One document matched: draft-malis-pwe3-sonet-03.txt
Differences from draft-malis-pwe3-sonet-02.txt
PWE3 Working Group Andrew G. Malis
Internet Draft Ken Hsu
Expiration Date: December 2002 Vivace Networks, Inc.
David Zelig Jeremy Brayley
Corrigent Systems, LTD. Steve Vogelsang
John Shirron
Jim Boyle Laurel Networks, Inc.
Protocol Driven Networks, Inc.
Luca Martini
Ron Cohen Craig White
Lycium Networks Level 3 Communications, LLC.
Prayson Pate Tom Johnson
Overture Networks, Inc. Marlene Drost
Ed Hallman
Litchfield Communications, Inc.
June 2002
SONET/SDH Circuit Emulation over Packet (CEP)
draft-malis-pwe3-sonet-03.txt
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of section 10 of [RFC2026].
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."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
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Abstract
Generic requirements and framework for Pseudo Wire Emulation Edge-to-
Edge (PWE3) have been described in [PWE3-REQ] and [PWE3-FW]. This
draft provides encapsulation formats and semantics for connecting
SONET/SDH edge networks through a packet network using IP or MPLS.
This basic application of SONET/SDH interworking will allow service
providers to take advantage of new technologies in the core in order to
provide traditional SONET/SDH services.
Table of Contents
1 Conventions used in this document 2
2 Introduction 2
3 Applicability Statement 3
4 Scope 5
5 CEP Encapsulation Format 7
6 CEP Operation 16
7 SONET/SDH Maintenance Signals 19
8 SONET/SDH Transport Timing 23
9 SONET/SDH Pointer Management 24
10 CEP Performance Monitors 25
11 Open Issues 27
12 Security Considerations 28
13 Intellectual Property Disclaimer 28
14 References 29
15 Acknowledgments 30
16 Author's Addresses 30
1 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].
2 Introduction
This document describes a protocol that performs SONET Emulation
over a variety of Packet-Switched Networks (PSNs) as part of the
PWE3 Working Group. The document assumes that the reader is
familiar with the PWE3 terminology and concepts described in PWE3
requirements and framework documents [PWE3-REQ] and [PWE3-FW] as
well as the PWE3 Protocol Layering Model [PWE3-LAYERS]. The
protocol is titled "Circuit Emulation over Packet" (CEP).
The transmission system for circuit-oriented TDM signals is the
Synchronous Optical Network [SONET], [GR253] / Synchronous Digital
Hierarchy (SDH) [G707]. To support TDM traffic (which includes
voice, data, and private leased line services) PSNs must emulate the
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circuit characteristics of SONET/SDH payloads. An RTP Header
[RFC1889] and a CEP Control Word are used to encapsulate the
SONET/SDH TDM signals for transmission over an arbitrary PSN.
This document also describes an optional extension to CEP called
Dynamic Bandwidth Allocation (DBA). This is a method for
dynamically reducing the bandwidth utilized by emulated SONET/SDH
circuits in the packet network. This bandwidth reduction is
accomplished by not sending the SONET/SDH payload through the packet
network under certain conditions such as AIS-P or STS SPE
Unequipped.
In addition, this document describes a technique for RTP header
compression/suppression based on [ROHC-LLA].
This document is based on a previous document describing a method
for encapsulating SONET signals for carriage over MPLS networks
[CEM].
This document is closely related to and references [MARTINI-TRANS],
which describes the control protocol methods used to signal the
usage of CEP, [MARTINI-ENCAP] which describes a related method of
encapsulating Layer 2 frames over MPLS and which shares the same
signaling, and [CEM-MIB] which describes a MIB for controlling and
observing CEM services.
This document is complimentary to [CESoPSN] and [CEP-VT] which
describe methods for transporting sub-STS-1 rate circuits in native
format or VT mapped respectively.
3 Applicability Statement
SONET/SDH Circuit Emulation over Packet (CEP) is an encapsulation
layer intended for emulating SONET/SDH circuits over a Packet
Switched Network.
This protocol provides a method for emulating the key elements of
traditional SONET/SDH SPE services across a packet-switched network.
Both large fixed-facility network operators and smaller network
operators using ad hoc facilities may use this service.
The protocol makes no assumptions as to the contents of the
SONET/SDH SPE, and therefore is applicable to SONET/SDH circuits
carrying any type of payload.
Because the protocol terminates the SONET/SDH section and line
before emulating the individual SPEs, the protocol allows the PSN to
operate as a distributed SONET/SDH cross-connect.
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3.1 Fidelity of Emulated SONET/SDH SPE services
The protocol does not make any assumptions about the capabilities of
the underlying PSN. However, the fidelity of the emulated service
will be dependent on the characteristics of the underlying PSN.
Emulated SONET/SDH SPE services may differ from native SONET/SDH
services on the following parameters: SPE timing, service
reliability, end-to-end delay, and bit-error-rate. Each of these
parameters is discussed below.
Because of the rigorous synchronization requirements implied by
SONET/SDH services, it is expected that the protocol will most
commonly be deployed in situations where a common timing reference
is available at the PW end-points. Large network operators have
well-defined methods for distributing Stratum timing references
(such as BITS, SASE, or GPS). Using these references is the most
direct technique that can be mathematically proven to meet the
relevant network synchronization specifications.
However, smaller network operators or remote locations in larger
networks may not have access to a common reference either by design
or due to a persistent fault in the timing distribution network. In
the absence of common references adaptive timing recovery techniques
may be employed. However, the fidelity of the recovered SPE timing
will be dependent on the packet-delay variation behavior of the
underlying PSN and the robustness of the timing recovery algorithm
used. As a result, it may be difficult in these circumstances to
mathematically prove that the recovered SPE timing is in compliance
with relevant synchronization standards.
Service Reliability may be impacted by two components: the
robustness of the underlying PSN and whether specific steps have
been taken to protect the emulated service (such as 1+1 protection
switching on the emulated service). The jitter buffer and packet
reordering mechanisms associated with the protocol increase
resilience of the emulated service to fast PSN rerouting events.
End-to-end delay will be impacted by both the transit delay through
the PSN and the packet-delay-variation characteristics of the PSN.
