| Rfc | 4197 |
|---|
| Title | Requirements for Edge-to-Edge Emulation of Time Division Multiplexed
(TDM) Circuits over Packet Switching Networks |
|---|
| Author | M. Riegel, Ed. |
|---|
| Date | October 2005 |
|---|
| Format: | TXT, HTML |
|---|
| Status: | INFORMATIONAL |
|---|
|
Network Working Group M. Riegel
Request for Comments: 4197 Siemens AG
Category: Informational October 2005
Requirements for Edge-to-Edge Emulation of
Time Division Multiplexed (TDM) Circuits over
Packet Switching Networks
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 (2005).
Abstract
This document defines the specific requirements for edge-to-edge
emulation of circuits carrying Time Division Multiplexed (TDM)
digital signals of the Plesiochronous Digital Hierarchy as well as
the Synchronous Optical NETwork/Synchronous Digital Hierarchy over
packet-switched networks. It is aligned to the common architecture
for Pseudo Wire Emulation Edge-to-Edge (PWE3). It makes references
to the generic requirements for PWE3 where applicable and complements
them by defining requirements originating from specifics of TDM
circuits.
Table of Contents
1. Introduction ....................................................3
1.1. TDM Circuits Belonging to the PDH Hierarchy ................3
1.1.1. TDM Structure and Transport Modes ...................4
1.2. SONET/SDH Circuits .........................................4
2. Motivation ......................................................5
3. Terminology .....................................................6
4. Reference Models ................................................7
4.1. Generic PWE3 Models ........................................7
4.2. Clock Recovery .............................................7
4.3. Network Synchronization Reference Model ....................8
4.3.1. Synchronous Network Scenarios ......................10
4.3.2. Relative Network Scenario ..........................12
4.3.3. Adaptive Network Scenario ..........................12
5. Emulated Services ..............................................13
5.1. Structure-Agnostic Transport of Signals out of the
PDH Hierarchy .............................................13
5.2. Structure-Aware Transport of Signals out of the
PDH Hierarchy .............................................14
5.3. Structure-Aware Transport of SONET/SDH Circuits ...........14
6. Generic Requirements ...........................................14
6.1. Relevant Common PW Requirements ...........................14
6.2. Common Circuit Payload Requirements .......................15
6.3. General Design Issues .....................................16
7. Service-Specific Requirements ..................................16
7.1. Connectivity ..............................................16
7.2. Network Synchronization ...................................16
7.3. Robustness ................................................16
7.3.1. Packet loss ........................................17
7.3.2. Out-of-order delivery ..............................17
7.4. CE Signaling ..............................................17
7.5. PSN Bandwidth Utilization .................................18
7.6. Packet Delay Variation ....................................19
7.7. Compatibility with the Existing PSN Infrastructure ........19
7.8. Congestion Control ........................................19
7.9. Fault Detection and Handling ..............................20
7.10. Performance Monitoring ...................................20
8. Security Considerations ........................................20
9. References .....................................................20
9.1. Normative References ......................................20
9.2. Informative References ....................................21
10. Contributors Section ..........................................22
1. Introduction
This document defines the specific requirements for edge-to-edge
emulation of circuits carrying Time Division Multiplexed (TDM)
digital signals of the Plesiochronous Digital Hierarchy (PDH) as well
as the Synchronous Optical NETwork (SONET)/Synchronous Digital
Hierarchy (SDH) over Packet-Switched Networks (PSN). It is aligned
to the common architecture for Pseudo Wire Emulation Edge-to-Edge
(PWE3) as defined in [RFC3985]. It makes references to requirements
in [RFC3916] where applicable and complements [RFC3916] by defining
requirements originating from specifics of TDM circuits.
The term "TDM" will be used in this documents as a general descriptor
for the synchronous bit streams belonging to either the PDH or the
SONET/SDH hierarchies.
1.1. TDM Circuits Belonging to the PDH Hierarchy
The bit rates traditionally used in various regions of the world are
detailed in the normative reference [G.702]. For example, in North
America, the T1 bit stream of 1.544 Mbps and the T3 bit stream of
44.736 Mbps are mandated, while in Europe, the E1 bit stream of 2.048
Mbps and the E3 bit stream of 34.368 Mbps are utilized.
