Internet Emergency Preparedness (IEPREP) Telephony Topology Terminology
Standard names for emergency phone call paths mixing circuit-switched and IP networks.
Polk, J., "Internet Emergency Preparedness (IEPREP) Telephony Topology Terminology", RFC 3523, DOI 10.17487/RFC3523, April 2003, <https://www.rfc-editor.org/info/rfc3523>.
@misc{rfc3523,
series = {Request for Comments},
number = 3523,
howpublished = {RFC 3523},
publisher = {RFC Editor},
doi = {10.17487/RFC3523},
url = {https://www.rfc-editor.org/info/rfc3523},
author = {J. Polk},
title = {{Internet Emergency Preparedness (IEPREP) Telephony Topology Terminology}},
pagetotal = 6,
year = 2003,
month = apr,
} RFC text
Contents
Network Working Group J. Polk
Request for Comments: 3523 Cisco Systems
Category: Informational April 2003
Internet Emergency Preparedness (IEPREP)
Telephony Topology Terminology
Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
This document defines the topology naming conventions that are to be
used in reference to Internet Emergency Preparedness (IEPREP) phone
calls. These naming conventions should be used to focus the IEPREP
Working Group during discussions and when writing requirements, gap
analysis and other solutions documents.
Table of Contents
1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1 Motivation. . . . . . . . . . . . . . . . . . . . . . . . 2
1.2 Terms and Definitions . . . . . . . . . . . . . . . . . . 2
2. IEPREP Topologies. . . . . . . . . . . . . . . . . . . . . . 2
2.1 Topology "IP Bridging" . . . . . . . . . . . . . . . . . 3
2.2 Topology "IP at the Start". . . . . . . . . . . . . . . . 3
2.3 Topology "IP at the End". . . . . . . . . . . . . . . . . 4
2.4 Topology "End-to-End IP". . . . . . . . . . . . . . . . . 4
3. Security Considerations . . . . . . . . . . . . . . . . . . . 5
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 5
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 5
7. Author's Address. . . . . . . . . . . . . . . . . . . . . . . 5
8. Full Copyright Statement. . . . . . . . . . . . . . . . . . . 6
1. Introduction
This document defines the topology naming conventions that are to be
used in reference to IEPREP phone calls. These naming conventions
should be used to focus the IEPREP Working Group (WG) during
discussions and when writing requirements, gap analysis and other
solutions documents.
There has been much confusion on the IEPREP list as well as within
each meeting about the topologies IEPREP is considering. Hopefully
this document will give each reader and author a reference set of
named architectures.
This memo attempts to be agnostic with regard to IP signaling or
control protocols (SIP, MEGACO, etc), as well as any underlying
Quality of Service (QOS) mechanisms (Diffserv, RSVP, NSIS, etc).
1.1 Motivation
Simply put, to get everyone referencing the same (named) topologies
in order to have useful and less confusing dialog to further this
working group's efforts.
1.2 Terms and Definitions
The following acronyms need to be exploded for clarity:
CSN = Circuit Switched Network
GW = Gateway (CSN to IP, or IP to CSN)
2. IEPREP Topologies
There are 4 often mentioned, but very little documented topologies
discussed within this WG's efforts so far. The following subsections
name and describe each of the topologies.
The 4 topologies are (quickly):
Topology "IP Bridging"
Topology "IP at the Start"
Topology "IP at the End"
Topology "End-to-End IP"
2.1 Topology "IP Bridging"
This topology is sometimes known as "IP in the Middle" of two CSNs.
In this topology, a CSN phone of any type initiates (dials) a call to
another CSN phone with an IP core between the two CSNs.
This topology should simplistically look like this:
Circuit Internet Circuit
Switched IP or IP Switched
Network Ingress IP Segment Egress Network
-----------+ +--------------+ +-----------
| +----+ | IP | +----+ |
CSN | | | | | | | | CSN
Phone ------->| GW |----------------------->| GW |-------->Phone
| | | | | | | |
| +----+ | | +----+ |
-----------+ +--------------+ +-----------
Figure 1. Topology "IP Bridging"
2.2 Topology "IP at the Start"
This topology has the initiating party placing (dialing) the call
from an IP Phone (PDA or computer), and the called party residing in
the CSN.
Internet Circuit
or CSN Switched
IP Segment Ingress Network
-------------------+ +---------------
| +----+ |
IP | | | | CSN
Phone ------------------>| GW |--------> Phone
| | | |
| +----+ |
-------------------+ +---------------
Figure 2. Topology "IP at the Start"
2.3 Topology "IP at the End"
This topology has the calling party placing the call from a CSN
phone, and the called party being in an IP network.
Circuit Internet
Switched CSN or
Network Egress IP Segment
-------------------+ +---------------
| +----+ |
CSN | | | | IP
Phone ------------------>| GW |--------> Phone
| | | |
| +----+ |
-------------------+ +---------------
Figure 3. Topology "IP at the End"
2.4 Topology "End-to-End IP"
This topology has no circuit switched sections in the call path.
Internet
or
IP Network
+-----------------------------------------+
| |
+---------+ +-----------+
| |
| IP IP |
| Phone --------------------------------------------> Phone |
| |
+---------+ +-----------+
| |
+-----------------------------------------+
Figure 4. Topology "End to End IP"
Although shown as one large IP cloud here, the Internet is composed
of a series of loosely connected IP domains. An End-to-End IP call
will likely traverse a number of these domains and/or multiple
network providers, which may impact the call.