The protocol makes no assumption regarding either of these
parameters. However, the tighter the bound on transit delay and
delay variation, the shorter the end-to-end delay of the emulated
circuit will be.
BER for emulated circuits will be dependent on the characteristics
of the PSN. Each packet dropped by the PSN will result in an
equivalent number of byte errors on the emulated SPE. Using smaller
packet sizes can reduce the effect of lost packets on the emulated
service but increases the ratio of overhead to payload. The
protocol allows flexibility in packet length to accommodate the
desired BER/Overhead working point.
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To the extent possible, the use of low-loss paths (for example, by
reserving link bandwidth and router/switch buffering) in the PSN
will enhance the fidelity of the emulated circuits.
3.2 Performance Monitoring and Fault Isolation
The protocol allows collection of SONET/SDH-like faults and
performance monitoring parameters. Similarity with existing
SONET/SDH services is increased by the protocol's ability to carry
'far end error' indications (i.e. RDI). The protocol performance
monitoring capabilities are based on SONET/SDH requirements as
reflected by the available standards, and adapted to the nature of
the protocol.
The protocol provides the ability to detect lost packets and hence
allows it to distinguish between PSN problems and problems external
to the PSN as causes of outages and/or degradations of the emulated
service. In addition, the protocol supports fast detection of
defects, enabling vendors to implement rapid fault recovery
mechanisms for the emulated circuit.
3.3 Other Considerations
The protocol allows for bandwidth conservation in the PSN by
carrying only AIS-P and/or STS SPE Unequipped indications instead of
empty payloads, thus providing for efficiency gains on the PW.
Additional payload conservation techniques may be defined in the
future.
Being a constant bit rate (CBR) service, the protocol cannot provide
TCP-friendly behavior under network congestion. It will operate
best in environments where the Diff-Serv EF PHB with allocated
bandwidth is available end-to-end between the PW endpoints and the
EF bandwidth is sized to meet the requirements of the emulated
SONET/SDH circuits, or over a well engineered path as available
through the relevant signaling protocols like RSVP-TE and CR-LDP for
MPLS PSNs. Using these methods will prevent contention between the
SONET Emulation protocol and TCP traffic. Unusable service
characteristics from the packet switched network may be used to
trigger circuit/PW teardown or switch-over.
4 Scope
This document describes how to provide CEP for the following digital
signals:
1. SONET STS-1 synchronous payload envelope (SPE)/SDH VC-3
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2. STS-Nc SPE (N = 3, 12, 48, or 192)/SDH VC-4, VC-4-4c, VC-4-16c,
or VC-4-64c
For the remainder of this document, these constructs will be
referred to as SONET/SDH channels.
Although this document currently covers up to OC-192c/VC-4-64c,
future revision MAY address higher rates.
Other SONET/SDH signals, such as virtual tributary (VT) structured
sub-rate mapping, are not explicitly discussed in this document;
however, it can be extended in the future to support VT and lower
speed non-SONET/SDH services.
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5 CEP Encapsulation Format
In order to transport SONET/SDH SPEs through a packet-oriented
network, the SPE is broken into fragments. A CEP Header is pre-
pended to each fragment. The resulting packet is encapsulated in
RTP for transmission over an arbitrary PSN.
(Note: under certain circumstances the RTP header may be suppressed
to conserve network bandwidth. See section 5.4.3 for details).
The basic CEP packet appears in Figure 1.
+-----------------------------------+
| PSN and Multiplexing Layer |
| Headers |
+-----------------------------------+
| RTP Header |
| (RFC1889) |
+-----------------------------------+
| CEP Header |
+-----------------------------------+
| |
| |
| SONET/SDH SPE Fragment |
| |
| |
+-----------------------------------+
Figure 1 - Basic CEP Packet
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5.1 SONET/SDH SPE Fragment
The SONET/SDH Fragments MUST be byte aligned with the SONET/SDH SPE.
The first bit received from each byte of the SONET/SDH SPE MUST be
the Most Significant Bit of each byte in the SONET/SDH SPE fragment.
SONET/SDH bytes are placed into the SONET/SDH fragment in the same
order in which they are received.
SONET/SDH optical interfaces use binary coding and therefore are
scrambled prior to transmission to insure an adequate number of
transitions. For clarity, this scrambling will be referred to as
physical layer scrambling/descrambling.
In addition, many payload formats (such as for ATM and HDLC) include
an additional layer of scrambling to provide protection against
transition density violations within the SPEs. This function will
be referred to as payload scrambling/descrambling.
CEP assumes that physical layer scrambling/descrambling occurs as
part of the SONET/SDH section/line termination Native Service
Processing (NSP) functions.
However, CEP makes no assumption about payload scrambling. The
SONET/SDH SPE fragments MUST be constructed without knowledge or
processing of any incidental payload scrambling.
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5.2 CEP Header
The CEP Header supports a basic and extended mode. The Basic CEP
Header provides the minimum functionality necessary to accurately
emulate a TDM SONET over a PSN if a common reference is available at
both ends of the PW.
Enhanced functionality and commonality with other real-time Internet
applications is provided by RTP encapsulation.
Bit 0 of the first 32-bit CEP header indicates whether or not the
extended header is present. When this bit is 0, then no extended
header is present. When this bit is 1, then an extended header is
present. At this time, the contents of the extended header are for
future study. However, it is expected that this field will provide
support for payload compression, header protection, enhanced
performance monitoring, and/or other extensions to the base
protocol.
The Basic CEP header has the following format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0|R|D|N|P| Structure Pointer[0:12] | Sequence Number[0:13] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2 - Basic CEP Header Format
The Extended CEP header appears below:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|R|D|N|P| Structure Pointer[0:12] | Sequence Number[0:13] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3 - Extended CEP Header Format
The above fields are defined as follows:
R bit: CEP-RDI. This bit is set to one to signal to the remote CEP
function that a loss of packet synchronization has occurred. See
section 6.4 for details.
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D bit: Signals DBA Mode. MUST be set to zero for Normal Operation.
MUST be set to one if CEP is currently in DBA mode. DBA is an
optional mode during which trivial SPEs are not transmitted into the
packet network. See Table 1 and section 6 for further details.
