Network Working Group Andrew G. Malis
Internet Draft Ken Hsu
Expiration Date: October 2001 Vivace Networks, Inc.
Jeremy Brayley
Steve Vogelsang
John Shirron
Laurel Networks, Inc.
Luca Martini
Level 3 Communications, LLC.
Tom Johnson
Marlene Drost
Ed Hallman
Litchfield Communications
April 2001
SONET/SDH Circuit Emulation Service Over MPLS (CEM) Encapsulation
draft-malis-sonet-ces-mpls-04.txt
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of section 10 of RFC 2026 [1].
Internet-Drafts are working documents of the Internet Engineering
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Internet-Drafts are draft documents valid for a maximum of six
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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.
1. Abstract
This document describes a method for encapsulating SONET/SDH Path
signals for transport across an MPLS network.
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2. 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].
3. Introduction
This document describes a method for encapsulating time division
multiplexed (TDM) digital signals for transmission over a packet-
oriented MPLS network. The transmission system for circuit-oriented
TDM signals is the Synchronous Optical Network (SONET) [3], [6] /
Synchronous Digital Hierarchy (SDH) [4]. To support TDM traffic,
which includes voice, data, and private leased line service, the
MPLS network must emulate the circuit characteristics of SONET/SDH
payloads. MPLS labels and a new circuit emulation header are used
to encapsulate TDM signals and provide the Circuit Emulation Service
over MPLS (CEM).
This document also describes an optional extension to CEM 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.
This document is closely related to references [5], which describes
the control protocol methods used to signal the usage of CEM, and
[7], which describes a related method of encapsulating Layer 2
frames over MPLS and which shares the same signaling.
4. Scope
This document describes how to provide CEM for the following digital
signals:
1. SONET STS-1 synchronous payload envelope (SPE)/SDH VC-3
2. STS-Nc SPE (N = 3, 12, or 48)/SDH VC-4, VC-4-4c, VC-4-16c
For the remainder of this document, these constructs will be
referred to as SONET/SDH channels.
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. OC-192c SPE/VC-4-64c are also not
included at this point, since most MPLS networks use OC-192c or
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slower trunks, and thus would not have sufficient capacity. As
trunk capacities increase in the future, the scope of this document
can be accordingly extended.
5. CEM Encapsulation Format
In order to transport SONET/SDH SPEs through a packet-oriented
network, the SPE is broken into fragments. A 32-bit CEM Header is
pre-pended to each fragment. The Basic CEM packet appears in Figure
1.
+-----------------------------------+
| CEM Header |
+-----------------------------------+
| |
| |
| SONET/SDH SPE Fragment |
| |
| |
+-----------------------------------+
Figure 1. Basic CEM Packet
The 32-bit CEM 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|D|Resvd| Sequence Num | Structure Pointer |N|P| ECC-6 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2. CEM Header Format
The above fields are defined as follows:
D bit: Signals DBA Mode. MUST be set to zero for Normal Operation.
MUST be set to one if CEM 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 sections 7 and 8 for further
details.
Reserved: These bits are reserved for future use, and MUST be set to
zero.
Sequence Number: This is a packet sequence number, which MUST
continuously cycle from 0 to 1023. It SHOULD begin at zero when a
CEM LSP is created.
Structure Pointer: The Structure Pointer MUST contain the offset of
the J1 byte within the CEM payload. The value is from 0 to 1,022,
where 0 means the first byte after the CEM header. The Structure
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Pointer MUST be set to 0x3FF (1,023) if a packet does not carry the
J1 byte. See [3], [4] and [6] for more information on the J1 byte
and the SONET/SDH payload pointer.
The N and P bits: Indicate negative and positive pointer adjustment
events. They are also used to relay SONET/SDH maintenance signals
such as AIS-P. See Table 1 and sections 7 and 8 for more details.
+---+---+---+----------------------------------------------+
| D | N | P | Interpretation |
+---+---+---+----------------------------------------------+
| 0 | 0 | 0 | Normal Mode û No Ptr Adjustment |
| 0 | 0 | 1 | Normal Mode û Positive Ptr Adjustment |
| 0 | 1 | 0 | Normal Mode û Negative Ptr Adjustment |
| 0 | 1 | 1 | Normal Mode û AIS-P |
| | | | |
| 1 | 0 | 0 | DBA Mode û STS SPE Unequipped |
| 1 | 0 | 1 | DBA Mode û STS SPE Unequipped Pos Ptr Adj |
| 1 | 1 | 0 | DBA Mode û STS SPE Unequipped Neg Ptr Adj |
| 1 | 1 | 1 | DBA Mode û AIS-P |
+---+---+---+----------------------------------------------+
Table 1. Interpretation of D, N, and P bits
ECC-6: An Error Correction Code to protect the CEM header. This
offers the ability to correct single bit errors and detect up to two
bit errors. The ECC algorithm is described in Appendix B.
