PWE3 Working Group Andrew G. Malis
Internet Draft Vivace Networks, Inc.
Expiration Date: July 2003
Prayson Pate
Overture Networks, Inc.
January 2003
SONET/SDH Circuit Emulation over Packet (CEP)
draft-ietf-pwe3-sonet-01.txt
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of section 10 of [RFC2026].
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Abstract
The generic requirements and architecture for Pseudo Wire Emulation
Edge-to-Edge (PWE3) have been described in [PWE3-REQ] and [PWE3-
ARCH]. Additional requirements for emulation of SONET/SDH and
lower-rate TDM circuits has been defined in [PWE3-TDM-REQ].
This draft provides encapsulation formats and semantics for
emulating SONET/SDH circuits and services over a packet-switched
network (PSN).
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Co-Authors
The following individuals are co-authors of this document
Craig White Level3 Communications
David Zelig Corrigent Systems
Ed Hallman Litchfield Communications
Jeremy Brayley Laurel Networks
Jim Boyle Protocol Driven Networks
John Shirron Laurel Networks
Luca Martini Level3 Communications
Marlene Drost Litchfield Communications
Ron Cohen Lycium Networks
Steve Vogelsang Laurel Networks
Tom Johnson (Editor) Litchfield Communications
Table of Contents
1 Conventions used in this document 2
2 Acronyms 3
3 Scope 4
4 CEP Encapsulation Format 5
4.1 SONET/SDH Fragment 6
4.2 CEP Header 7
4.3 RTP Header 9
4.4 PSN Encapsulation 10
4.5 L2TP Encapsulation 13
5 SONET/SDH Transport Timing 14
6 SONET/SDH Pointer Management 14
6.1 Explicit Pointer Adjustment Relay (EPAR) 14
6.2 Adaptive Pointer Management (APM) 15
7 CEP Performance Monitors 15
7.1 Near-End Performance Monitors 16
7.2 Far-End Performance Monitors 17
8 Payload Compression Options 17
8.1 Dynamic Bandwidth Allocation 18
8.2 Service-Specific Payload Formats 19
9 Open Issues 26
10 Security Considerations 27
11 Intellectual Property Disclaimer 27
12 References 27
13 Author's Addresses 29
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].
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2 Acronyms
ADM Add Drop Multiplexer
AIS Alarm Indication Signal
AU-n Administrative Unit-n (SDH)
AUG Administrative Unit Group (SDH)
BIP Bit Interleaved Parity
BITS Building Integrated Timing Supply
CEP Circuit Emulation over Packet
DBA Dynamic Bandwidth Allocation - see [CEP-SPE]
EBM Equipped Bit Mask
LOF Loss of Frame
LOS Loss of Signal
LTE Line Terminating Equipment
PSN Packet Switched Network
POH Path Overhead
PTE Path Terminating Equipment
PWE3 Pseudo-Wire Emulation Edge-to-Edge
RDI Remote Defect Indication
SDH Synchronous Digital Hierarchy
SONET Synchronous Optical Network
STM-n Synchronous Transport Module-n (SDH)
STS-n Synchronous Transport Signal-n (SONET)
TDM Time Division Multiplexing
TOH Transport Overhead
TU-n Tributary Unit-n (SDH)
TUG-n Tributary Unit Group-n (SDH)
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VC-n Virtual Container-n (SDH)
VT Virtual Tributary (SONET)
VTG Virtual Tributary Group (SONET)
3 Scope
This document describes how to emulate the following digital signals
over a packet switched network:
1. Synchronous Payload Envelope (SPE): STS-1/VC-3, STS-3c/VC-4, STS-
12c/VC-4-4c, STS-48c/VC-4-16c, STS-192c/VC-4-64c.
2. Virtual Tributary (VT): VT1.5/VC-11, VT2/VC-12, VT3, VT6/VC-2
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.
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4 CEP Encapsulation Format
In order to transport SONET/SDH circuits through a packet-oriented
network, the SPE (or VT) is broken into fragments, and 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 4.4.3 for details).
The basic CEP packet appears in Figure 1.
+-----------------------------------+
| PSN and Multiplexing Layer |
| Headers |
+-----------------------------------+
| RTP Header |
| (RFC1889) |
+-----------------------------------+
| CEP Header |
+-----------------------------------+
| |
| |
| SONET/SDH |
| Fragment |
| |
| |
+-----------------------------------+
Figure 1 - Basic CEP Packet
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4.1 SONET/SDH Fragment
The SONET/SDH Fragments MUST be byte aligned with the SONET/SDH SPE
(or VT).
