Internet DRAFT - draft-munozcalle-pwe3-multiplexing-isdn-interfaces
draft-munozcalle-pwe3-multiplexing-isdn-interfaces
Network Working Group Javier Munoz-Calle
Internet Draft Juan M. Vozmediano
Intended status: Standards Track Universidad de Sevilla
Expires: July 2013 January 11, 2013
Efficient Multiplexing of ISDN User-Network Interfaces over TDM
Pseudo-wires
draft-munozcalle-pwe3-multiplexing-isdn-interfaces-00.txt
Abstract
This document defines a mechanism to efficiently multiplex a number
of ISDN interfaces over a single TDMPW. This mechanism would allow a
single Access Gateway to remotely terminate a high number of distant
local loops.
Status of this Memo
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Table of Contents
1. Introduction ................................................ 2
2. Terminology ................................................. 4
3. Requirements for an efficient concentration ................. 4
4. Choosing the type of TDMPW .................................. 4
5. Transport of the TDM error indicators ....................... 5
6. AAL2 TDMoIP payload for ISDN UNIs............................ 6
6.1. AAL2 TDMoIP payload for ISDN BRIs ...................... 6
6.2. AAL2 TDMoIP payload for ISDN PRIs ...................... 8
7. Security Considerations..................................... 10
8. IANA Considerations ........................................ 10
9. References ................................................. 10
9.1. Normative References................................... 10
9.2. Informative References .............................. 11
Acknowledgments ............................................... 11
1. Introduction
The ISDN transmission capacity is slotted into fixed-rate channels.
B-channels are tailored to carry toll-quality digitalized voice,
while D-channels enclose control flows. These two channel types are
marketed into two possible ISDN user-to-network interfaces (UNI),
namely a Basic Rate (BRI [I.430]) and a Primary (PRI [I.431])
interface.
The SigTran architecture [RFC2719] allows the transparent
integration of ISDN terminals in the packet networks by terminating
the ISDN local loops at Access Gateways (AGW). These network elements
may be implemented either monolithically, or decomposed into a Media
Gateway and a Media Gateway Controller (MGC), where the later
controls the former.
TDM Pseudowires (TDMPWs) [RFC4197] represent an ideal candidate to
efficiently multiplex a number of ISDN interfaces (Figure 1A). This
would allow a single AGW (Egress PE) to remotely terminate a high
number of distant local loops (Figure 1B). This multiplexing may be
applied to both decomposed and monolithic (showed at Figure) access
gateways.
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+---+ +-----+ +---+ +-------+ +------+
|TEs|....| | |TEs|....| | | |
+---+ | | +---+ | | TDMPWs | |
ISDN | AGW | ISDN |Ingress|........| |
UNIs | | UNIs | PE | | |
+---+ | | +---+ | | | |
|TEs|....| | |TEs|....| | | |
+---+ +-----+ +---+ +-------+ | |
| |
+---+ +-----+ +---+ +-------+ | |
|TEs|....| | |TEs|....| | | |
+---+ | | +---+ | | TDMPWs | |
ISDN | AGW | ISDN |Ingress|........|Egress|
UNIs | | UNIs | PE | | PE |
+---+ | | +---+ | | | (AGW)|
|TEs|....| | |TEs|....| | | |
+---+ +-----+ +---+ +-------+ | |
| |
+---+ +-----+ +---+ +-------+ | |
|TEs|....| | |TEs|....| | | |
+---+ | | +---+ | | TDMPWs | |
ISDN | AGW | ISDN |Ingress|........| |
UNIs | | UNIs | PE | | |
+---+ | | +---+ | | | |
|TEs|....| | |TEs|....| | | |
+---+ +-----+ +---+ +-------+ +------+
(A) (B)
TEs: ISDN Terminal Equipments AGW: Access GateWay
UNIs: User-Network Interfaces PE: Provider Edge
Figure 1 Concentration of ISDN Access Gateways using TDMPWs
This multiplexing may be used to concentrate various AGWs on a single
piece of equipment, providing the following improvements:
o It reduces infrastructure costs (AGWs are more complex equipment
than PEs).
o In decomposed gateways, it reduces the number of equipments to be
managed by the Media Gateway Controller (MGC), and thus its
complexity.
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2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC-2119 [RFC2119].
3. Requirements for an efficient concentration
To achieve an efficient concentration of various AGWs, TDMPWs:
o Must carry only the in-use ISDN channels (channels associated to
a call), discarding idle physical B or D-channels (i.e., in a PRI
with only 2 channel in use, carrying the full PRI would waste
bandwidth in the IP network). The ISDN channels may be dynamically
assigned to a call by Q.931 signaling.
o May apply Voice Activity Detection (VAD) to individual voice
calls to save bandwidth. The VAD is only applicable to voice
channels (always using single-channel ISDN bearer services) and
the ISDN channels bound to voice calls may be dynamically signaled
by Q.931.
o Must allow multiplexing several UNIs over each TDMPW. For BRIs,
this avoids tiny payloads and low transmission efficiency. For
PRIs, the use of a TDMPW per PRI can be acceptable, but if the
TDMPW only carries the in-use ISDN channels, low efficiency
situations may also arise.
4. Choosing the type of TDMPW
o Transparent TDMPWs (SAToP [RFC4553] and AAL1 TDMoIP without
Structured Data Transport [RFC5087]) only emulate complete E1/T1
PRI interfaces, without identifying their elementary channels.