The N and P bits: MAY be used to explicitly relay negative and
positive pointer adjustment events across the PSN. They are also
used to relay SONET/SDH maintenance signals such as AIS-P. See
Table 1 and sections 7 and 9 for more details.
+---+---+---+----------------------------------------------+
| D | N | P | Interpretation |
+---+---+---+----------------------------------------------+
| 0 | 0 | 0 | Normal Mode - No Ptr Adjustment |
| 0 | 0 | 1 | Normal Mode - Positive Ptr Adjustment |
| 0 | 1 | 0 | Normal Mode - Negative Ptr Adjustment |
| 0 | 1 | 1 | Normal Mode - AIS-P |
| | | | |
| 1 | 0 | 0 | DBA Mode - STS SPE Unequipped |
| 1 | 0 | 1 | DBA Mode - STS SPE Unequipped Pos Ptr Adj |
| 1 | 1 | 0 | DBA Mode - STS SPE Unequipped Neg Ptr Adj |
| 1 | 1 | 1 | DBA Mode - AIS-P |
+---+---+---+----------------------------------------------+
Table 1. Interpretation of D, N, and P bits
Sequence Number[0:13]: This is a packet sequence number, which MUST
continuously cycle from 0 to 0x3FFF. It is generated and processed
in accordance with the rules established in [RFC1889]. When the RTP
header is used, this sequence number MUST match the LSBs of the RTP
sequence Number.
Structure Pointer[0:12]: The Structure Pointer MUST contain the
offset of the J1 byte within the CEP SPE Fragment. The value is
from 0 to 0x1FFE, where 0 means the first byte after the CEP header.
The Structure Pointer MUST be set to 0x1FFF if a packet does not
carry the J1 byte. See [SONET], [GR253], and [G707] for more
information on the J1 byte and the SONET/SDH payload pointer.
Implementations MUST support SPE Fragments of 783 bytes and MAY
support SPE fragments of from 8 to 8191 bytes.
Note 1: Implementations that choose to support programmable payload
lengths SHOULD support payloads that are an integer multiple of 8
bytes.
Note 2: CEP packets are fixed in length for all of the packets of a
particular emulated TDM stream. This length is statically
provisioned for each TDM stream. Therefore, the length of each CEP
packet does not need to be carried in the CEP header.
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5.3 RTP Header
CEP uses the fixed RTP Header as shown below.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|V=2|P|X| CC |M| PT | sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| synchronization source (SSRC) identifier |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
o V (version) is always set to 2
o P (padding) is always set to 0
o X (header extension) is always set to 0
o CC (CSRC count) is always set to 0
o M (marker) is set to 0 for CEP packets.
o PT (payload type) is used to identify packets carrying the
packetized SONET/SDH data. One PT value should be allocated from
the range of dynamic values (see [RTP-TYPES]) for every CEP PW.
Allocation is done during the PW setup and MUST be the same for both
PW directions. The PE at the PW ingress MUST set the PT value in the
RTP header to the allocated value.
o Sequence Number is used primarily to provide the common PW
sequencing function as well as detection of lost packets. It is
generated and processed in accordance with the rules established in
[RFC1889].
o Timestamp is used primarily for carrying timing information over
the network. Their values are used in accordance with the rules
established in [RFC1889]. Frequency of the clock used for
generating timestamps MUST be 19.44 MHz based on a local reference.
O SSRC (synchronization source) value in the RTP header MAY be used
for detection of misconnections.
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5.4 PSN Encapsulation
In principle, CEP packets can be carried over any packet-oriented
network. The following sections describe specifically how CEP
packets MUST be encapsulated for carriage over MPLS or IP networks.
5.4.1 IP Encapsulation
CEP uses the standard IP/UDP/RTP encapsulation scheme as shown
below. The UDP destination port MUST be used to Demultiplex
individual SONET channels.
+-----------------------------------+
| |
| IPv6/v4 Header |
| |
+-----------------------------------+
| UDP Header |
+-----------------------------------+
| RTP Header |
+-----------------------------------+
| CEP Header |
+-----------------------------------+
| |
| |
| SONET/SDH SPE Fragment |
| |
| |
+-----------------------------------+
Figure 4 - IP Transport Encapsulation
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5.4.2 MPLS Encapsulation
RTP MAY be directly encapsulated in MPLS as shown below. To
transport a CEP packet over an MPLS network, an MPLS label-stack
MUST be pushed on top of the CEP packet. The bottom label in the
MPLS label stack MUST be used to demultiplex individual SONET
channels. In keeping with the conventions used in [MARTINI-TRANS],
this demultiplexing label is referred to as the VC Label and the
upper labels are referred to as Tunnel Labels.
+-----------------------------------+
| One or more MPLS Tunnel Labels |
+-----------------------------------+
| VC Label |
+-----------------------------------+
| RTP Header |
+-----------------------------------+
| CEP Header |
+-----------------------------------+
| |
| |
| SONET/SDH SPE Fragment |
| |
| |
+-----------------------------------+
Figure 5 - Typical MPLS Transport Encapsulation
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5.4.3 RTP Header Suppression
In addition to normal RTP header compression mechanisms as described
in [RFC2508] and [RFC3095], an additional option may be used in CEP
which suppresses transmission of the RTP header altogether.
This mode may be used when both SONET Emulation PEs have access to a
common reference clock and both support RTP Header Suppression.
Under these conditions the following encapsulation formats may be
used.
The choice to utilize RTP Header Suppression may be statically
configured using [CEM-MIB], or signaled using a PW maintenance
protocol such as [MARTINI-TRANS].
+-----------------------------------+
| |
| IPv6/v4 Header |
| |
+-----------------------------------+
| UDP Header |
+-----------------------------------+
| CEP Header |
+-----------------------------------+
| |
| |
| SONET/SDH SPE Fragment |
| |
| |
+-----------------------------------+
Figure 6 - IP Transport Encapsulation w/ RTP Header Suppression
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+-----------------------------------+
| One or more MPLS Tunnel Labels |
+-----------------------------------+
| VC Label |
+-----------------------------------+
| CEP Header |
+-----------------------------------+
| |
| |
| SONET/SDH SPE Fragment |
| |
| |
+-----------------------------------+
Figure 7 - MPLS Transport Encapsulation w/ RTP Header Suppression
5.5 L2TP Encapsulation
Encapsulation for L2TP PSNs is for future study.
