN switches and databases. Security in this case can be enforced initially by physical separation: a dedicated Signaling Internet can be used between the Gateway and the RAS, carrying only signaling traffic. In the future, this can be supplemented by use of authentication methods such as IPSec. 3.4 Advantages 3.4.1 Scaling As discussed above, SS7 was designed for signaling between large telecom switching systems, concentrating signaling for many lines Dalias, et al Informational [Page 8] INTERNET-DRAFT draft-ong-ss7-internet-gateway-00.txt March 1998 onto a common signaling channel. The Gateway design allows a single Gateway to support interconnection scaling up to large numbers of remote access server devices, as needed to support Internet access for that particular provider. 3.4.2 Redundancy Due to the mission critical nature of the gateway, it must support some form of redundancy in all configurations. There are 2 options for initially for providing redundancy. In both cases, established calls are unaffected by gateway failure. Highly Available - This option will require one gateway with a hot standby gateway, multiple interfaces, and the appropriate software to control the switchover in time of failure. Calls in the process of being setup may be lost during service interruptions but these will be minimal. Fault tolerant - This option will require a much higher level of sophistication. This option can be a single or multiple gateway configuration with the appropriate software however calls in the process of setup will not be lost during gateway switchover and the availability is much higher than option 1. Future use of distributed gateways is for further study. 3.4.3 Flexible Deployment Since the Gateway and RAS are connected via Internet protocols, there is a great deal of flexibility for locating and matching Gateway and RAS. For example, the Gateway and RAS could be co- located close to the telecom switch, acting as a single logical peer switch. Dalias, et al Informational [Page 9] INTERNET-DRAFT draft-ong-ss7-internet-gateway-00.txt March 1998 ............................ ...... . . . . +------+ . . . SS7 | / | . . . Network | STP | . . . __| / |____________ . / +------+ . . \ .........../.../............ . \ / / . +---------+ __________ / / A-link . | Gateway | / / . +---------+ / / . | \ +------+ +------+ . | +-----+ | PSTN | | PSTN |-----TDM---------------- +-----+| RAS | | SCP | |Switch|-------------------------| RAS |+-----+ +------+ +------+ Circuits . +-----+ Figure 4: Gateway/RAS as Peer Switch Alternatively, the Gateway could provide a central interface point for many RAS devices scattered in multiple locations, acting more like a gateway Signal Transfer Point (STP) in SS7. ............................ ...... . . . . +------+ . . . SS7 | / | . . +---------+ . Network | STP |--------------| Gateway | . __| / | . . +---------+ . / +------+ . . | .........../.../............ . | Signaling / / . ISP | Internet / / A-link . _____|_______|___ / / . | | / / . |ASP |ASP / +------+ . | | / | PSTN | TDM . +-----+ | / |Switch|---------------------| RAS | | / +------+ Circuits . +-----+ | +------+ . | | PSTN | TDM . +-----+ |Switch|--------------------------------------| RAS | +------+ Circuits . +-----+ Figure 5: Gateway Serving Multiple Switches/RAS Dalias, et al Informational [Page 10] INTERNET-DRAFT draft-ong-ss7-internet-gateway-00.txt March 1998 Acronyms SS7 - Signaling System 7 RAS - Remote Access Server SCP - Service Control Point ASP - Access Signaling Protocol STP - Signal Transfer Point TDM - Time Division Multiplexed ISP - Internet Service Provider PSTN - Public Switched Telecom Network CIC - Circuit Identification Code IAM - Initial Address Message ANM - Answer Message REL - Release Message RLC - Release Complete Message Contact Addresses Robert Dalias Jiri Matousek Lyndon Ong Bay Networks, Inc. Bay Networks, Inc. Bay Networks, Inc. 5 Federal Street 5 Federal Street 4401 Gt America Pkwy Billerica, MA 01821 Billerica, MA 01821 Santa Clara, CA 95052 rdalias@baynetworks jiri@baynetworks.com long@baynetworks.com .com INTERNET DRAFT EXPIRES NOVEMBER 1998 Dalias, et al Informational [Page 11]