Although TDM can be used to carry unstructured bit streams at the
rates defined in [G.702], there is a standardized method of carrying
bit streams in larger units called frames, each frame contains the
same number of bits.
Related to the sampling frequency of voice traffic the bitrate is
always a multiple of 8000, hence the T1 frame consists of 193 bits
and the E1 frame of 256 bits. The number of bits in a frame is
called the frame size.
The framing is imposed by introducing a periodic pattern into the bit
stream to identify the boundaries of the frames (e.g., 1 framing bit
per T1 frame, a sequence of 8 framing bits per E1 frame). The
details of how these framing bits are generated and used are
elucidated in [G.704], [G.706], and [G.751]. Unframed TDM has all
bits available for payload.
Framed TDM is often used to multiplex multiple channels (e.g., voice
channels each consisting of 8000 8-bit-samples per second) in a
sequence of "timeslots" recurring in the same position in each frame.
This multiplexing is called "channelized TDM" and introduces
additional structure.
In some cases, framing also defines groups of consecutive frames
called multiframes. Such grouping imposes an additional level of
structure on the TDM bit-stream.
1.1.1. TDM Structure and Transport Modes
Unstructured TDM:
TDM that consists of a raw bit-stream of rate defined in [G.702],
with all bits available for payload.
Structured TDM:
TDM with one or more levels of structure delineation, including
frames, channelization, and multiframes (e.g., as defined in [G.704],
[G.751], and [T1.107]).
Structure-Agnostic Transport:
Transport of unstructured TDM, or of structured TDM when the
structure is deemed inconsequential from the transport point of view.
In structure-agnostic transport, any structural overhead that may be
present is transparently transported along with the payload data, and
the encapsulation provides no mechanisms for its location or
utilization.
Structure-Aware Transport:
Transport of structured TDM taking at least some level of the
structure into account. In structure-aware transport, there is no
guarantee that all bits of the TDM bit-stream will be transported
over the PSN network (specifically, the synchronization bits and
related overhead may be stripped at ingress and usually will be
regenerated at egress) or that transported bits will be situated in
the packet in their original order (but in this case, bit order is
usually recovered at egress; one known exception is loss of
multiframe synchronization between the TDM data and CAS bits
introduced by a digital cross-connect acting as a Native Service
Processing (NSP) block, see [TR-NWT-170]).
1.2. SONET/SDH Circuits
The term SONET refers to the North American Synchronous Optical
NETwork as specified by [T1.105]. It is based on the concept of a
Nx783 byte payload container repeated every 125us. This payload is
referred to as an STS-1 SPE and may be concatenated into higher
bandwidth circuits (e.g., STS-Nc) or sub-divided into lower bandwidth
circuits (Virtual Tributaries). The higher bandwidth concatenated
circuits can be used to carry anything from IP Packets to ATM cells
to Digital Video Signals. Individual STS-1 SPEs are frequently used
to carry individual DS3 or E3 TDM circuits. When the 783 byte
containers are sub-divided for lower rate payloads, they are
frequently used to carry individual T1 or E1 TDM circuits.
The Synchronous Digital Hierarchy (SDH) is the international
equivalent and enhancement of SONET and is specified by [G.707].
Both SONET and SDH include a substantial amount of transport overhead
that is used for performance monitoring, fault isolation, and other
maintenance functions along different types of optical or electrical
spans. This also includes a pointer-based mechanism for carrying
payloads asynchronously. In addition, the payload area includes
dedicated overhead for end-to-end performance monitoring, fault
isolation, and maintenance for the service being carried. If the
main payload area is sub-divided into lower rate circuits (such as
T1/E1), additional overhead is included for end-to-end monitoring of
the individual T1/E1 circuits.
This document discusses the requirements for emulation of SONET/SDH
services. These services include end-to-end emulation of the SONET
payload (STS-1 SPE), emulation of concatenated payloads (STS-Nc SPE),
as well as emulation of a variety of sub-STS-1 rate circuits jointly
referred to as Virtual Tributaries (VT) and their SDH analogs.
2. Motivation
[RFC3916] specifies common requirements for edge-to-edge emulation of
circuits of various types. However, these requirements, as well as
references in [RFC3985], do not cover specifics of PWs carrying TDM
circuits.