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6 CEP Operation
The following sections describe CEP operation.
6.1 Introduction and Terminology
CEP MUST support a normal mode of operation and MAY support an
optional extension called Dynamic Bandwidth Allocation (DBA).
During normal operation, SONET/SDH payloads are fragmented, pre-
pended with the appropriate headers and then transmitted into the
packet network. During DBA mode, only the headers are transmitted.
This is done to conserve bandwidth when meaningful user data is not
present in the SPE, such as during AIS-P or STS SPE Unequipped.
6.1.1 CEP Packetizer and De-Packetizer
As with all adaptation functions, CEP has two distinct components:
adapting TDM SONET/SDH into a CEP packet stream, and converting the
CEP packet stream back into a TDM SONET/SDH. The first function
will be referred to as CEP Packetizer and the second as CEP De-
Packetizer. This terminology is illustrated in Figure 8.
+------------+ +---------------+
| | | |
SONET --> | CEP | --> PSN --> | CEP | --> SONET
SDH | Packetizer | | De-Packetizer | SDH
| | | |
+------------+ +---------------+
Figure 8 - CEP Terminology
Note: the CEP de-packetizer requires a buffering mechanism to
account for delay variation in the CEP packet stream. This
buffering mechanism will be generically referred to as the CEP
jitter buffer.
6.1.2 CEP DBA
DBA is an optional mode of operation that only transmits the headers
into the packet network under certain circumstances such as AIS-P or
STS Unequipped.
If DBA is supported by a CEP implementation, the user SHOULD be able
to configure if DBA will be triggered by AIS-P, STS Unequipped,
both, or neither on a per channel basis.
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If DBA is supported, the determination of AIS-P and STS Unequipped
MUST be based on the state of SONET/SDH Section, Line, and Path
Overhead bytes.
During AIS-P, there is no valid payload pointer, so pointer
adjustments cannot occur. During STS Unequipped, the SONET/SDH
payload pointer is valid, and therefore pointer adjustments MUST be
supported even during DBA. See Table 1 for details.
6.2 Description of Normal CEP Operation
During normal operation, the CEP packetizer will receive a fixed
rate byte stream from a SONET/SDH interface. When a packets worth
of data has been received from a SONET/SDH channel, the necessary
headers are pre-pended to the SPE fragment and the resulting CEP
packet is transmitted into the packet network. Because all CEP
packets associated with a specific SONET/SDH channel will have the
same length, the transmission of CEP packets for that channel SHOULD
occur at regular intervals.
At the far end of the packet network, the CEP de-packetizer will
receive packets into a jitter buffer and then play out the received
byte stream at a fixed rate onto the corresponding SONET/SDH
channel. The jitter buffer SHOULD be adjustable in length to
account for varying network delay behavior. The receive packet rate
from the packet network should be exactly balanced by the
transmission rate onto the SONET/SDH channel, on average. The time
over which this average is taken corresponds to the depth of the
jitter buffer for a specific CEP channel.
The RTP sequence numbers provide a mechanism to detect lost and/or
mis-ordered packets. The sequence number in the CEP header may be
used when transmission of the RTP header is suppressed (see section
5.4.3 for details). The CEP de-packetizer MUST detect lost or mis-
ordered packets. The CEP de-packetizer SHOULD play out an all ones
pattern (AIS) in place of any dropped packets. The CEP de-
packetizer MAY re-order packets received out of order. If the CEP
de-packetizer does not support re-ordering, it must drop mis-ordered
packets.
6.3 Description of CEP Operation during DBA
There are several issues that should be addressed by a workable CEP
DBA mechanism. First, when DBA is invoked, there should be a
substantial savings in bandwidth utilization in the packet network.
The second issue is that the transition in and out of DBA should be
tightly coordinated between the local CEP packetizer and CEP de-
packetizer at the far side of the packet network. A third is that
the transition in and out of DBA should be accomplished with minimal
disruption to the adapted data stream.
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Another goal is that the reduction of CEP traffic due to DBA should
not be mistaken for a fault in the packet network or vice-versa.
Finally, the implementation of DBA should require minimal
modifications beyond what is necessary for the nominal CEP case.
The mechanism described below is a reasonable balance of these
goals.
During DBA, packets MUST be emitted at exactly the same rate as they
would be during normal operation. This SHOULD be accomplished by
transmitting each DBA packet after a complete packet of data has
been received from the SONET/SDH channel. The only change from
normal operation is that the CEP packets during DBA MUST only
suppress the transmission of the SPE while still sending the
appropriate headers. Because some links have a minimum supported
packet size, the CEP packetizer MAY append a configurable number of
bytes immediately after the CEP header to pad out the CEP packet to
reach the mimumum supported packet size. The D-bit MUST be set to
one, to indicate that DBA is active.
The CEP de-packetizer MUST assume that each packet received with the
D-bit set represents a normal-sized packet containing an AIS-P or
SPE Unequipped payload as noted by N and P. See Table 1. The CEP
de-packetizer MUST accept DBA packets with or without padding.
This allows the CEP packetization and de-packetization logic during
DBA to be similar to the nominal case. It ensures that the correct
SONET/SDH indication is reliably transmitted between CEP adaptation
points. It minimizes the risk of under or over running the jitter
buffer during the transition in and out of DBA, since packets are
continuously transmitted during DBA. And, it guarantees that faults
in the packet network are recognized as distinctly different from
line conditioning on the SONET/SDH interfaces.
6.4 Packet Synchronization
A key component in declaring the state of a CEP service is whether
or not the CEP de-packetizer is in or out of packet synchronization.
The following paragraphs describe how that determination is made.
As discussed in section 6, a CEP de-packetizer MAY or MAY NOT
support re-ordering of mis-ordered packets.
As packets are received from the PSN, they are placed into a jitter
buffer prior to play out on the SONET interface. If a CEP de-
packetizer supports re-ordering, any packet received before its play
out time will still be considered valid.
