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Versions: 00 01                                                         
Internet-Draft          Expires: November 1998          Internet-Draft




Transport Working Group                                    Authors:
Internet-Draft                                            R. Dalias
Category: Informational                                 J. Matousek
May   1998                                                   L. Ong
                                                       Bay Networks

           Bay Networks SS7-Internet Gateway Architecture
                draft-ong-ss7-internet-gateway-01.txt


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
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   ftp.ietf.org (US East Coast), or ftp.isi.edu (US West Coast).

Copyright Notice

   Copyright  (C) The Internet Society (1998).  All Rights Reserved.

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.





















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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

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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.









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         ........................      ......
         .                      .      .
         .             +------+ .      .
         .    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.


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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.



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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:


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- 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.




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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

The SS7 gateway acts as a fire-wall between the SS7 networks and
the IP signaling network. Remote Access Servers on the IP signaling
network do not have direct access to the SS7 network - the gateway
must proxy for all the required functionality.

Both incoming and outgoing calls (from the RAS perspective) will be
supported by the protocol. However, configuration option in the
gateway and in the RAS must be provided, which will allow the
system administrator to inhibit the placement of outgoing calls
(from the RAS into the PSTN network).

It is also assumed that the IP signaling network will be physically
separate from the RAS user traffic. The RAS needs to support two


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Ethernet interfaces and must ensure that the IP traffic from the
user traffic Ethernet cannot be forwarded to the IP signaling
traffic Ethernet and vice versa.

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
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.





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         ............................      ......
         .                          .      .
         .               +------+   .      .
         .    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

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Acronyms

ANM - Answer Message                    REL - Release Message
ASP - Access Signaling Protocol         RLC - Release Complete
CIC - Circuit Identification Code       SCP - Service Control Point
IAM - Initial Address Message           SS7 - Signaling System 7
ISP - Internet Service Provider         STP - Signal Transfer Point
PSTN - Public Switched Telecom Network  TDM - Time Division Mult.
RAS - Remote Access Server


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

Full Copyright Statement

   Copyright (C) The Internet Society (1998).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
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   The limited permissions granted above are perpetual and will not be
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