The need for a specific document to complement [RFC3916] addressing
of edge-to-edge emulation of TDM circuits arises from the following:
o Specifics of the TDM circuits. For example,
* the need for balance between the clock of ingress and egress
attachment circuits in each direction of the Pseudo Wire (PW),
* the need to maintain jitter and wander of the clock of the
egress end service, within the limits imposed by the
appropriate normative documents, in the presence of the packet
delay variation produced by the PSN.
o Specifics of applications using TDM circuits. For example, voice
applications,
* put special emphasis on minimization of one-way delay, and
* are relatively tolerant to errors in data.
o Other applications might have different specifics. For example,
transport of signaling information
* is relatively tolerant to one-way delay, and
* is sensitive to errors in transmitted data.
o Specifics of the customers' expectations regarding end-to-end
behavior of services that contain emulated TDM circuits. For
example, experience with carrying such services over SONET/SDH
networks increases the need for
* isolation of problems introduced by the PSN from those
occurring beyond the PSN bounds,
* sensitivity to misconnection,
* sensitivity to unexpected connection termination, etc.
3. Terminology
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 [RFC2119].
The terms defined in [RFC3985], Section 1.4 are used consistently.
However some terms and acronyms are used in conjunction with the TDM
services. In particular:
TDM networks employ Channel-Associated Signaling (CAS) or Common
Channel Signaling (CCS) to supervise and advertise status of
telephony applications, provide alerts to these applications (as to
requests to connect or disconnect), and to transfer routing and
addressing information. These signals must be reliably transported
over the PSNs for the telephony end-systems to function properly.
CAS (Channel-Associated Signaling)
CAS is carried in the same T1 or E1 frame as the voice signals,
but not in the speech band. Since CAS signaling may be
transferred at a rate slower than the TDM traffic in a timeslot,
one need not update all the CAS bits in every TDM frame. Hence,
CAS systems cycle through all the signaling bits only after some
number of TDM frames, which defines a new structure known as a
multiframe or superframe. Common multiframes are 12, 16, or 24
frames in length, corresponding to 1.5, 2, and 3 milliseconds in
duration.
CCS (Common Channel Signaling)
CCS signaling uses a separate digital channel to carry
asynchronous messages pertaining to the state of telephony
applications over related TDM timeslots of a TDM trunk. This
channel may be physically situated in one or more adjacent
timeslots of the same TDM trunk (trunk associated CCS) or may be
transported over an entirely separate network.
CCS is typically HDLC-based, with idle codes or keep-alive
messages being sent until a signaling event (e.g., on-hook or
off-hook) occurs. Examples of HDLC-based CCS systems are SS7
[Q.700] and ISDN PRI signaling [Q.931].
Note: For the TDM network, we use the terms "jitter" and "wander" as
defined in [G.810] to describe short- and long-term variance of the
significant instants of the digital signal, while for the PSN we use
the term packet delay variation (PDV) (see [RFC3393]).
4. Reference Models
4.1. Generic PWE3 Models
Generic models that have been defined in [RFC3985] in sections
- 4.1 (Network Reference Model),
- 4.2 (PWE3 Pre-processing),
- 4.3 (Maintenance Reference Model),
- 4.4 (Protocol Stack Reference Model) and
- 4.5 (Pre-processing Extension to Protocol Stack Reference Model).
They are fully applicable for the purposes of this document without
modification.
All the services considered in this document represent special cases
of the Bit-stream and Structured bit-stream payload type defined in
Section 3.3 of [RFC3985].
4.2. Clock Recovery
Clock recovery is extraction of the transmission bit timing
information from the delivered packet stream. Extraction of this
information from a highly jittered source, such as a packet stream,
may be a complex task.
4.3. Network Synchronization Reference Model
Figure 1 shows a generic network synchronization reference model.