The first bit received from each byte of the SONET/SDH MUST be the
Most Significant Bit of each byte in the SONET/SDH 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 fragments MUST be constructed without knowledge or
processing of any incidental payload scrambling.
CEP implementations MUST include the H3 (or V3) byte in the
SONET/SDH fragment during negative pointer adjustment events, and
MUST remove the stuff-byte from the SONET/SDH fragment during
positive pointer adjustment events.
All CEP Implementations MUST support a packet size of 783 Bytes and
MAY support other packet sizes.
VT emulation implementations MUST support payload size of 1/4 VT
superframe fragment, and MAY support 1/2 and full VT superframe
payload sizes.
OPTIONAL SONET/SDH Fragment formats have been defined to reduce the
bandwidth requirements of the emulated SONET/SDH circuits under
certain conditions. These OPTIONAL Formats are included in Appendix
B.
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4.2 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. Extended headers are utilized for the some of the OPTIONAL
SONET/SDH fragment formats described in Appendix B.
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 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.
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 payloads are not transmitted into
the packet network. See Table 1 and section 8.1 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/V. See
Table 1 and section 6.1 for more details.
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+---+---+---+----------------------------------------------+
| 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/V |
| | | | |
| 1 | 0 | 0 | DBA Mode - STS Unequipped |
| 1 | 0 | 1 | DBA Mode - STS Unequipped Pos Ptr Adj |
| 1 | 1 | 0 | DBA Mode - STS Unequipped Neg Ptr Adj |
| 1 | 1 | 1 | DBA Mode - AIS-P/V |
+---+---+---+----------------------------------------------+
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 first byte of the payload structure. For SPE
emulation, the structure pointer locates the J1 byte within the CEP
SONET/SDH Fragment. (For VT emulation the structure pointer locates
the V5 byte within the SONET/SDH 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 (or V5) byte.
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4.3 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 |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
Figure 3 - RTP Header
V : (version) always set to 2
P : (padding) always set to 0
X : (header extension) always set to 0
CC: (CSRC count) always set to 0
M : (marker) set to 0 for CEP packets.
PT: (payload type) 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.
Sequence Number : 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].
Timestamp : 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.
SSRC : (synchronization source) MAY be used for detection of
misconnections.
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4.4 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.
4.4.1 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 Fragment |
| |
| |
+-----------------------------------+
Figure 4 - IP Transport Encapsulation
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4.4.2 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 [PWE3-CONTROL],
this demultiplexing label is referred to as the PW Label and the
upper labels are referred to as Tunnel Labels.
+-----------------------------------+
| One or more MPLS Tunnel Labels |
+-----------------------------------+
| PW Label |
+-----------------------------------+
| RTP Header |
+-----------------------------------+
| CEP Header |
+-----------------------------------+
| |
| |
| SONET/SDH Fragment |
| |
| |
+-----------------------------------+
Figure 5 - Typical MPLS Transport Encapsulation
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4.4.3 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 [PWE3-CONTROL].
+-----------------------------------+
| |
| IPv6/v4 Header |
| |
+-----------------------------------+
| UDP Header |
+-----------------------------------+
| CEP Header |
+-----------------------------------+
| |
| |
| SONET/SDH Fragment |
| |
| |
+-----------------------------------+
Figure 6 - IP Transport Encapsulation w/ RTP Header Suppression
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+-----------------------------------+
| One or more MPLS Tunnel Labels |
+-----------------------------------+
| PW Label |
+-----------------------------------+
| CEP Header |
+-----------------------------------+
| |
| |
| SONET/SDH Fragment |
| |
| |
+-----------------------------------+
Figure 7 - MPLS Transport Encapsulation w/ RTP Header Suppression
4.5 L2TP Encapsulation
Encapsulation for L2TP PSNs is for future study.
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5 SONET/SDH Transport Timing
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 , or is addressed through an adaptive timing
recovery algorithm, and is therefore outside of the scope of this
specification.
6 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 (or VT) will
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 (or V3) 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 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 (or
V3) 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 (and V3) byte and stuff byte
during positive and negative pointer adjustments can be found in
[SONET], [GR253], and [G707].
6.1 Explicit Pointer Adjustment Relay (EPAR)
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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
MUST either set the N and P bits to zero or utilize them to relay
AIS-P/V and STS/VT Unequipped as shown in table 1.
For SPE Emulation, 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.
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.
For VT emulation, a pointer adjustment event MUST be transmitted in
all packets carrying a single VT superframe, starting from the
packet carrying the V5 byte and not including the packet carrying
the V5 byte of the next VT superframe. Pointer adjustment events at
the VT and STS-1 levels are mapped to N and P indications. Pointer
adjustments at the VT level are mapped 1:1 to CEP indications, while
STS-1 indications are mapped according to the ratio of VT/SPE byte
rates.