Note: CEM packets are fixed in length for all of the packets of a
particular emulated TDM stream. This length is signaled using the
CEM Payload Bytes parameter defined in [5], or is statically
provisioned for each TDM stream. Therefore, the length of each CEM
packet does not need to be carried in the CEM header.
5.1 Transport Encapsulation
In principle, CEM packets can be transported over any packet-
oriented network. The following sections describe specifically how
CEM packets MUST be encapsulated for transport over MPLS or IP
networks.
5.1.1 MPLS Transport
To transport a CEM packet over an MPLS network, an MPLS label-stack
MUST be pushed on top of the CEM packet.
The last two labels prior to the CEM header are referred to as the
Tunnel and VC labels.
The VC label is required, and is the last label prior to the CEM
Header. The VC label MUST be used to identify the CEM connection
within the MPLS tunnel.
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The optional tunnel label is immediately above the VC label on the
label stack. If present, the tunnel label MUST be used to identify
the MPLS LSP used to tunnel the TDM packets through the MPLS network
(the tunnel LSP).
This is similar to the label stack usage defined in [5] and [7].
+-----------------------------------+
| Additional MPLS Labels (Optional) |
+-----------------------------------+
| Tunnel Label (Optional) |
+-----------------------------------+
| VC Label |
+-----------------------------------+
| CEM Header |
+-----------------------------------+
| |
| |
| SONET/SDH SPE Fragment |
| |
| |
+-----------------------------------+
Figure 3. Typical MPLS Transport Encapsulation
5.1.1 IP Transport
It is highly desirable to define a single encapsulation format that
will work for both IP and MPLS. Furthermore, it is desirable that
the encapsulation mechanism be as efficient as possible.
One way to achieve these goals is to map CEM directly onto IP by
mapping the previously described MPLS packets onto IP.
A mechanism for carrying MPLS over IP is described in [8].
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Using this encapsulation scheme would result in the packet format
illustrated in figure 4.
+-----------------------------------+
| |
| IPv6/v4 Header [8] |
| |
+-----------------------------------+
| Tunnel Label (Optional) |
+-----------------------------------+
| VC Label |
+-----------------------------------+
| CEM Header |
+-----------------------------------+
| |
| |
| SONET/SDH SPE Fragment |
| |
| |
+-----------------------------------+
Figure 4. MPLS Transport Encapsulation
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6. CEM Operation
The following sections describe CEM operation.
6.1 Introduction and Terminology
CEM 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 CEM Header and the MPLS label-stack, and then
transmitted into the packet network. During DBA mode, only the CEM
header and MPLS label stack 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 CEM Packetizer and De-Packetizer
As with all adaptation functions, CEM has two distinct components:
adapting TDM SONET/SDH into a CEM packet stream, and converting the
CEM packet stream back into a TDM SONET/SDH. The first function
will be referred to as CEM Packetizer and the second as CEM De-
Packetizer. This terminology is illustrated in figure 5.
+------------+ +---------------+
| | | |
SONET --> | CEM | --> MPLS --> | CEM | --> SONET
SDH | Packetizer | | De-Packetizer | SDH
| | | |
+------------+ +---------------+
Figure 5. CEM Terminology
Note: the CEM receive function requires a buffering mechanism to
account for delay variation in the CEM packet stream. This
buffering mechanism will be generically referred to as the CEM
jitter buffer.
6.1.2 CEM DBA
CEM DBA is an optional mode of operation that only transmits the CEM
Header and MPLS Label Stack into the packet network under certain
circumstances such as AIS-P or STS Unequipped.
If DBA is supported by a CEM 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.
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. DBA based on pattern detection within the SPE (i.e.
all zeros, 7Es, or ATM idle cells) is for further study.
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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 CEM Operation
During normal operation, the CEM 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 CEM Header
and MPLS label stack are pre-pended to the SPE fragment and the
resulting CEM packet is transmitted into the MPLS network. Because
all CEM packets associated with a specific SONET/SDH channel will
have the same length, the transmission of CEM packets for that
channel SHOULD occur at regular intervals.