+---------------+ +---------------+
| PE1 | | PE2 |
K | +--+ | | +--+ | G
| | | J| | | | H| | |
v | v | | | v | | v
+---+ | +-+ +-+ +-+ | +--+ +--+ | +-+ +-+ +-+ | +---+
| | | |P| |D| |P| | | | | | | |P| |E| |P| | | |
| |<===|h|<:|e|<:|h|<:::| |<::| |<:::|h|<:|n|<=|h|<===| |
| | | |y| |c| |y| | | | | | | |y| |c| |y| | | |
| C | | +-+ +-+ +-+ | | | | | | +-+ +-+ +-+ | | C |
| E | | | |S1| |S2| | | | E |
| 1 | | +-+ +-+ +-+ | | | | | | +-+ +-+ +-+ | | 2 |
| | | |P| |E| |P| | | | | | | |P| |D| |P| | | |
| |===>|h|=>|n|:>|h|:::>| |::>| |:::>|h|:>|e|=>|h|===>| |
| | | |y| |c| |y| | | | | | | |y| |c| |y| | | |
+---+ | +-+ +-+ +-+ | +--+ +--+ | +-+ +-+ +-+ | +---+
^ ^ | | ^ | | | ^ | ^ ^
| | | |B | |<------+------>| | | | | |
| A | +--+ | | | +--+-E | F |
| +---------------+ +-+ +---------------+ |
| ^ |I| ^ |
| | +-+ | |
| C D |
+-----------------------------L-----------------------------+
Figure 1: The Network Synchronization Reference Model
The following notation is used in Figure 1:
CE1, CE2
Customer edge devices terminating TDM circuits to be emulated.
PE1, PE2
Provider edge devices adapting these end services to PW.
S1, S2
Provider core routers.
Phy
Physical interface terminating the TDM circuit.
Enc
PSN-bound interface of the PW, where the encapsulation takes
place.
Dec
CE-bound interface of the PW, where the decapsulation takes place.
It contains a compensation buffer (also known as the "jitter
buffer") of limited size.
"==>"
TDM attachment circuits.
"::>"
PW providing edge-to-edge emulation for the TDM circuit.
The characters "A" - "L" denote various clocks:
"A"
The clock used by CE1 for transmission of the TDM attachment
circuit towards CE1.
"B"
The clock recovered by PE1 from the incoming TDM attachment
circuit. "A" and "B" always have the same frequency.
"G"
The clock used by CE2 for transmission of the TDM attachment
circuit towards CE2.
"H"
The clock recovered by PE2 from the incoming TDM attachment
circuit. "G" and "H" always have the same frequency.
"C", "D"
Local oscillators available to PE1 and PE2, respectively.
"E"
Clock used by PE2 to transmit the TDM attachment service circuit
to CE2 (the recovered clock).
"F"
Clock recovered by CE2 from the incoming TDM attachment service
("E and "F" have the same frequency).
"I"
If the clock exists, it is the common network reference clock
available to PE1 and PE2.
"J"
Clock used by PE1 to transmit the TDM attachment service circuit
to CE1 (the recovered clock).
"K"
Clock recovered by CE1 from the incoming TDM attachment service
("J" and "K" have the same frequency).
"L"
If it exists, it is the common reference clock of CE1 and CE2.
Note that different pairs of CE devices may use different common
reference clocks.
A requirement of edge-to-edge emulation of a TDM circuit is that
clock "B" and "E", as well as clock "H" and "J", are of the same
frequency. The most appropriate method will depend on the network
synchronization scheme.
The following groups of synchronization scenarios can be considered:
4.3.1. Synchronous Network Scenarios
Depending on which part of the network is synchronized by a common
clock, there are two scenarios:
o PE Synchronized Network:
Figure 2 is an adapted version of the generic network reference
model, and presents the PE synchronized network scenario.
The common network reference clock "I" is available to all the PE
devices, and local oscillators "C" and "D" are locked to "I":
* Clocks "E" and "J" are the same as "D" and "C", respectively.
* Clocks "A" and "G" are the same as "K" and "F", respectively
(i.e., CE1 and CE2 use loop timing).
+-----+ +-----+
+-----+ | |- - -|=================|- - -| | +-----+
| /-- |<---------|............PW1..............|<---------| <-\ |
|| CE | | | PE1 | | PE2 | | |CE2 ||
| \-> |--------->|............PW2..............|--------->| --/ |
+-----+ | |- - -|=================|- - -| | +-----+
+-----+ +-----+
^ ^
|C |D
+-----------+-----------+
|
+-+
|I|
+-+
Figure 2: PE Synchronized Scenario
o CE Synchronized Network:
Figure 3 is an adapted version of the generic network reference
model, and presents the CE synchronized network scenario.
