PWE3 Working Group Yaakov (Jonathan) Stein
Internet Draft Ronen Shashoua
draft-stein-tdm-as-00.txt Ron Insler
Expires: December 2002 RAD Data Communications
June 2002
TDM Applicability Statement
draft-stein-tdm-as-00.txt
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TDMoIP [PAGE 1] TDM Applicability Statement June, 2002
Abstract
This document is an applicability statement for emulation of time
division multiplexed (TDM) digital voice and data signals over
Pseudo Wires.
Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL"
in this document are to be interpreted as described in RFC 2119.
Table of Contents
1. Introduction .................................................2
2. Applicability Statement ......................................2
3. References ...................................................5
4. Contact Information ..........................................5
1. Introduction
Telephony traffic is conventionally carried over connection-
oriented synchronous or plesiosynchronous networks (which will be
loosely called TDM networks herein). With the proliferation of
packet-switched networks (PSNs), telephony carriers desire
integration of TDM services into a unified PSN infrastructure.
This integration requires emulation of TDM circuits within the
PSN, a function that can be carried out using Pseudo Wires (PWs),
as described in the PWE3 requirements [PWE-REQ] and framework
[PWE-FR] documents. This emulation must ensure QoS and voice
quality similar to those of existing circuit-based networks as
well as preserving signaling features.
2. Applicability Statement
2.1 Reliability
A method of transporting TDM traffic over PWs must take into
account a customer's expectations regarding end-to-end behavior of
TDM services. Customers have become accustomed to "five nine"
reliability, meaning that the infrastructure functions correctly
99.999 percent of the time.
Due to the limitations of packet switched networks (PSNs), it will
not be possible to maintain this level of service reliability in
PW emulations of TDM services. Instead we suggest requiring "three
nines" reliability. This means in particular, that no more than
Stein et al. [PAGE 2] TDM Applicability Statement June, 2002
one in a thousand calls will be disconnected prematurely, no more
than one in a thousand initiations will be misrouted, etc.
When transporting SS7 signaling, the availability and
dependability performance objectives of [Q.766] are not expected
to be met unless further mechanisms are put in place.
2.2 TDM traffic types
A method for transporting TDM traffic over PWs MUST be able to
handle all of the following types of TDM traffic
[G.703,G.704,G.751,T1.107]
n*64K
unstructured T1
unstructured E1
structured T1 without CAS
structured E1 without CAS
structured T1 with CAS
structured E1 with CAS
T3
E3
The traffic may consist of any combination of multiplexed voice,
voice-band data, or leased-line services.
2.3 Timing transport
TDM circuits need to maintain jitter and wander to within limits
specified by the relevant standards [G.823,G.824]. In all cases a
jitter buffer will be required to be supplied by the edge devices,
in order to allow compensation for the packet delay variation of
the underlying PSN.
If a reliable local source of timing is not available, clock
information MUST be transferred edge to edge. Although some
degradation in the stratum level is to be expected, the recovered
clock must conform to the standards.
2.4 TDM Alarms
Standard TDM alarms MUST be both transferred edge to edge and MUST
be generated when required.
2.5 Latency
Latency and hence round trip delay may increase when transporting
over a PW. Since round-trip delay may not conform to what is
termed "acceptable for most user applications" in [G.114],
mechanisms for reducing latency (such as trading off reduced
efficiency for reduced latency) SHOULD be provided.
Stein et al. [PAGE 3] TDM Applicability Statement June, 2002
2.6 Audio Quality
For voice traffic the voice quality MUST be similar to those of
existing circuit-based networks, but may suffer minor degradation
due to increased delay and jitter. [P.800]
Non-voice, in-band signals required for the proper operation of
the public switched telephone network MUST be transported
sufficiently accurately as to enable their detection and disallow
their false-alarms, in order to maintain proper operation
according to their normative standards. Such signals include dual
tone multifrequency (DTMF) tones, the 2100 Hz tone including any
associated phase reversals and amplitude modulations,
multifrequency tones incorporated in the various signaling systems
of the PSTN, and tones used in national signaling systems for
ring-back, busy and congestions indication.