If a CEP de-packetizer does not support re-ordering, a number of
approaches may be used to minimize the impact of mis-ordered or lost
packets on the final re-assembled SONET stream. For example, [AAL1]
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uses a simple state-machine to re-order packets in a sub-set of
possible cases.
However, the final determination as to whether or not to declare
acquisition or loss of packet synchronization MUST be based on the
same criteria regardless of whether an implementation supports or
does not support re-ordering.
Therefore, the determination of acquisition or loss of packet
synchronization is always made at SONET play-out time. During SONET
play-out, the CEP de-packetizer will play received CEP packets onto
the SONET interface. However, if the jitter buffer is empty or the
packet to be played out has not been received, the CEP de-packetizer
will play out an empty packet onto the SONET interface in place of
the unavailable packet.
The acquisition of packet synch is based on the number of sequential
CEP packets that are played onto the SONET interface. While, loss
of packet synch is based on the number of sequential 'empty' packets
that are played onto the SONET interface. Specific details of these
two cases is described below.
6.4.1 Acquisition of Packet Synchronization
At startup, a CEP de-packetizer will be out of packet
synchronization by default. To declare packet synchronization at
startup or after a loss of packet synchronization, the CEP de-
packetizer must play-out a configurable number of CEP packets with
sequential sequence numbers towards the SONET interface.
6.4.2 Loss of Packet Synchronization
Once a CEP de-packetizer is in packet sync, it may encounter a set
of events that will cause it to lose packet synchronization.
If the CEP de-packetizer encounters more than a configurable number
of sequential empty packets, the CEP de-packetizer MUST declare loss
of packet synchronization (LOPS) defect.
Loss of Packet Synchronization (LOPS) failure is declared after 2.5
+/- 0.5 seconds of LOPS defect, and cleared after 10 seconds free of
LOPS defect state. The VC is considered down as long as LOPS failure
is declared.
7 SONET/SDH Maintenance Signals
There are several issues that must be considered in the mapping of
maintenance signals between SONET/SDH and a PSN. A description of
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how these signals and conditions are mapped between the two domains
is described below.
For clarity, the mappings are split into two groups: SONET/SDH to
PSN, and PSN to SONET/SDH.
7.1 SONET/SDH to PSN
The following sections describe how SONET/SDH Maintenance Signals
and Alarm conditions are mapped into a Packet Switched Network.
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7.1.1 AIS-P Indication
In a SONET/SDH network, SONET Path outages are signaled using
maintenance alarms such as Path AIS (AIS-P). In particular, AIS-P
indicates that the SONET/SDH Path is not currently transmitting
valid end-user data, and the SPE contains all ones.
It should be noted that nearly every type of service-affecting
section or line defect will result in an AIS-P condition.
The SONET/SDH hierarchy is illustrated below.
+----------+
| PATH |
+----------+
^
|
AIS-P
|
|
+----------+
| LINE |
+ ---------+
^ ^
| |
AIS-L +------ LOP
|
|
+----------+
| SECTION |
+----------+
^ ^
| |
| |
LOS LOF
Figure 9 - SONET/SDH Fault Hierarchy
Should the Section Layer detect a Loss of Signal (LOS) or Loss of
Frame (LOF) condition, it sends AIS-L up to the Line Layer. If the
Line Layer detects AIS-L or Loss of Path (LOP), it sends AIS-P to
the Path Layer.
In normal mode during AIS-P, CEP packets are generated as usual.
The N and P bits MUST be set to 11 binary to signal AIS-P explicitly
through the packet network. The D-bit MUST be set to zero to
indicate that the SPE is being carried through the packet network.
Normal CEP packets with the SPE fragment, CEP Header, the Circuit ID
Word, and PSN Header MUST be transmitted into the packet network.
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However, to conserve network bandwidth during AIS-P, DBA MAY be
employed. If DBA has been enabled for AIS-P and AIS-P is currently
occurring, the N and P bits MUST be set to 11 binary to signal AIS,
and the D-bit MUST be set to one to indicate that the SPE is not
being carried through the packet network. Only the CEP header, the
Circuit ID Word, and the PSN Header MUST be transmitted into the
packet network.
7.1.2 STS SPE Unequipped Indication
The declaration of STS SPE unequipped MUST conform to [GR253].
Quoted below:
"R6-135 [481] STS PTE shall detect an STS Path Unequipped (UNEQ-P)
defect within 10 ms of the onset of at least five consecutive
samples (which may or may not be consecutive frames) of unequipped
STS Signal Labels (C2 byte), as specified in Table 6-2"
The termination of STS SPE unequipped MUST also conform to [GR253].
"R6-137 [485v2] STS PTE shall terminate an UNEQ-P defect within 10
ms of the onset of at least five consecutive samples (which may or
may not be consecutive frames) of STS Signal Labels that are not
unequipped or all-ones, as specified in Table 6-2"
For normal operation during SPE Unequipped, the N and P bits MUST be
interpreted as usual. The SPE MUST be transmitted into the packet
network along with the appropriate headers, and the D-Bit MUST be
set to zero.
If DBA has been enabled for STS SPE Unequipped and the Unequipped is
occurring on the SONET/SDH channel, the D-bit MUST be set to one to
indicate DBA is active. Only the necessary headers are transmitted
into the packet network. The N and P bits MAY be used to signal
pointer adjustments as normal. See Table 1 and section 6 for
details.
7.1.3 CEP-RDI
The CEP function MUST send CEP-RDI towards the packet network during
loss of packet synchronization. This MUST be accomplished by
setting the R bit to one in the CEP header.
7.2 PSN to SONET/SDH
The following sections discuss how the various conditions on the
packet network are converted into SONET/SDH indications.
7.2.1 AIS-P Indication
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There are several conditions in the packet network that will cause
the CEP de-packetization function to play out an AIS-P indication
towards a SONET/SDH channel.
The first of these is the receipt of CEP packets with the N and P
bits set to one, and the D-bit set to zero. This is an explicit
indication of AIS-P being received at the far-end of the packet
network, with DBA disabled for AIS-P. The CEP de-packetizer MUST
play out the received SPE fragment (which will incidentally be
carrying all ones), and MUST configure the SONET/SDH Overhead to
signal AIS-P as defined in [SONET], [GR253], and [G707].