If both bits are set, then an AIS-P/V event has been detected at the
PW ingress, making the pointer invalid.
When DBA is invoked (i.e. the D-bit = 1), N and P have additional
meanings. See Table 1 and section Appendix C for more details.
6.2 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 not considered to be within
scope for this document.
7 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
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performance of SONET/SDH services. These counters are referred to
as Performance Monitors.
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.
7.1 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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7.2 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.
8 Payload Compression Options
In addition to pure emulation, CEP also offers a number of options
for reducing the total bandwidth utilized by the emulated circuit.
These options fall into two categories: Dynamic Bandwidth Allocation
and Service-Specific Payload Formats.
Dynamic Bandwidth Allocation suppresses transmission of the CEP
payload altogether under certain circumstances such as AIS-P/V and
STS/VT Unequipped. Service-Specific Payload formats reduce
bandwidth by suppressing transmission of portions of the SPE based
on specific knowledge of the SPE payload.
Details on these payload compression options are described in the
following subsections.
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8.1 Dynamic Bandwidth Allocation
Dynamic Bandwidth Allocation (DBA) is an OPTIONAL mechanism for
suppressing the transmission of the SPE (or VT) fragment when one of
two trigger conditions are met, AIS-P/V or STS/VT Unequipped.
Implementations that support DBA MUST include a mechanism for
disabling DBA on a channel-by-channel basis to allow for
interoperability with implementations that do not support DBA.
When a DBA trigger is recognized at a PW ingress, the CEP packets
will be constructed as shown in figure C.1.
Optional padding bytes may be included if the intervening packet
network has a minimum packet size which is less than the DBA packet.
+-----------------------------------+
| PSN and Multiplexing Layer |
| Headers |
+-----------------------------------+
| RTP Header |
| (RFC1889) |
+-----------------------------------+
| CEP Header |
+-----------------------------------+
| (Optional) Padding |
+-----------------------------------+
Figure 8 - Basic CEP-DBA Packet
If RTP Header suppression is utilized, the CEP packets will be
constructed as shown in figure A2.2
+-----------------------------------+
| PSN and Multiplexing Layer |
| Headers |
+-----------------------------------+
| CEP Header |
+-----------------------------------+
| (Optional) Padding |
+-----------------------------------+
Figure 9 - CEP-DBA Packet with RTP Header Suppression
Other than the suppression of the CEP payload, the CEP behavior
during DBA should be equivalent to normal CEP behavior. In
particular, the packet transmission rate during DBA should be
equivalent to the normal packet transmission rate.
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8.2 Service-Specific Payload Formats
In addition to the standard payload encapsulations for SPE and VT
transport, several OPTIONAL payload formats have been defined to
provide varying amounts of payload compression depending on the type
and amount of user traffic present within the SPE. These are
described below:
8.2.1 Fractional STS-1 (VC-3) Encapsulation
Fractional STS-1 (VC-3) encapsulation carries only selected set of
VTs within an STS-1 container. This mode is applicable for STS-1
with POH signal label byte C2=2 (VT-structured SPE) and for C2=3
(Locked VT mode). The mapping of VTs into an STS-1 container is
described in section 3.2.4 of [GR253] and the mapping into VC-3 is
defined in section 7.2.4 in [G.707]. The CEP packetizer removes all
fixed column bytes (columns 30 and 59) and all bytes that belong to
the removed VTs. Only STS-1 POH bytes and bytes that belong to the
selected VTs are carried within the payload. The CEP de-packetizer
adds the fixed stuff bytes and generates unequipped VT data
replacing the removed VT bytes.
Figure 21 below describes VT mapping into an STS-1 SPE.
1 2 3 * * * 29 30 31 32 * * * 58 59 60 61 * * * 87
+--+------------------+-+------------------+-+------------------+
1 |J1| Byte 1 (V1-V4) |R| | | | |R| | | | |
+--+---+---+------+---+-+------------------+-+------------------+
2 |B3|VT | | | |R| | | | |R| | | | |
+--+1.5| | | +-+---+---+------+---+-+------------------+
3 |C2| | | | |R| | | | |R| | | | |
+--+ | | | +-+---+---+------+---+-+------------------+
4 |G1| | | | |R| | | | |R| | | | |
+--+ | | | +-+---+---+------+---+-+------------------+
5 |F2| | | | |R| | | | |R| | | | |
+--|1-1|2-1| * * *|7-4|-|1-1|2-1| * * *|7-4|-|1-1|2-1| * * *|7-4|
6 |H4| | | | |R| | | | |R| | | | |
+--+ | | | +-+---+---+------+---+-+------------------+
7 |Z3| | | | |R| | | | |R| | | | |
+--+ | | | +-+---+---+------+---+-+------------------+
8 |Z4| | | | |R| | | | |R| | | | |
+--+ | | | +-+---+---+------+---+-+------------------+
9 |Z5| | | | |R| | | | |R| | | | |
+--+---+---+------+---+-+---+---+------+---+-+------------------+
| | |
+-- Path Overhead +--------------------+-- Fixed Stuffs
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Figure 10 - SONET SPE Mapping of VT1.5
The SPE always contains seven interleaved VT groups (VTGs). Each VTG
contains a single type of VT, and each VTG occupies 12 columns (108
bytes) within each SPE. A VTG can contain 4 VT1.5s (T1s), 3 VT2s
(E1s), 2 VT3s or a single VT6. Altogether, the SPE can carry 28 T1s
or carry 21 E1s.