The common network reference clock "L" is available to all the CE
devices, and local oscillators "A" and "G" are locked to "L":
* Clocks "E" and "J" are the same as "G" and "A", respectively
(i.e., PE1 and PE2 use loop timing).
+-----+ +-----+
+-----+ | |- - -|=================|- - -| | +-----+
| |<---------|............PW1..............|<---------| |
| CE1 | | | PE1 | | PE2 | | | CE2 |
| |--------->|............PW2..............|--------->| |
+-----+ | |- - -|=================|- - -| | +-----+
^ +-----+ +-----+ ^
|A G|
+----------------------------+------------------------------+
|
+-+
|L|
+-+
Figure 3: CE Synchronized Scenario
No timing information has to be transferred in these cases.
4.3.2. Relative Network Scenario
In this case, each CE uses its own transmission clock source that
must be carried across the PSN and recovered by the remote PE,
respectively. The common PE clock "I" can be used as reference for
this purpose.
Figure 4 shows the relative network scenario.
The common network reference clock "I" is available to all the PE
devices, and local oscillators "C" and "D" are locked to "I":
o Clocks "A" and "G" are generated locally without reference to a
common clock.
o Clocks "E" and "J" are generated in reference to a common clock
available at all PE devices.
In a slight modification of this scenario, one (but not both!) of the
CE devices may use its receive clock as its transmission clock (i.e.,
use loop timing).
|G
+-----+ +-----+ v
+-----+ | |- - -|=================|- - -| | +-----+
| |<---------|............PW1..............|<---------| |
| CE1 | | | PE1 | | PE2 | | | CE2 |
| |--------->|............PW2..............|--------->| |
+-----+ | |- - -|=================|- - -| | +-----+
^ +-----+<-------+------->+-----+
|A |
+-+
|I|
+-+
Figure 4: Relative Network Scenario Timing
In this case, timing information (the difference between the common
reference clock "I" and the incoming clock "A") MUST be explicitly
transferred from the ingress PE to the egress PE.
4.3.3. Adaptive Network Scenario
The adaptive scenario is characterized by:
o No common network reference clock "I" is available to PE1 and PE2.
o No common reference clock "L" is available to CE1 and CE2.
Figure 5 presents the adaptive network scenario.
|J |G
v |
+-----+ +-----+ v
+-----+ | |- - -|=================|- - -| | +-----+
| |<---------|............PW1..............|<---------| |
| CE1 | | | PE1 | | PE2 | | | CE2 |
| |--------->|............PW2..............|--------->| |
+-----+ | |- - -|=================|- - -| | +-----+
^ +-----+ +-----+
| ^
A| E|
Figure 5: Adaptive Scenario
Synchronizing clocks "A" and "E" in this scenario is more difficult
than it is in the other scenarios.
Note that the tolerance between clocks "A" and "E" must be small
enough to ensure that the jitter buffer does not overflow or
underflow.
In this case, timing information MAY be explicitly transferred from
the ingress PE to the egress PE, e.g., by RTP.
5. Emulated Services
This section defines requirements for the payload and encapsulation
layers for edge-to-edge emulation of TDM services with bit-stream
payload as well as structured bit-stream payload.
Wherever possible, the requirements specified in this document SHOULD
be satisfied by appropriate arrangements of the encapsulation layer
only. The (rare) cases when the requirements apply to both the
encapsulation and payload layers (or even to the payload layer only)
will be explicitly noted.
The service-specific encapsulation layer for edge-to-edge emulation
comprises the following services over a PSN.
5.1. Structure-Agnostic Transport of Signals out of the PDH Hierarchy
Structure-agnostic transport is considered for the following signals:
o E1 as described in [G.704].
o T1 (DS1) as described in [G.704].
o E3 as defined in [G.751].
o T3 (DS3) as described in [T1.107].
5.2. Structure-Aware Transport of Signals out of the PDH Hierarchy
Structure-aware transport is considered for the following signals:
o E1/T1 with one of the structures imposed by framing as described
in [G.704].
o NxDS0 with or without CAS.
5.3. Structure-Aware Transport of SONET/SDH Circuits
Structure-aware transport is considered for the following SONET/SDH
circuits:
o SONET STS-1 synchronous payload envelope (SPE)/SDH VC-3.
o SONET STS-Nc SPE (N = 3, 12, 48, 192) / SDH VC-4, VC-4-4c,
VC-4-16c, VC-4-64c.
o SONET VT-N (N = 1.5, 2, 3, 6) / SDH VC-11, VC-12, VC-2.
o SONET Nx VT-N / SDH Nx VC-11/VC-12/VC-2/VC-3.