The second case is the receipt of CEP packets with the N and P bits
set to one, and the D-bit set to one. This indicates that AIS-P is
being received at the far-end of the packet network, with DBA
enabled for AIS-P. The CEP de-packetizer MUST play out one packet's
worth of all ones for each packet received, and MUST configure the
SONET/SDH Overhead to signal AIS-P as defined in [SONET], [GR253],
and [G707].
A third case that will cause a CEP de-packetization function to play
out an AIS-P indication onto a SONET/SDH channel is during loss of
packet synchronization. The CEP de-packetizer MUST configure the
SONET/SDH Overhead to signal AIS-P as defined in [SONET], [GR253],
and [G707].
7.2.2 STS SPE Unequipped Indication
There are three conditions in the packet network that will cause the
CEP function to transmit STS SPE Unequipped indications onto the
SONET/SDH channel.
The first, which is transparent to CEP, is the receipt of regular
CEP packets that happen to be carrying an SPE that contains the
appropriate Path overhead to signal STS SPE unequipped. This case
does not require any special processing on the part of the CEP de-
packetizer.
The second case is the receipt of CEP packets that have the D-bit
set to one to indicate DBA active and the N and P bits set to 00
binary, 01 binary, or 10 binary to indicate SPE Unequipped with or
without pointer adjustments. The CEP de-packetizer MUST use this
information to transmit a packet of all zeros onto the SONET/SDH
interface, and adjust the payload pointer as necessary.
The third case when a CEP de-packetizer MUST play out an STS SPE
Unequipped Indication towards the SONET interface is when the VC-
label has been withdrawn due to de-provisioning of the circuit.
8 SONET/SDH Transport Timing
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It is assumed that the distribution of SONET/SDH Transport timing
information is addressed through external mechanisms such as
Building Integrated Timing System (BITS), Stand Alone
Synchronization Equipment (SASE), Global Positioning System (GPS) or
other such methods and is therefore outside of the scope of this
specification.
9 SONET/SDH Pointer Management
A pointer management system is defined as part of the definition of
SONET/SDH. Details on SONET/SDH pointer management can be found in
[SONET], [GR253], and [G707]. If there is a frequency offset
between the frame rate of the transport overhead and that of the
SONET/SDH SPE, then the alignment of the SPE shall periodically slip
back or advance in time through positive or negative stuffing.
The emulation of this aspect of SONET networks may be accomplished
using a variety of techniques including (but not limited to)
explicit pointer adjustment relay (EPAR) and adaptive pointer
management (APM).
In any case, the handling of the SPE data by the CEP packetizer is
the same.
During a negative pointer adjustment event, the CEP packetizer MUST
incorporate the H3 byte from the SONET/SDH stream into the CEP
packet payload in order with the rest of the SPE. During a positive
pointer adjustment event, the CEP de-packetizer MUST strip the stuff
byte from the CEP packet payload.
When playing out a negative pointer adjustment event, the
appropriate byte of the CEP payload MUST be placed into the H3 byte
of the SONET/SDH stream. When playing out a positive pointer
adjustment, the CEP de-packetizer MUST insert a stuff-byte into the
appropriate position within the SONET/SDH stream.
The details regarding the use of the H3 byte and stuff byte during
positive and negative pointer adjustments can be found in [SONET],
[GR253], and [G707].
9.1 Explicit Pointer Adjustment Relay (EPAR)
CEP provides an OPTIONAL mechanism to explicitly relay pointer
adjustment events from one side of the PSN to the other. This
technique will be referred to as Explicit Pointer Adjustment Relay
(EPAR). The mechanics of EPAR are described below.
The following text only applies to implementations that choose to
implement EPAR. Any CEP implementation that does not support EPAR
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MUST either set the N and P bits to zero or utilize them to relay
AIS-P and STS Unequipped as shown in table 1.
If EPAR is being used, the pointer adjustment event MUST be
transmitted in three consecutive packets by the packetizer. The de-
packetizer MUST play out the pointer adjustment event when any one
packet with N/P bit set is received.
References [SONET], [GR253], and [G707] specify that pointer
adjustment events MUST be separated by three SONET/SDH frames
without a pointer adjustment event. In order to explicitly relay
all legal pointer adjustment events, the packet size for a specific
circuit SHOULD be no larger than (783 * 4 * N)/3, where N is the
STS-Nc multiplier.
However, there are SONET implementations that allow pointer
adjustments to occur in back to back SONET/SDH frames. In order to
support this possibility, EPAR implementations SHOULD set the packet
size for a particular circuit to be no larger than (783*N)/3. Where
N is the STS-Nc multiplier.
Since the minimum value of N is one, EPAR implementations SHOULD
support a minimum payload length of 783/3 or 261 bytes.
For EPAR implementations, the CEP de-packetizer MUST utilize the CEP
sequence numbers to insure that SONET/SDH pointer adjustment events
are not played any more frequently than once per every three CEP
packets transmitted by the remote CEP packetizer.
If both bits are set, then an AIS-P event has occurred (this is
further discussed in section 7).
When DBA is invoked (i.e. the D-bit = 1), N and P have additional
meanings. See Table 1 and section 6.
9.2 Adaptive Pointer Management (APM)
Another OPTIONAL method that may be used to emulate SONET pointer
management is Adaptive Pointer Management (APM). In basic terms,
APM uses information about the depth of the CEP jitter buffers to
introduce pointer adjustments in the reassembled SONET SPE.
Details about specific APM algorithms is for future study.
10 CEP Performance Monitors
SONET/SDH as defined in [SONET], [GR253], and [G707] includes the
definition of several counters that may be used to monitor the
performance of SONET/SDH services. These counters are referred to
as Performance Monitors.
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In order for CEP to be utilized by traditional SONET/SDH network
operators, CEP SHOULD provide similar functionality. To this end,
the following sections describe a number of counters that will
collectively be referred to as CEP Performance Monitors.
10.1 Near-End Performance Monitors
These performance monitors are maintained by the CEP De-Packetizer
during reassembly of the SONET stream.
The performance monitors are based on two types of defects.
Type 1 defect is defined as: missing or dropped packet.
Type 2 defect is defined as: buffer under run, buffer over-run,
LOPS.