The fractional STS-1 encapsulation can optionally carry a bit mask
that specifies which VTs are carried within the STS-1 payload and
which VTs have been removed. This optional bit mask attribute allows
the ingress circuit emulation node to remove an unequipped VT on the
fly, providing the egress circuit emulation node enough information
for reconstructing the VTs in the right order. The use of bit mask
enables "on the fly" compression, whereby only equipped VTs (carrying
actual data) are sent.
8.2.1.1 Fractional STS-1 CEP header
The fractional STS-1 CEP header uses the STS-1 CEP header
encapsulation as defined in this draft. Optionally, an additional 4
byte header extension word is added. The extended header is
described in Figure 23.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|R|D|N|P| Structure Pointer[0:12] | Sequence Number[0:13] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|E|0|0|0| Equipped Bit Mask (EBM) [0:27] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 11 - Extended Fractional STS-1 Header
The following fields are used within the extended header:
- R, D, N, P, Structured Pointer and Sequence Number: All
fields are used as defined in this draft for STS-1
encapsulation.
- E: Extension bit.
E=0: indicates that extended header is not used.
E=1: indicates that extended header is carried within the
packet.
The E bit in the first word is set to 1 to indicate use
of the Equipped Bit Mask (EBM). The E bit in the second
word indicates whether the extended header (to be defined
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in future revision of this draft) is used.
- EBM: Each bit within the bit mask refers to a different VT
within the STS-1 container. A bit set to 1 indicates that
the corresponding VT is equipped, hence carried within the
fractional STS-1 payload.
The format of the EBM is defined in Figure 24.
0 1 2
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| VTG7 | VTG6 | VTG5 | VTG4 | VTG3 | VTG2 | VTG1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 12 - Equipped Bit Mask (EBM) for fractional STS-1
The 28 bits of the EBM are divided into groups of 4 bits, each
corresponding to a different VTG within the STS container. All 4 bits
are used to indicate whether VT1.5 (T1) tributaries are carried
within a VTG. The first 3 bits read from right to left are used to
indicate whether VT2 (E1) tributaries are carried within a VTG. For
example, the EBM of a fully occupied STS-1 with VT1.5 is all ones,
while that of an STS-1 fully occupied with VT2 (E1) tributaries has
the binary value 0111011101110111011101110111.
8.2.1.2 B3 Compensation
Fractional STS-1 encapsulation can be implemented in Line
Terminating Equipment (LTE) or in Path Terminating Equipment (PTE).
PTE implementations terminate the path layer at the ingress PE and
generate a new path layer at the egress PE.
LTE implementations do not terminate the path layer, and therefore
need to keep the content and integrity of the POH bytes across the
PSN. In LTE implementations, special care must be taken to maintain
the B3 bit-wise parity POH byte. The B3 compensation algorithm is
defined below.
Since the BIP-8 value in a given frame reflects the parity check
over the previous frame, the changes made to BIP-8 bits in the
previous frame shall also be considered in the compensation of BIP-8
in the current frame. Therefore the following equation shall be used
for compensation of the individual bits of the BIP-8:
B3[i]'(t) = B3[i](t-1) || B3[i]'(t-1) || B3[i](t) || B*3[i](t-1)
Where:
B3[i] = the existing B3[i] value in the incoming signal
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B3[i]' = the new (compensated) B3[i] value.
B3*[i] = the B3[i] value of the unequipped VT(VC)s in the
incoming signal
|| = exclusive OR operator.
t = the time of the current frame.
t-1 = the time of the previous frame.
The egress PE MUST reconstruct the unequipped VTs and modify the B3
parity value in the same manner to accommodate for the additional VTs
added. In this way the end-to-end BIP is preserved.