Note: There is no requirement for the structure-agnostic transport of
SONET/SDH. For this case, it would seem that structure must be taken
into account.
6. Generic Requirements
6.1. Relevant Common PW Requirements
The encapsulation and payload layers MUST conform to the common PW
requirements defined in [RFC3916]:
1. Conveyance of Necessary Header Information:
A. For structure-agnostic transport, this functionality MAY be
provided by the payload layer.
B. For structure-aware transport, the necessary information MUST
be provided by the encapsulation layer.
C. Structure-aware transport of SONET/SDH circuits MUST preserve
path overhead information as part of the payload. Relevant
components of the transport overhead MAY be carried in the
encapsulation layer.
2. Support of Multiplexing and Demultiplexing if supported by the
native services. This is relevant for Nx DS0 circuits (with or
without signaling) and Nx VT-x in a single STS-1 SPE or VC-4.:
A. For these circuits, the combination of encapsulation and
payload layers MUST provide for separate treatment of every
sub-circuit.
B. Enough information SHOULD be provided by the pseudo wire to
allow multiplexing and demultiplexing by the NSP. Reduction
of the complexity of the PW emulation by using NSP circuitry
for multiplexing and demultiplexing MAY be the preferred
solution.
3. Intervention or transparent transfer of Maintenance Messages of
the Native Services, depending on the particular scenario.
4. Consideration of Per-PSN Packet Overhead (see also Section 7.5
below).
5. Detection and handling of PW faults. The list of faults is given
in Section 7.9 below.
Fragmentation indications MAY be used for structure-aware transport
when the structures in question either exceed desired packetization
delay or exceed Path MTU between the pair of PEs.
The following requirement listed in [RFC3916] is not applicable to
emulation of TDM services:
o Support of variable length PDUs.
6.2. Common Circuit Payload Requirements
Structure-agnostic transport treats TDM circuits as belonging to the
'Bit-stream' payload type defined in [RFC3985].
Structure-aware transport treats these circuits as belonging to the
"Structured bit-stream" payload type defined in [RFC3985].
Accordingly, the encapsulation layer MUST provide the common
Sequencing service and SHOULD provide Timing information
(Synchronization services) when required (see Section 4.3 above).
Note: Length service MAY be provided by the encapsulation layer, but
is not required.
6.3. General Design Issues
The combination of payload and encapsulation layers SHOULD comply
with the general design principles of the Internet protocols as
presented in Section 3 of [RFC1958] and [RFC3985].
If necessary, the payload layer MAY use some forms of adaptation of
the native TDM payload in order to achieve specific, well-documented
design objectives. In these cases, standard adaptation techniques
SHOULD be used.
7. Service-Specific Requirements
7.1. Connectivity
1. The emulation MUST support the transport of signals between
Attachment Circuits (ACs) of the same type (see Section 5) and,
wherever appropriate, bit-rate.
2. The encapsulation layer SHOULD remain unaffected by specific
characteristics of connection between the ACs and PE devices at
the two ends of the PW.
7.2. Network Synchronization
1. The encapsulation layer MUST provide synchronization services
that are sufficient to:
A. match the ingress and egress end service clocks regardless of
the specific network synchronization scenario, and
B. keep the jitter and wander of the egress service clock within
the service-specific limits defined by the appropriate
normative references.
2. If the same high-quality synchronization source is available to
all the PE devices in the given domain, the encapsulation layer
SHOULD be able to make use of it (e.g., for better reconstruction
of the native service clock).
7.3. Robustness
The robustness of the emulated service depends not only upon the
edge-to-edge emulation protocol, but also upon proper implementation
of the following procedures.
7.3.1. Packet loss
Edge-to-edge emulation of TDM circuits MAY assume very low
probability of packet loss between ingress and egress PE. In
particular, no retransmission mechanisms are required.
In order to minimize the effect of lost packets on the egress
service, the encapsulation layer SHOULD:
1. Enable independent interpretation of TDM data in each packet by
the egress PE (see [RFC2736]). This requirement MAY be
disregarded if the egress PE needs to interpret structures that
exceed the path MTU between the ingress and egress PEs.