At the far end of the packet network, the CEM 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 CEM channel.
6.3 Description of CEM Operation during DBA
There are several issues that should be addressed by a workable CEM
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 CEM packetizer and CEM 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.
Another goal is that the reduction of CEM 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 CEM 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 CEM packets during DBA MUST only carry
the CEM header and the MPLS label stack. The D-bit MUST be set to
one, to indicate that DBA is active.
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The CEM 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.
This allows the CEM packetization and de-packetization logic during
DBA to be virtually identical to the nominal case. It insures that
the correct SONET/SDH indication is reliably transmitted between CEM
adaptation points. It minimizes the risk of under or over running
the jitter buffer during the transition in and out of DBA. And, it
guarantees that faults in the packet network are recognized as
distinctly different from line conditioning on the SONET/SDH
interfaces.
7. SONET/SDH Maintenance Signals
There are several issues that must be considered in the mapping of
maintenance signals between SONET/SDH and MPLS. A description of
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
MPLS, and MPLS to SONET/SDH.
7.1 SONET/SDH to MPLS
The following sections describe how SONET/SDH Maintenance Signals
and Alarm conditions are mapped into MPLS.
7.1.1 AIS-P Indication
In a SONET/SDH network, circuit 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-effecting
section or line defect will result in an AIS-P condition.
The SONET/SDH hierarchy is illustrated below.
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+----------+
| PATH |
+----------+
^
|
AIS-P
|
|
+----------+
| LINE |
+ ---------+
^ ^
| |
AIS-L +------ LOP
|
|
+----------+
| SECTION |
+----------+
^ ^
| |
| |
LOS LOF
Figure 6. SONET/SDH Fault Hierarchy.
Should the Section Layer detect a Loss of Section (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, CEM 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 CEM packets with the SPE fragment, CEM Header, and MPLS label
stack MUST be transmitted into the packet network.
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 CEM header and
the MPLS label stack MUST be transmitted into the packet network.
Also note that this differs from the outage mechanism in [5], which
withdraws labels as a result of an endpoint outage. TDM circuit
emulation requires the ability to distinguish between the de-
provisioning of a circuit, which would cause the labels to be
withdrawn, and temporary outages, which are signaled using AIS-P.
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7.1.2 STS SPE Unequipped Indication
The STS SPE Unequipped Indication is a slightly different case than
AIS-P. When byte C2 of the Path Overhead (STS path signal label) is
00h and Byte B3 (STS Path BIP-8) is valid, it indicates that the SPE
is unequipped. Note: this is typically signaled by setting the
entire SPE to zeros.
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 CEM Header and MPLS label stack, 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 CEM Header and MPLS Label Stack
must be transmitted into the packet network. The N and P bits MUST
be used to signal pointer adjustments as normal. See Table 1 and
section 8 for details.
7.1.3 RDI-P Indication
The CEM function MUST send RDI-P towards the packet network under a
variety of network errors such as loss of packet synchronization.
This MUST be accomplished by modifying the SONET/SDH Path Overhead
within the CEM packets. Specifically the G1 byte must be updated to
signal RDI-P and the B3 (Path BIP-8) must be re-computed. See [3],
[4], and [6] for details.
7.2 MPLS 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
There are several conditions in the packet network that will cause
the CEM de-packetization function to send an AIS-P indication onto a
SONET/SDH channel.
The first of these is the receipt of CEM 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 CEM 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 [3], [4], and [6].
The second case is the receipt of CEM packets with the N and P bits
set to one, and the D-bit set to one. This is an explicit
indication of AIS-P being received at the far-end of the packet
network, with DBA enabled for AIS-P. The CEM de-packetizer MUST
play out one packetÆs worth of all ones for each packet received,
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and MUST configure the SONET/SDH Overhead to signal AIS-P as defined
in [3], [4], and [6].
Additional conditions that SHOULD trigger the transmission of AIS-P
onto a SONET/SDH channel include loss of packet synchronization and
jitter buffer under-run. The definition of these conditions are
under investigation and will be clarified in a subsequent revision
of this draft.
7.2.2 STS SPE Unequipped Indication
There are two conditions in the packet network that will cause the
CEM function to transmit STS SPE Unequipped indications onto the
SONET/SDH channel.
The first, which is transparent to CEM, is the receipt of regular
CEM 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 CEM de-
packetizer.
The second case is the receipt of CEM 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 CEM 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.