The specific performance monitors that are defined for CEP are as
follows:
ES-CEP - CEP Errored Seconds
SES-CEP - CEP Severely Errored Seconds
UAS-CEP - CEP Unavailable Seconds
Each second that contain at least one type 1 defect SHALL be
declared as ES-CEP.
Each second that contain type 2 defect, or missing packets above
pre-defined, configurable threshold of missing/dropped packets SHALL
be declared both SES-CEP and ES-CEP. Default value for missing
packet to SES is 3.
UAS-CEP SHALL be declared after X consecutives SES-CEP, cleared
after X consecutive seconds without SES-CEP. Default value of X is
10 seconds.
Once unavailability is declared, ES and SES counts SHALL be
inhibited up to the point where the unavailability was started. Once
unavailability is removed, ES that occurred along the X seconds
clearing period SHALL be added to the ES counts. An update is
required even for closed intervals if necessary.
FC-CEP is the number of time type 1 or type 2 defect states were
declared. The NE SHALL have thresholding on ES-CEP, SES-CEP and
UAS-CEP (thresholding mean activate a notification if more than pre-
defined # of seconds are declared as ES, etc. in 15 minutes
interval).
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10.2 Far-End Performance Monitors
These performance monitors provide insight into the CEM De-
packetizer at the far-end of the PSN.
Far end statistics are based on the RDI-CEP bit. Limited
functionality is supported compared to [GR-253] for simplicity and
because it is assumed that all relevant statistics are available
from the end point of the PW. CEP-FE defect is declared when CEP-RDI
is set in the incoming CEP packets.
CEP-FE failure declared after 2.5 +/- 0.5 seconds of CEP-FE defect,
and cleared after 10 seconds free of CES-FE defect state. Sending
notification to the OS for CEP-FE failure is local policy.
This draft does not attempt to define SES-CEPFE, UAS-CEPFE and FC-
CEPFE, but they can be added if to fully emulate GR-253 far end PM
(thresholding is required too here except for FC-CEPFE). (Note that
ES-CEPFE is not relevant since CEP does not report back missing
packets - only LOPS which is SES).
The definition of additional performance monitors is for future
study.
11 Open Issues
This version of the draft does not address payload compression
within the emulated SONET. Payload compression is expected to be
supported by future versions of this draft by utilizing the extended
CEP header.
This version of the draft does not tie into PWE3 maintenance
mechanisms for the setup and tear down of services. That short-
coming will be addressed in future revisions of this document.
Underlying MPLS QoS requirements are not covered by this revision of
the draft. Future revisions may discuss underlying QoS
requirements.
Support for VT and lower speed non-SONET/SDH services are not
covered in this revision of the draft. Future revisions may address
VT and non-SONET/SDH TDM services.
An alternate version of DBA has been suggested that would suppress
transmission of the entire CEP packet stream under certain
circumstances. Future versions of this draft may define such a
mechanism.
It is possible to define SONET Emulation specific redundancy
mechanisms, such as 1+1 or N:1. Future versions of this draft may
define such mechanisms.
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12 Security Considerations
This document does not address or modify security issues within the
relevant PSNs.
13 Intellectual Property Disclaimer
This document is being submitted for use in IETF standards
discussions. Vivace Networks, Inc. has filed one or more patent
applications relating to the CEP technology outlined in this
document. Vivace Networks, Inc. will grant free unlimited licenses
for use of this technology.
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14 References
[RFC2026] Bradner, S., "The Internet Standards Process -- Revision
3", BCP 9, RFC2026, October 1996.
[PWE3-REQ] XiPeng Xiao et al, Requirements for Pseudo Wire Emulation
Edge-to-Edge (PWE3), Work in Progress, July-2001, draft-ietf-pwe3-
requirements-01.txt
[PWE3-FW] Prayson Pate et al, Framework for Pseudo Wire Emulation
Edge-to-Edge (PWE3), Work in progress, February 2002, draft-ietf-
pwe3-framework-00.txt
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[PWE3-LAYERS], Stewart Bryant et al., Protocol Layering in PWE3, Work
in Progress, February 2002, pwe3-protocol-layering-01.txt
[SONET] American National Standards Institute, "Synchronous Optical
Network (SONET) - Basic Description including Multiplex Structure,
Rates and Formats," ANSI T1.105-1995.
[GR253] Telcordia Technologies, "Synchronous Optical Network (SONET)
Transport Systems: Common Generic Criteria", GR-253-CORE, Issue 3,
September 2000.
[G707] ITU Recommendation G.707, "Network Node Interface For The
Synchronous Digital Hierarchy", 1996.
[RFC1889] H. Schulzrinne et al, RTP: A Transport Protocol for Real-
Time Applications, RFC 1889, IETF, 1996
[ROHC-LLA] Lars-Eric Jonsson et al, A Link-Layer Assisted ROHC
Profile for IP/UDP/RTP draft-ietf-rohc-rtp-lla-03.txt.
[CEM] Malis et al, "SONET/SDH Circuit Emulation Service Over MPLS
(CEM) Encapsulation", draft-malis-sonet-ces-mpls-05.txt, work in
progress, July 2001.
[CEM-MIB] Danenberg et al, "SONET/SDH Circuit Emulation Service Over
PSN (CEP) Management Information Base Using SMIv2", draft-danenberg-
pw-cem-mib-02.txt, work in progress, Feb 2002.
[MARTINI-TRANS] Martini et al, "Transport of Layer 2 Frames Over
MPLS", draft-martini-l2circuit-trans-mpls-06.txt, work in progress,
July 2001.
[MARTINI-ENCAP] Martini et al, "Encapsulation Methods for Transport
of Layer 2 Frames Over MPLS", draft-martini-l2circuit-encap-mpls-
02.txt, work in progress, July 2001.
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[CESoPSN] Vainshtein et al, "TDM Circuit Emulation Service over
Packet Switched Network", draft-vainshtein-cesopsn-02.txt, work in
progress, February 2002.
[CES-VT] Pate et al, "TDM Service Specification for Pseudo-Wire
Emulation Edge-to-Edge", draft-pate-pwe3-tdm-03.txt, work in
progress, January 2001.