2. Allow reliable detection of lost packets (see next section). In
particular, it SHOULD allow estimation of the arrival time of the
next packet and detection of lost packets based on this estimate.
3. Minimize possible effect of lost packets on recovery of the
circuit clock by the egress PE.
4. Increase the resilience of the CE TDM interface to packet loss by
allowing the egress PE to substitute appropriate data.
7.3.2. Out-of-order delivery
The encapsulation layer MUST provide the necessary mechanisms to
guarantee ordered delivery of packets carrying the TDM data over the
PSN. Packets that have arrived out-of-order:
1. MUST be detected, and
2. SHOULD be reordered if not judged to be too late or too early for
playout.
Out-of-order packets that cannot be reordered MUST be treated as
lost.
7.4. CE Signaling
Unstructured TDM circuits would not usually require any special
mechanism for carrying CE signaling as this would be carried as part
of the emulated service.
Some CE applications using structured TDM circuits (e.g., telephony)
require specific signaling that conveys the changes of state of these
applications relative to the TDM data.
The encapsulation layer SHOULD support signaling of state of CE
applications for the relevant circuits providing for:
1. Ability to support different signaling schemes with minimal
impact on encapsulation of TDM data,
2. Multiplexing of application-specific CE signals and data of the
emulated service in the same PW,
3. Synchronization (within the application-specific tolerance
limits) between CE signals and data at the PW egress,
4. Probabilistic recovery against possible, occasional loss of
packets in the PSN, and
5. Deterministic recovery of the CE application state after PW setup
and network outages.
CE signaling that is used for maintenance purposes (loopback
commands, performance monitoring data retrieval, etc.) SHOULD use the
generic PWE3 maintenance protocol.
7.5. PSN Bandwidth Utilization
1. The encapsulation layer SHOULD allow for an effective trade-off
between the following requirements:
A. Effective PSN bandwidth utilization. Assuming that the size
of the encapsulation layer header does not depend on the size
of its payload, an increase in the packet payload size
results in increased efficiency.
B. Low edge-to-edge latency. Low end-to-end latency is the
common requirement for Voice applications over TDM services.
Packetization latency is one of the components comprising
edge-to-edge latency, and it decreases with the packet
payload size.
The compensation buffer used by the CE-bound IWF increases
latency to the emulated circuit. Additional delays introduced by
this buffer SHOULD NOT exceed the packet delay variation observed
in the PSN.
2. The encapsulation layer MAY provide for saving PSN bandwidth by
not sending corrupted TDM data across the PSN.
3. The encapsulation layer MAY provide the ability to save the PSN
bandwidth for the structure-aware case by not sending channels
that are permanently inactive.
4. The encapsulation layer MAY enable the dynamic suppression of
temporarily unused channels from transmission for the structure-
aware case.
If used, dynamic suppression of temporarily unused channels
MUST NOT violate the integrity of the structures delivered over
the PW.
5. For NxDS0, the encapsulation layer MUST provide the ability to
keep the edge-to-edge delay independent of the service rate.
7.6. Packet Delay Variation
The encapsulation layer SHOULD provide for the ability to compensate
for packet delay variation, while maintaining jitter and wander of
the egress end service clock with tolerances specified in the
normative references.
The encapsulation layer MAY provide for run-time adaptation of delay
introduced by the jitter buffer if the packet delay variation varies
with time. Such an adaptation MAY introduce a low level of errors
(within the limits tolerated by the application) but SHOULD NOT
introduce additional wander of the egress end service clock.
7.7. Compatibility with the Existing PSN Infrastructure
The combination of encapsulation and PSN tunnel layers used for edge-
to-edge emulation of TDM circuits SHOULD be compatible with existing
PSN infrastructures. In particular, compatibility with the
mechanisms of header compression over links where capacity is at a
premium SHOULD be provided.
7.8. Congestion Control
TDM circuits run at a constant rate, and hence offer constant traffic
loads to the PSN. The rate varying mechanism that TCP uses to match
the demand to the network congestion state is, therefore, not
applicable.
The ability to shut down a TDM PW when congestion has been detected
MUST be provided.
Precautions should be taken to avoid situations wherein multiple TDM
PWs are simultaneously shut down or re-established, because this
leads to PSN instability.