7.2.3 RDI-P Indication
The CEM function MUST send an RDI-P towards a SONET/SDH channel
under the conditions defined for SONET/SDH Line Terminating
equipment in [3], [4], and [6].
8. Clocking Modes
It is necessary to be able to regenerate the input service clock at
the output interface. Two clocking modes are supported: synchronous
and asynchronous.
8.1 Synchronous
When synchronous SONET/SDH timing is available at both ends of the
circuit, the N and P bits are used to signal negative or positive
pointer justification events.
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 N and P bits are used to replay
the stuff indicators and eliminate transport jitter.
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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.
Furthermore, it is possible for pointer adjustments to occur in back
to back SONET/SDH frames. In order to support this possibility, the
packet size for a particular circuit MUST be no larger than
(783*N)/3. Where N is the STS-Nc multiplier.
Since the minimum value of N is one, all CEM implementations MUST
support a minimum payload length of 783/3 or 261 bytes. Smaller
payload lengths MAY be supported as an option.
The CEM de-packetizer MUST utilize the CEM sequence numbers to
insure that SONET/SDH pointer adjustment events are not played any
more frequently than once per every three CEM packets transmitted by
the remote CEM 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 7.
8.2 Asynchronous
If synchronous timing is not available, the N and P bits are not
used for frequency justification and adaptive methods are used to
recover the timing. The N and P bits are only used for the
occurrence of a path AIS event. An example adaptive method can be
found in section 3.4.2 of [9].
9. CEM LSP Signaling
For maximum network scaling, CEM LSP signaling may be performed
using the LDP Extended Discovery mechanism as augmented by the VC
FEC Element defined in [5]. MPLS traffic tunnels may be dedicated
to CEM, or shared with other MPLS-based services. The value 8008 is
used for the VC Type in the VC FEC Element in order to signify that
the LSP being signaled is to carry CEM. Note that the generic
control word defined in [6] is not used, as its functionality is
included in the CEM encapsulation header.
Alternatively, static label assignment may be used, or a dedicated
traffic engineered LSP may be used for each CEM circuit.
CEM packets are fixed in length for all of the packets of a
particular emulated TDM stream. This length is signaled using the
CEM Payload Bytes parameter defined in [5], or is statically
provisioned for each TDM stream.
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The use of DBA is signaled by the use of the CEM Options parameter
defined in [5], or is statically provisioned for each TDM stream.
10. Open Issues
Future revisions of this document may discuss the following items.
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.
The current draft only considers DBA based on SONET/SDH Overhead.
It would be very desirable to extending DBA to include pattern-based
suppression such as long runs of HDLC flags (i.e. 0x7E). One issue
that complicates pattern-based DBA is that the path overhead appears
every Nx87 bytes within the SPE. One solution may be to have a
special mode of DBA, where the Path Overhead is explicitly
transported within the packet along with the specific pattern (e.g.
7E).
This revision of the draft does not provide a definition for æloss
of packet synchronizationÆ or æjitter buffer under-runÆ. Details
for declaring these conditions at the de-packetizer will be
addressed in future revisions.
11. Security Considerations
As with [5], this document does not affect the underlying security
issues of MPLS.
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12. 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 CEM technology outlined in this
document. Vivace Networks, Inc. will grant free unlimited licenses
for use of this technology.
13. References
[1] Bradner, S., "The Internet Standards Process -- Revision 3",
BCP 9, RFC 2026, October 1996.
[2] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997
[3] American National Standards Institute, "Synchronous Optical
Network (SONET) - Basic Description including Multiplex
Structure, Rates and Formats," ANSI T1.105-1995.
[4] ITU Recommendation G.707, "Network Node Interface For The
Synchronous Digital Hierarchy", 1996.
[5] Martini et al, "Transport of Layer 2 Frames Over MPLS", draft-
martini-l2circuit-trans-mpls-05.txt, work in progress, February
2001.
[6] Telcordia Technologies, ôSynchronous Optical Network (SONET)
Transport Systems: Common Generic Criteriaö, GR-253-CORE, Issue
3, September 2000.
[7] Martini et al, "Encapsulation Methods for Transport of Layer 2
Frames Over MPLS", draft-martini-l2circuit-encap-mpls-01.txt,
work in progress, February 2001.
[8] Worster, ôMPLS Label Stack Encapsulation in IPö, draft-worster-
mpls-in-ip-04, work in progress, Expires August 2001.
[9] ATM Forum, "Circuit Emulation Service Interoperability
Specification Version 2.0", af-vtoa-0078.000, January 1997.