8.2.2 Asynchronous T3/E3 STS-1 (VC-3) Encapsulation
Asynchronous T3/E3 STS-1 (VC-3) encapsulation is applicable for STS-
1 with POH signal label byte C2=4, carrying asynchronous mapping of
T3 or E3 signals.
A T3 is encapsulated asynchronously into an STS-1 SPE using the
format defined in section 3.4.2.1 of [GR253]. The STS-1 SPE is then
encapsulated as defined in this draft.
Optionally, the STS-1 SPE can be compressed by removing the fixed
columns leaving only data columns. STS-1 columns are numbered 1 to
87, starting from the POH column numbered 1. The following columns
have fixed values and are removed:
2, 3, 30, 31, 59, 60
Figure 13 - Fixed columns removed within T3 mapping to STS-1
Bandwidth saving is approximately 7% (6 columns out of 87). The B3
parity byte need not be modified as the parity of the fixed columns
amounts to zero.
A T3 is encapsulated asynchronously into a VC-3 container as
described in section 10.1.2.1 of [G.707]. VC-3 container has only 85
data columns, which is identical to the STS-1 container with the two
fixed stuff columns 30 and 59 removed. Other than that, the mapping
is identical.
An E3 is encapsulated asynchronously into a VC-3 SPE using the
format defined in section 10.1.2.2 of [G.707]. The VC-3 SPE is then
encapsulated as defined in this draft.
Optionally, the VC3 SPE can be compressed by removing the fixed
columns leaving only data columns. VC-3 columns are numbered 1 to 85
(and not 87), starting from the POH column numbered 1. The following
columns have fixed values and are removed:
2, 6, 10, 14, 18, 19, 23, 27, 31, 35, 39, 44, 48,
52, 56, 60, 61, 65, 69, 73, 77, 81
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Figure 14 - Fixed columns removed within E3 mapping to VC-3
Bandwidth saving is approximately 28% (24 columns out of 85). The B3
parity byte need not be modified as the parity of the fixed columns
amounts to zero.
Implementations of asynchronous T3/E3 STS-1 (VC-3) encapsulation
MUST support payload length of one SPE and MAY support payload
length of 1/3 SPE. The actual payload size are smaller and are
described in appendix B.
8.2.3 Fractional VC-4 Encapsulation
SDH defines a mapping of VC-11, VC-12, VC-2 and VC-3 directly into
VC-4. This mapping does not have an equivalent within the SONET
hierarchy. The VC-4 mapping is common in SDH implementations.
VC-4 <--x3-- TUG-3 <----x1-------- TU-3 <---- VC-3 >---- E3/T3
|
+--x7-- TUG-2 <--x1- TU-2 <-- VC-2 <---- DS-2
|
+----x3---- TU-12 <-- VC-12<---- E1
|
+----x4---- TU-11 <-- VC-11<---- T1
Figure 15 - Mapping of VCs into VC-4
Figure 27 describes the mapping options of VCs into VC-4. A VC-4
contains three TUG-3s. Each TUG-3 is composed of either a single TU-
3, or 7 TUG-2s. A TU-3 contains a single VC-3. A TUG-2 contains
either 4 VC-11s (T1s), 3 VC-12s (E1s) or one VC-2. Therefore a VC-4
may contain 3 VC-3s, 1 VC-3 and 42 VC-12s, 63 VC-12s, etc.
Fractional VC-4 encapsulation carries only selected set of VCs
within a VC-4 container. This mode is applicable for VC-4 with POH
signal label byte C2=2 (TUG structure) and for C2=3 (Locked TU-n).
The mapping of VCs into a VC-4 container is described in section 7.2
of [G.707]. The CEP packetizer removes all fixed column bytes and
all bytes that belong to the removed VCs. Only VC-4 POH bytes and
bytes that belong to the selected VCs are carried within the
payload. The CEP de-packetizer adds the fixed stuff bytes and
generates unequipped VC data replacing the removed VC bytes.
The fractional VC-4 encapsulation can optionally carry a bit mask
that specifies which VCs are carried within the VC-4 payload and
which VCs have been removed. This optional bit mask attribute allows
the ingress circuit emulation node to remove an unequipped VCs on the
fly, providing the egress circuit emulation node enough information
for reconstructing the VCs in the right order. The use of bit mask
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enables "on the fly" compression, whereby only equipped VCs (carrying
actual data) are sent.
VC-3 carrying asynchronous T3/E3 signals within the VC-4 container
can optionally be compressed by removing the fixed column bytes as
described in section 7.2.2, providing additional bandwidth saving.