Further congestion considerations are discussed in chapter 6.5 of
[RFC3985].
7.9. Fault Detection and Handling
The encapsulation layer for edge-to-edge emulation of TDM services
SHOULD, separately or in conjunction with the lower layers of the
PWE3 stack, provide for detection, handling, and reporting of the
following defects:
1. Misconnection, or Stray Packets. The importance of this
requirement stems from customer expectation due to reliable
misconnection detection in SONET/SDH networks.
2. Packet Loss. Packet loss detection is required to maintain clock
integrity, as discussed in Section 7.3.1 above. In addition,
packet loss detection mechanisms SHOULD provide for localization
of the outage in the end-to-end emulated service.
3. Malformed packets.
7.10. Performance Monitoring
The encapsulation layer for edge-to-edge emulation of TDM services
SHOULD provide for collection of performance monitoring (PM) data
that is compatible with the parameters defined for 'classic',
TDM-based carriers of these services. The applicability of [G.826]
is left for further study.
8. Security Considerations
The security considerations in [RFC3916] are fully applicable to the
emulation of TDM services. In addition, TDM services are sensitive
to packet delay variation [Section 7.6], and need to be protected
from this method of attack.
9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
9.2. Informative References
[RFC3916] Xiao, X., McPherson, D., and P. Pate, "Requirements for
Pseudo-Wire Emulation Edge-to-Edge (PWE3)", RFC 3916,
September 2004.
[RFC3985] Bryant, S. and P. Pate, "Pseudo Wire Emulation Edge-to-
Edge (PWE3) Architecture", RFC 3985, March 2005.
[G.702] ITU-T Recommendation G.702 (11/88) - Digital hierarchy
bit rates
[G.704] ITU-T Recommendation G.704 (10/98) - Synchronous frame
structures used at 1544, 6312, 2048, 8448 and 44 736
Kbit/s hierarchical levels
[G.706] ITU-T Recommendation G.706 (04/91) - Frame alignment and
cyclic redundancy check (CRC) procedures relating to
basic frame structures defined in Recommendation G.704
[G.707] ITU-T Recommendation G.707 (10/00) - Network node
interface for the synchronous digital hierarchy (SDH)
[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.810] ITU-T Recommendation G.810 (08/96) - Definitions and
terminology for synchronization networks
[G.826] ITU-T Recommendation G.826 (02/99) - Error performance
parameters and objectives for international, constant
bit rate digital paths at or above the primary rate
[Q.700] ITU-T Recommendation Q.700 (03/93) - Introduction to
CCITT Signalling System No. 7
[Q.931] ITU-T Recommendation Q.931 (05/98) - ISDN user-network
interface layer 3 specification for basic call control
[RFC1958] Carpenter, B., "Architectural Principles of the
Internet", RFC 1958, June 1996.
[RFC2736] Handley, M. and C. Perkins, "Guidelines for Writers of
RTP Payload Format Specifications", BCP 36, RFC 2736,
December 1999.
[RFC3393] Demichelis, C. and P. Chimento, "IP Packet Delay
Variation Metric for IP Performance Metrics (IPPM)", RFC
3393, November 2002.
[T1.105] ANSI T1.105 - 2001 Synchronous Optical Network (SONET) -
Basic Description including Multiplex Structure, Rates,
and Formats, May 2001
[T1.107] ANSI T1.107 - 1995. Digital Hierarchy - Format
Specification
[TR-NWT-170] Digital Cross Connect Systems - Generic Requirements and
Objectives, Bellcore, TR-NWT-170, January 1993
10. Contributors Section
The following have contributed to this document:
Sasha Vainshtein
Axerra Networks
EMail: sasha@axerra.com
Yaakov Stein
RAD Data Communication
EMail: yaakov_s@rad.com
Prayson Pate
Overture Networks, Inc.
EMail: prayson.pate@overturenetworks.com
Ron Cohen
Lycium Networks
EMail: ronc@lyciumnetworks.com
Tim Frost
Zarlink Semiconductor
EMail: tim.frost@zarlink.com
Author's Address
Maximilian Riegel
Siemens AG
St-Martin-Str 76
Munich 81541
Germany
Phone: +49-89-636-75194
EMail: maximilian.riegel@siemens.com
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