13. Acknowledgments
The authors would like to thank Mitri Halabi and Bob Colvin, both of
Vivace Networks, for their comments and suggestions.
14. Authors' Addresses
Andrew G. Malis
Vivace Networks, Inc.
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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
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
Thomas K. Johnson
Litchfield Communications
76 Westbury Park Rd.
Watertown, CT 06795
Email: tom_johnson@litchfieldcomm.com
Ed Hallman
Litchfield Communications
76 Westbury Park Rd.
Watertown, CT 06795
Email: ed_hallman@litchfieldcomm.com
Marlene Drost
Litchfield Communications
76 Westbury Park Rd.
Watertown, CT 06795
Email: marlene_drost@litchfieldcomm.com
Malis, et al. Expires October 2001 [Page 16]
SONET/SDH Circuit Emulation Over MPLS April 2001
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 ´s 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 exactly 64 times larger than the
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/SDH Circuit Emulation Over MPLS April 2001
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 MPLS networks.
Appendix B. ECC-6 Definition
ECC-6 is an Error Correction Code to protect the CEM header. This
provides single bit correction and the ability to detect up to two
bit errors.
Error Correction Code:
|---------------Header bits 0-25 -------------------| ECC-6 code|
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 1 1 1 1 0 0 0 1 0 0 0 1 1 1 1 1 0 1 0 0 0 1 0 1 1|1 0 0 0 0 0|
|1 1 1 1 0 1 0 0 0 1 0 0 1 0 0 0 0 1 0 1 1 1 1 1 1 1|0 1 0 0 0 0|
|1 0 0 0 1 1 1 1 0 0 1 0 1 1 1 0 0 0 1 1 1 1 0 0 1 1|0 0 1 0 0 0|
|0 1 0 0 1 1 1 1 0 0 0 1 1 0 0 1 1 1 1 1 0 0 1 1 0 1|0 0 0 1 0 0|
|0 0 1 0 0 0 1 0 1 1 1 1 1 1 0 0 1 1 1 1 1 0 1 0 1 0|0 0 0 0 1 0|
|0 0 0 1 0 0 0 1 1 1 1 1 0 0 1 1 0 0 1 1 0 1 1 1 1 1|0 0 0 0 0 1|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 7. ECC-6 Check Matrix X
The ECC-6 code protects the 32 bit CEM header as follows:
The encoder generates the 6 bit ECC using the matrix shown in Figure
7. In brief, the encoder builds another 26 column by 6 row matrix
and calculates even parity over the rows. The matrix columns
represent CEM header bits 0 through 25.
Denote each column of the ECC-6 check matrix by X[], and each column
of the intermediate encoder matrix as Y[]. CEM[] denotes the CEM
header and ECC[] is the error correction code that is inserted into
CEM header bits 26 through 31.
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SONET/SDH Circuit Emulation Over MPLS April 2001
for i = 0 to 25 {
if CEM[i] = 0 {
Y[i] = 0;
} else {
Y[i] = X[i];
}
}
In other words, for each CEM header bit (i) set to 1, set the
resulting matrix column Y[i] according to Figure 7.
The final ECC-6 code is calculated as even parity of each row in Y
(i.e. ECC[k]=CEM[25+k]=even parity of row k).
The receiver also uses matrix X to calculate an intermediate matrix
YÆ based on all 32 bits of the CEM header. Therefore YÆ is 32
columns wide and includes the ECC-6 code.
for i = 0 to 31 {
if CEM[i] = 0 {
YÆ[i] = 0;
} else {
YÆ[i] = X[i];
}
}
The receiver then appends the incoming ECC-6 code to Y as column 32
(ECC[0] should align with row 0) and calculates even parity for each
row. The result is a single 6 bit column Z. If all 6 bits are 0,
there are no bit errors (or at least no detectable errors).
Otherwise, it uses Z to perform a reverse lookup on X[] from Figure
7. If Z matches column X[i], then there is a single bit error. The
receiver should invert bit CEM[i] to correct the header. If Z fails
to match any column of X, then the CEM header contains more than one
bit error and the CEM packet MUST be discarded.
Note that the ECC-6 code provides single bit correction and 2-bit
detection of errors within the received ECC-6 code itself
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developing Internet standards in which case the procedures for
Malis, et al. Expires October 2001 [Page 19]
SONET/SDH Circuit Emulation Over MPLS April 2001
copyrights defined in the Internet Standards process must be
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Acknowledgement
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Malis, et al. Expires October 2001 [Page 20]