Implementations of fractional VC-4 encapsulation MUST support
payload length of 1/3 SPE and MAY support payload lengths of 4/9,
5/9, 6/9, 7/9, 8/9 and full SPE. The actual payload size of
fractional VC-4 encapsulation depends on the number of VCs carried
within the payload. The possible actual payload sizes are described
in appendix B.
8.2.3.1 Fractional VC-4 Mapping
[G.707] defines the mapping of TUG-3 to a VC-4 in section 7.2.1. Each
TUG-3 includes 86 columns. TUG-3#1, TUG-3#2 and TUG-3#3 are byte
multiplexed, starting from column 4. Column 1 is the VC-4 POH, while
columns 2 and 3 are fixed, and therefore removed within the
fractional VC-4 encapsulation.
The mapping of TU-3 into TUG-3 is defined in section 7.2.2 of
[G.707]. The TU-3 consists of the VC-3 with a 9-byte VC-3 POH and
the TU-3 pointer. The first column of the 9-row by 86-column TUG-3
is allocated to the TU-3 pointer (bytes H1, H2, H3) and fixed stuff.
The phase of the VC-3 with respect to the TUG-3 is indicated by the
TU-3 pointer.
The mapping of TUG-2 into TUG-3 is defined in section 7.2.3 of
[G707]. The first two columns of the TUG-3 are fixed and therefore
removed in the fractional VC-4 encapsulation. The 7 TUG-2, each 12
columns wide, are byte multiplexed starting from column 3 of the TUG-
3. This is the equivalent of multiplexing 7 VTGs within STS-1
container in SONET terminology, except for the location of the fixed
columns.
The static fractional VC-4 mapping assumes that both the ingress and
egress nodes are preconfigured with the set of equipped VCs carried
within the fractional VC-4 encapsulation. The ingress emulation edge
removes the fixed columns as well as columns of the VCs agreed upon
by the two edges, and updates the B3 VC-4 byte. The egress side adds
the fixed columns and the unequipped VCs and updates B3.
8.2.3.2 Fractional VC-4 CEP Header
The fractional VC-4 CEP header uses the VC-4 CEP header encapsulation
defined Section 3.3 in this draft. Optionally, an additional 12 byte
header extension word is added. The extended header is described in
Figure 28.
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|R|D|N|P| Structure Pointer[0:12] | Sequence Number[0:13] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0| Equipped Bit Mask #1 (EBM) [0:29] TUG-3#1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0| Equipped Bit Mask #2 (EBM) [0:29] TUG-3#2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|E|0| Equipped Bit Mask #3 (EBM) [0:29] TUG-3#3 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 16 - Extended Fractional VC-4 Header
The following fields are used within the extended header:
- R, D, N, P, Structured Pointer and Sequence Number: All
fields are used as defined in this draft for VC-4
encapsulation.
- E: Extension bit.
E=0: indicates that extended header is not used.
E=1: indicates that extended header is carried within the
packet.
The E bit in the first word is set to 1 to indicate use
of the Equipped Bit Mask (EBM). The E bit in the forth
word indicates whether the extended header (to be defined
in future revision of this draft) is used. The MSB bit of
word two and three is always set to 1 to indicate that
header is extended.
- EBM: The Equipped Bit Mask indicate which tributaries are
carried within the fractional VC-4 payload.
Three EBM fields are used. Each EBM field corresponds to a different
TUG-3 within the VC-4. The EBM includes 7 groups of 4 bits per TUG-2.
A bit set to 1 indicates that the corresponding VC is equipped, hence
carried within the fractional VC-4 payload. Additional 2 bit within
the EBM indicates whether VC-3 carried within the TUG-3 is equipped
and whether it is in AIS mode.
The format of the EBM for fractional VC-4 is defined corresponding to
one of the TUG-3 is defined in Figure 29.
0 1 2
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|A|T|TUG2#7 |TUG2#6 |TUG2#5 |TUG2#4 |TUG2#3 |TUG2#2 |TUG2#1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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Figure 17 - Equipped Bit Mask (EBM) for fractional VC-4
The 30 bits of the EBM are divided into two bits that control the VC-
3 within the TUG-3 and 7 groups of 4 bits, each corresponding to a
different TUG-2 within the TUG-3 container.
For a TUG-3 containing TUG-2, the first two A and T bits MUST be set
to zero. The TUG-2 bits indicate whether the VCs within the TUG-2 are
equipped. All 4 bits are used to indicate whether VC11 (T1)
tributaries are carried within a TUG-2. The first 3 bits read right
to left are used to indicate whether VC12 (E1) tributaries are
carried within a TUG-2. The first bit is used to indicate a VC-2 is
carried within a TUG-2. For example, 28 bits of the EBM of a fully
occupied TUG-3 with VC11 is all ones, while that of a TUG-3 fully
occupied with VC12 (E1) tributaries has the binary value
0111011101110111011101110111.