3. Security Considerations
This document merely suggests a common naming convention within
IEPREP WG discussions, therefore there are no special security
considerations.
4. IANA Considerations
There are no IANA considerations within this document.
5. Acknowledgements
To Scott Bradner, Kimberly King and Mike Pierce for their comments
and suggestions.
6. References
There are no references at the present time.
7. Author's Address
James M. Polk
Cisco Systems
2200 East President George Bush Turnpike
Richardson, Texas 75082 USA
EMail: jmpolk@cisco.com
8. Full Copyright Statement
Copyright (C) The Internet Society (2003). All Rights Reserved.
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Acknowledgement
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AI ELI5
On this page
§1Introduction and Motivation
The IEPREP (Internet Emergency Preparedness) working group focuses on enabling emergency communications—calls made during natural disasters, accidents, and other crises where normal network infrastructure may be overloaded or damaged. The group discusses architectures that blend traditional circuit-switched telephone networks with modern IP-based telephony.
However, working group discussions and documents referred to different topologies without consistent naming. Some mentioned "IP in the middle," others discussed "IP on one end," creating confusion about whether documents were addressing the same scenario or different ones. This memo provides a common naming convention so authors and participants can reference the same topologies, focusing discussions and ensuring requirements documents and solutions address the intended architectures.
The memo is deliberately agnostic about the specific signaling protocols (SIP, MEGACO, etc.) or quality-of-service mechanisms (Diffserv, RSVP, NSIS, etc.) used within each topology, so the naming applies regardless of implementation choices.
Emergency responders talk about phone systems during disasters. Without shared names for different designs, people kept misunderstanding each other. This memo gives the designs names so everyone can talk about the same thing.
§2Four Standard Topologies
The RFC defines four topology types, each combining circuit-switched networks (CSNs) and IP segments with gateways (GWs) that translate between them.
Topology "IP Bridging" (also called "IP in the Middle") has two circuit-switched network endpoints with IP in between. A CSN phone dials another CSN phone; a gateway at one CSN egress point converts the call to IP, the call traverses an IP network, and another gateway at the other CSN's ingress point converts the call back to circuit-switched telephony. This topology is useful when connecting two isolated circuit-switched networks via an IP backbone.
Topology "IP at the Start" has the calling party using an IP phone (computer, PDA, or IP phone hardware), and the called party using a CSN phone. A gateway at the CSN egress point converts the call from IP to circuit-switched when routing to the destination. This topology represents a call from an emergency worker with a mobile device calling someone in the traditional telephone network.
Topology "IP at the End" is the reverse: the caller is on a CSN phone, and the called party is on the IP network. A gateway at the CSN egress ingests the circuit-switched call and converts it to IP for transmission to the IP-based endpoint. Emergency coordinators on the traditional network might use this topology to reach responders with IP phones.
Topology "End-to-End IP" has both the calling and called parties on IP networks with no circuit-switched portions. Both endpoints communicate over IP directly or through IP domains. This is the simplest topology architecturally but may traverse multiple independent IP networks and providers, which can affect call quality and reliability.
Four common setups: IP bridges two phone networks, IP starts a call and phones finish it, phones start it and IP finishes it, or IP is the whole call end-to-end.
| Topology | Calling Party | Intermediate Network(s) | Called Party |
|---|---|---|---|
| IP Bridging | CSN phone | GW -> IP -> GW | CSN phone |
| IP at the Start | IP phone | GW -> CSN | CSN phone |
| IP at the End | CSN phone | GW -> IP | IP phone |
| End-to-End IP THIS RFC | IP phone | IP network(s) | IP phone |
§3Key Definitions
The RFC introduces two key acronyms for consistency: CSN (Circuit-Switched Network) refers to traditional telephony infrastructure that routes calls on dedicated circuits. GW (Gateway) is a device that converts between circuit-switched and IP protocols, translating call signaling and media streams so endpoints on different networks can communicate.
The document notes that these topologies are media-protocol and signaling-protocol agnostic. An IP segment could use SIP, MEGACO, or another protocol; quality of service could be managed with Diffserv, RSVP, NSIS, or other mechanisms. The topology names refer only to where circuit-switched and IP sections appear in the path, not to the technologies used within those sections.
CSN is the old phone system; GW is a translator box that lets old phones talk to new IP phones.
§4Impact on IEPREP Work
By establishing these four standard topology names, the RFC provides a foundation for IEPREP working group discussions. Requirements documents can explicitly state which topologies they address. Gap analysis can identify which topologies lack standardized solutions. Solution proposals can be evaluated against all four topologies or focused on specific ones, with clear understanding of scope.
For example, a security solution might address all four topologies equally, or it might be specific to End-to-End IP calls (which have different trust boundaries than topologies that cross CSN/IP boundaries). An interoperability proposal might identify which gateway-to-gateway signaling issues apply to IP Bridging but not IP at the Start. Without these standard names, such distinctions would be buried in lengthy explanations or misunderstood entirely.
The RFC explicitly does not address policy—how to decide which topology to use in a given emergency, or how to prioritize emergency calls within each topology. These policy questions are expected to be addressed in later IEPREP requirements and solution documents, with the topology naming providing the foundation for clear communication.
Now when the working group writes about emergency phone systems, they can say "we are solving IP Bridging" and everyone knows exactly what they mean.