[RFC2508] S.Casner, V.Jacobson, Compressing IP/UDP/RTP Headers for
Low-Speed Serial Links, RFC 2508, IETF, 1999
[RFC3095] C.Bormann (Ed.), RObust Header Compression (ROHC):
Framework and four profiles: RTP, UDP, ESP, and uncompressed, RFC
3095, IETF, 2001
[AAL1] ITU-T, "Recommendation I.363.1, B-ISDN Adaptation Layer
Specification: Type AAL1", Appendix III, August 1996.
15 Acknowledgments
The authors would like to thank all of the members of the PWE3
working group who have contributed to the development of this draft,
and specifically Danny McPherson and Allison Mankin for their advice
and assistance.
16 Author's Addresses
Andrew G. Malis
Vivace Networks, Inc.
2730 Orchard Parkway
San Jose, CA 95134
Email: Andy.Malis@vivacenetworks.com
Ken Hsu
Vivace Networks, Inc.
2730 Orchard Parkway
San Jose, CA 95134
Email: Ken.Hsu@vivacenetworks.com
Jeremy Brayley
Laurel Networks, Inc.
2706 Nicholson Rd.
Sewickley, PA 15143
Email: jbrayley@laurelnetworks.com
Steve Vogelsang
Laurel Networks, Inc.
2706 Nicholson Rd.
Sewickley, PA 15143
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Email: sjv@laurelnetworks.com
John Shirron
Laurel Networks, Inc.
2607 Nicholson Rd.
Sewickley, PA 15143
Email: jshirron@laurelnetworks.com
Luca Martini
Level 3 Communications, LLC.
1025 Eldorado Blvd.
Broomfield, CO 80021
Email: luca@level3.net
Tom Johnson
Litchfield Communications, Inc.
76 Westbury Park Rd.
Watertown, CT 06795
Email: tom_johnson@litchfieldcomm.com
Ed Hallman
Litchfield Communications, Inc.
76 Westbury Park Rd.
Watertown, CT 06795
Email: ed_hallman@litchfieldcomm.com
Marlene Drost
Litchfield Communications, Inc.
76 Westbury Park Rd.
Watertown, CT 06795
Email: marlene_drost@litchfieldcomm.com
Jim Boyle
Protocol Driven Networks, Inc.
1381 Kildaire Farm #288
Cary, NC 27511
Email: jboyle@pdnets.com
David Zelig
Corrigent Systems LTD.
126, Yigal Alon st.
Tel Aviv, ISRAEL
Email: davidz@corrigent.com
Ron Cohen
Lycium Networks
Hamanofim 9, POB 12256
Herzeliya, Israel 46733
Email: ronc@lyciumnetworks.com
Prayson Pate
Overture Networks
P. O. Box 14864
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RTP, NC, USA 27709
Email: prayson.pate@overturenetworks.com
Craig White
Level3 Communications, LLC.
1025 Eldorado Blvd,
Broomfield CO 80021
Email: Craig.White@Level3.com
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Appendix A. SONET/SDH Rates and Formats
For simplicity, the discussion in this section uses SONET
terminology, but it applies equally to SDH as well. SDH-equivalent
terminology is shown in the tables.
The basic SONET modular signal is the synchronous transport signal-
level 1 (STS-1). A number of STS-1s may be multiplexed into higher-
level signals denoted as STS-N, with N synchronous payload envelopes
(SPEs). The optical counterpart of the STS-N is the Optical Carrier-
level N, or OC-N. Table 2 lists standard SONET line rates discussed
in this document.
OC Level OC-1 OC-3 OC-12 OC-48 OC-192
SDH Term - STM-1 STM-4 STM-16 STM-64
Line Rate(Mb/s) 51.840 155.520 622.080 2,488.320 9,953.280
Table 2. Standard SONET Line Rates
Each SONET frame is 125 us and consists of nine rows. An STS-N frame
has nine rows and N*90 columns. Of the N*90 columns, the first N*3
columns are transport overhead and the other N*87 columns are SPEs.
A number of STS-1s may also be linked together to form a super-rate
signal with only one SPE. The optical super-rate signal is denoted
as OC-Nc, which has a higher payload capacity than OC-N.
The first 9-byte column of each SPE is the path overhead (POH) and
the remaining columns form the payload capacity with fixed stuff
(STS-Nc only). The fixed stuff, which is purely overhead, is N/3-1
columns for STS-Nc. Thus, STS-1 and STS-3c do not have any fixed
stuff, STS-12c has three columns of fixed stuff, and so on.
The POH of an STS-1 or STS-Nc is always nine bytes in nine rows. The
payload capacity of an STS-1 is 86 columns (774 bytes) per frame.
The payload capacity of an STS-Nc is (N*87)-(N/3) columns per frame.
Thus, the payload capacity of an STS-3c is (3*87 - 1)*9 = 2,340
bytes per frame. As another example, the payload capacity of an STS-
192c is 149,760 bytes, which is 64 times the capacity of an STS-3c.
There are 8,000 SONET frames per second. Therefore, the SPE size,
(POH plus payload capacity) of an STS-1 is 783*8*8,000 = 50.112
Mb/s. The SPE size of a concatenated STS-3c is 2,349 bytes per frame
or 150.336 Mb/s. The payload capacity of an STS-192c is 149,760
bytes per frame, which is equivalent to 9,584.640 Mb/s. Table 2
lists the SPE and payload rates supported.
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SONET STS Level STS-1 STS-3c STS-12c STS-48c STS-192c
SDH VC Level - VC-4 VC-4-4c VC-4-16c VC-4-64c
Payload Size(Bytes) 774 2,340 9,360 37,440 149,760
Payload Rate(Mb/s) 49.536 149.760 599.040 2,396.160 9,584.640
SPE Size(Bytes) 783 2,349 9,396 37,584 150,336
SPE Rate(Mb/s) 50.112 150.336 601.344 2,405.376 9,621.504
Table 2. Payload Size and Rate
To support circuit emulation, the entire SPE of a SONET STS or SDH
VC level is encapsulated into packets, using the encapsulation
defined in section 5, for carriage across packet-switched networks.
Full Copyright Statement
Copyright (C) The Internet Society (2001). All Rights Reserved. This
document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
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pwe3-sonet Expires December 2002 [Page 34]
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