For a TUG-3 containing VC-3, all TUG-2 bits MUST be set to zero. The
A and T bits are defined as follows:
T : TUG-3 carried bit. If set to 1, the VC-3 payload is carried
within the TUG-3 container. If set to 0, all the TUG-3 columns are
not carried within the fractional VC-4 encapsulation. The TUG-3
columns are removed either because the VC-3 is unequipped or in AIS
mode.
A: VC-3 AIS bit. The A bit MUST be set to 0 when the T bit is 1 (i.e.
when the TUG-3 columns are carried within the fractional VC-4
encapsulation). The A bit indicate the reason for removal of the
entire TUG-3 columns. If set to 0, the TUG-3 columns were removed
because the VC-3 is unequipped. If set to 1, the TUG-3 columns were
removed because the VC-3 is in AIS mode.
8.2.3.3 B3
Fractional VC-4 encapsulation can be implemented in Line Terminating
Equipment (LTE) or in Path Terminating Equipment (PTE). PTE
implementations terminate the path layer at the ingress PE and
generate a new path layer at the egress PE. LTE implementations do
not terminate the path layer, and therefore need to keep the content
and integrity of the POH bytes across the PSN. In LTE
implementations, special care must be taken to maintain the B3 bit-
wise parity POH byte. The same procedures for B3 compensation as
described in section 7.2.1.2 for fractional STS-1 encapsulation are
used.
9 Open Issues
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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.
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.
10 Security Considerations
This document does not address or modify security issues within the
relevant PSNs.
11 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. Also, Lycium Networks has filed one or
more patent applications that may be related to the CEP technology
outlined in this document. Lycium Networks grants free unlimited
licenses for use of this technology.
12 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-TDM-REQ] Max Riegel, Requirements for Edge-to-Edge Emulation
of TDM Circuits over Packet Switching Networks (PSN), Work in
Progress, December 2002, draft-riegel-pwe3-tdm-requirements-01.txt.
[PWE3-ARCH] Stewart Bryant and Prayson Pate, PWE3 Architecture, Work
in progress, November 2002, draft-ietf-pwe3-arch-01.txt
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[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[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.
[CEP-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.
[PWE3-CONTROL] Martini et al, " Transport of Layer 2 Frames Over
MPLS", draft-ietf-pwe3-control-protocol-01.txt, work in progress,
November 2002.
[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.
[T1.403] ANSI, "Network and Customer Installation Interfaces - DS1
Electrical Interfaces", T1.403-1999, May 24, 1999.
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13 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
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 (Editor)
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
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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
14 Hatidhar St., P.O.Box 2088
Ra'anana 43000, Israel
Email: ronc@lyciumnetworks.com
Prayson Pate
Overture Networks
P. O. Box 14864
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 3 - 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 4, for carriage across packet-switched networks.
VTs are organized in SONET superframes, where a SONET superframe is
a sequence of four SONET SPEs. The SPE path overhead byte H4
indicates the SPE number within the superframe. The VT data can
float relative to the SPE position. The overhead bytes V1, V2 and
V3 are used as pointer and stuffing byte similar to the use of the
H1, H2 and H3 TOH bytes.
Table 3 below indicates the number of bytes occupied by a VT within
a superframe.
Mapping VT size Reference
-------------------------------------------------------------
VT1.5/VC-11 104 bytes [GR253] Section 3.4.1.1
VT2/VC-12 140 bytes [G.707] Section 10.1.4.1
VT3 212 bytes [GR253] Section 3.4.1.3
VT6/VC-2 428 bytes [GR253] Section 3.4.1.4
Table 4 - Number of Bytes in a VT Superframe
To support circuit emulation, the entire SONET SPE or VT or SDH VC
level is encapsulated into packets, using the encapsulation defined
in section 3, for carriage across packet-switched networks.
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Appendix B. Payload sizes
CEP packets are normally fixed in length for all packets of a
particular emulated SONET/SDH stream. The exceptions are DBA CEP
packets and on the fly compression within the fractional STS-1/VC-
3/VC-4 mode. When the fractional encapsulation does not carry the
equipped flag indications, the EBM to be transmitted MUST be
statically provisioned at both ends. The length of each CEP packet
does not need to be carried in the CEP header because it can be
uniquely determined for each CEP packet as a function of the
provisioned payload size, the type of VTs carried within the STS-1
signal, the DBA indication and the equipped flags (either
dynamically or statically provisioned).