Voice-band modem transmissions MUST be transported sufficiently
accurately as to enable their essentially unimpaired operation.
2.7 Echo cancellation
Echo cancellation conforming to [G.168] MUST be provided whenever
the emulated service would have provided it. In addition, due to
latency often exceeding that of the emulated service, echo
cancellation MUST be provided whenever the latency exceeds 30
milliseconds and echo cancellation is not otherwise provided.
2.8 CAS signaling
When present, CAS signaling MUST be transparently transferred edge
to edge. The precise phase of signaling bits inside multiframes
need not always be retained.
Since CAS signaling bits can be used to transport hooking, hook-
flash, and pulse dialing indications, CAS transition timings and
durations MUST be maintained.
2.9 CCS signaling
Trunk associated CCS signaling (e.g. [Q.700]) MUST be transferred
edge to edge. For HDLC type payloads the precise number of idle
flags may not be preserved, and to conserve bandwidth, unnecessary
flags SHOULD be removed.
2.10 PSN Bandwidth Conservation
In order to conserve PSN bandwidth the encapsulation overhead
SHOULD be kept to a minimum.
Whenever possible, PSN bandwidth conservation techniques such as
dynamic bandwidth allocation (whereby only active timeslots are
transported), SHOULD be employed.
Stein et al. [PAGE 4] TDM Applicability Statement June, 2002
3. References
[G.114] ITU-T Recommendation G.114 (05/00)
One-way transmission time
[G.168] ITU-T Recommendation G.168 (04/00)
Digital network echo cancellers
[G.703] ITU-T Recommendation G.703 (10/98)
Physical/electrical characteristics of hierarchical digital
interfaces
[G.704] ITU-T Recommendation G.704 (10/98)
Synchronous frame structures used at 1544, 6312, 2048, 8448 and
44736 Kbit/s hierarchical levels
[G.751] ITU-T Recommendation G.751 (11/88)
Digital multiplex equipments operating at the third order bit rate
of 34 368 Kbit/s and the fourth order bit rate of 139 264 Kbit/s
and using positive justification
[G.823] ITU-T Recommendation G.823 (03/00)
The control of jitter and wander within digital networks which are
based on the 2048 Kbit/s hierarchy
[G.824] ITU-T Recommendation G.824 (03/00)
The control of jitter and wander within digital networks which are
based on the 1544 Kbit/s hierarchy
[P.800] ITU-T Recommendation P.800 (08/96)
Methods for Subjective Determnation of Transmission Quality
[Q.700] ITU-T Recommendation Q.700 (03/93)
Introduction to CCITT Signalling System No. 7
[Q.766] ITU-T Recommendation Q.766 (03/93)
Performance Objectives in the Integrated Services Digital Network
Application (SS7 ISUP)
[T1.107] ANS - I T1.107 1995 (07/95) Digital Hierarchy - Formats
Specifications
4. Contact Information
Yaakov (Jonathan) Stein
RAD Data Communications
24 Raoul Wallenburg St., Bldg C
Tel-Aviv 69719 ISRAEL
Phone: +972 3 645-5389
Email: yaakov_s@rad.co.il
Stein et al. [PAGE 5] TDM Applicability Statement June, 2002
Ronen Shashoua
RAD Data Communications
24 Raoul Wallenburg St., Bldg C
Tel-Aviv 69719 ISRAEL
Phone: +972 3 645-5447
Email: ronen_s@rad.co.il
Ron Insler
RAD Data Communications
24 Raoul Wallenburg St., Bldg C
Tel-Aviv 69719 ISRAEL
Phone: +972 3 645-5445
Email: ron_i@rad.co.il
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Stein et al. [PAGE 6]