The following payload lengths can be statically provisioned for
fractional STS-1 encapsulations:
1. Full SPE length (783 bytes)
2. Third of SPE length (261 bytes)
The actual payload sizes would be smaller, depending on the number
of virtual tributaries carried within the fractional SPE. Figure 21
provides the actual payload length as a function of N, the number of
tributaries carried within the fractional STS-1. In particular, when
all VTs are equipped, the fractional STS-1 full SPE payload size is
765 bytes.
VT Type Full SPE SPE/3
----------------------------------------------
VT1.5 (T1) 27*N+9 9*N+3 N=0..28
VT2 (E1) 36*N+9 12*N+3 N=0..21
Figure 18 - Fractional STS-1 Actual Payload Size
The following payload lengths can be statically provisioned for
fractional VC-4 encapsulation:
1. Full SPE length (2349 bytes)
2. 8/9 SPE length (2088 bytes)
3. 7/9 SPE length (1827 bytes)
4. 6/9 SPE length (1566 bytes)
5. 5/9 SPE length (1305 bytes)
6. 4/9 SPE length (1044 bytes)
7. 1/3 SPE length (783 bytes)
The actual payload sizes would be smaller, depending on the number
of virtual tributaries carried within the fractional SPE. Each
equipped VC contributes the following number of bytes per SPE:
Each VC-11 contributes 27 bytes
Each VC-12 contributes 36 bytes
Each VC-2 contributes 108 bytes
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Each VC-3(DS-3) contributes 738 bytes (including pointers)
Each VC-3(E-3) contributes 576 bytes (including pointers)
Each VC-3(not compressed) contributes 774 bytes (including
pointers)
Figure 19 - Fractional VC-4 Actual Payload Size
For example, the payload size of a fractional VC-4 configured to
third SPE encapsulation that carries a single T3 VC-3 and 6 VC-12s
would be: 9 (VC-4 POH) + 6 * 36 + 738 / 3 = 221 bytes payload per
each packet.
The following payload lengths can be statically provisioned for
asynchronous T3/E3 STS-1/VC-3 encapsulations:
1. Full SPE length (783 bytes)
2. Third of SPE length (261 bytes)
The actual payload sizes would be smaller as described in figure 23.
Signal Full SPE SPE/3
----------------------------------------------
T3 729 243
E3 567 189
Figure 20 - Asynchronous T3/E3 STS-1/VC-3 Actual Payload Size
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Appendix C: Example Network Diagram
Figure 7 below shows an example of SONET interconnect. Site A and
Site B are connected back to a Hub Site, Site C by means of a SONET
infrastructure. The OC3 from Site A and the OC12 from Site B are
partially equipped. Each of them is transported through a SONET
network back to a hubbing site (C). Equipped SPEs (or VTs) are then
groomed onto the OC-12 towards site C.
SONET Network
____ ___ ____
_/ \___/ \ _/ \__
+------+ Physical / \__/ \
|Site A| OC-12 / +---+ OC-12 \ Hub Site
| |=================|\S/|-------------+-----+ \ +------+
| | \ |/ \|=============|\ /| \ | |
+------+ /\ +---+-------------| \ / | / OC12 | |
/ | S |=========|Site C|
+------+ Physical/ +---+-------------| / \ | \ | |
|Site B| OC-12 \ |\S/|=============|/ \| \ | |
| |=================|/ \|-------------+-----+ / +------+
| | \ +---+ OC12 __ /
+------+ \ __/ \ /
\ ___ ___ / \_/
\_/ \____/ \___/
Figure 21 - SONET Interconnect Example Diagram
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Figure 8 below shows the same pair of OC12s being emulated over a
PSN. This configuration frees up bandwidth in the grooming network,
since only equipped SPEs (or VTs) are sent through the PSN.
Additional bandwidth savings can be realized by taking advantage of
the various payload compression options described in section 8.
SONET/TDM/Packet Network
____ ___ ____
_/ \___/ \ _/ \__
+------+ Physical /+-+ \__/ \_
|Site A| OC12 / | | +---+ \ Hub Site
| |=============|P|=| R | +---+ +-+ +-----+ \ +------+
| | \ |E| | |===| | | |=|\ /| \ | |
+------+ /\+-+ +---+ | | | | | \ / | / OC12| |
/ | R |=|P| | S |========|Site C|
+------+ Physical/ +-+ +---+ | | |E| | / \ | \ | |
|Site B| OC12 \ |P| | R |===| | | |=|/ \| \ | |
| |=============|E|=| | +---+ +-+ +-----+ / +------+
| | \ | | +---+ __ /
+------+ \ +-+ __/ \ /
\ ___ ___ / \_/
\_/ \____/ \___/
Figure 22 - SONET Interconnect Emulation Example Diagram
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Full Copyright Statement
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