Lacking BRI support and extraction of in-use channels, they should
be discarded.
o Static structure-aware TDMPWs (CESoPSN [RFC5086] and AAL1 TDMoIP
with Structured Data Transport [RFC5087]) have a constant payload
size (Constant Basic Rate). Thus, they can not carry only the in-
use ISDN channels for switched calls, which are dynamically set up
and torn down. Neither VAD nor UNI multiplexing are supported.
Support the 16-kb/s D-channels of ISDN BRIs is also missing.
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o Dynamic structure-aware TDMPWs (AAL2 TDMoIP [RFC5087]) have a
dynamic payload size (Variable Basic Rate). The number of in-use
ISDN channels may follow the setting up and tearing down of
switched calls. VAD and UNI multiplexing are also possible with
the aid of the AAL2-header's CID (Channel Identifier) field. Also,
they support the 16-kb/s D-channels of ISDN BRIs.
Consequently, AAL2 TDMoIP pseudo-wires are the best choice to achieve
an efficient transport. Each ISDN UNI carried over a TDMPW would be
identified by a unique CID. It is still necessary to solve the next
problems:
o Transportation of the TDM error indicators "LM" of every ISDN UNI
multiplexed over a TDMPW (bit "R" is common for all UNIs
multiplexed over the same TDMPW). They will be carried in the
AAL2-header UUI field.
o Identification of the in-use channels carried and, optionally, of
the VAD-enabled voice B-channels. As the Q.931 signaling may
change them at any time, they must be identified in the TDMPW's
payload. So, new AAL2 TDMoIP payload types must be defined. The
D-channel will be carried in transparent mode (should the D-
channel exists in a UNI, it is always in-use).
5. Transport of the TDM error indicators
TDM error indicators "LM" of every multiplexed ISDN UNI will be
carried in the AAL2-header UUI field, as shown at Figure 2 (BRIs) and
Figure 3 (PRIs). The "IC" flags identify the channels carried in the
payload, as indicated at Chapter 6.1.
0 1 2 3 4
+-+-+-+-+-+ L: Local TDM fail
|L| M |IC | M: Defect Modifier
+-+-+-+-+-+ IC: In-use channels
Figure 2 Flags in the AAL2-header UUI field for ISDN BRIs
0 1 2 3 4
+-+-+-+-+-+ L: Local TDM fail
|L| M |x x| M: Defect Modifier
+-+-+-+-+-+ x: Reserved
Figure 3 Flags in the AAL2-header UUI field for ISDN PRIs
The TDMPW-header CW field must carry the "M" flag (Remote Receive
failure). Its "LM" flags must be set to zero.
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6. AAL2 TDMoIP payload for ISDN UNIs
Every ISDN UNI will be identified by a unique AAL2 CID value. In-use
channels of an ISDN UNI must be carried in the payload of the AAL2
PDUs identified by its corresponding CID. The payload may carry an
integer number "m" of "in-use channel structures", with "N" octets
per structure. The method to determine the in-use channels it out of
the scope of this draft. The format of these structures depends on
the ISDN UNI type.
6.1. AAL2 TDMoIP payload for ISDN BRIs
For ISDN BRIs, each structure carries the channels in use of two
physical frames (bi-frame structures), with 1 octect for D-channel
(i.e. a nibble from each physical frame) and 4 octects per in-use B-
channel (i.e. 2 octects from each physical frame). Figure 4 shows the
format for a BRI with both B-channels in-use.
0 1 2 3 4 5 6 7
--- +-+-+-+-+-+-+-+-+
ISDN Frames #1,#2 | D #1 | D #2 |
--- +-+-+-+-+-+-+-+-+
| B1 |
ISDN Frame #1 +---------------+
| B1 |
--- +-+-+-+-+-+-+-+-+
| B1 |
ISDN Frame #2 +---------------+
| B1 |
--- +-+-+-+-+-+-+-+-+
| B2 |
ISDN Frame #1 +---------------+
| B2 |
--- +-+-+-+-+-+-+-+-+
| B2 |
ISDN Frame #2 +---------------+
| B2 |
--- +-+-+-+-+-+-+-+-+
Figure 4 Format for the in-use channels' bi-frame structure in a 2B+D
ISDN BRI
In-use channels in the BRI will be indicated by the two bits "IC" in
the AAL2-header UUI field (Figure 2), with the following meaning:
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|======================================================|
| | | Bi-frame structure (octets) |
| IC | BRI structure |--------------|----------------|
| | | D-channel | B-channels |
|====|=================|==============|================|
| 00 | No frame | 0 | 0 |
|----|-----------------|--------------|----------------|
| 01 | 0B + D | 1 | 0 |
|----|-----------------|--------------|----------------|
| 10 | 1B + D | 1 | 4 |
|----|-----------------|--------------|----------------|
| 11 | 2B + D | 1 | 8 |
|======================================================|
Table 1 Interpretation of bits IC in the AAL2-header UUI field
Optionally, the AAL2 payload may begin with a one-octet long VAD-mask
to mark the VAD-enabled channels (Figure 5). When the voice call in
channel Bi uses VAD, the bit in position 'i' is set. The PEs may
negotiate the use of VAD using the sub-AVP "TDMoIP AAL2 Options" [RFC
5287].
0 1 2 3 4 5 6 7
+--+--+--+--+--+--+--+--+ B1: VAD for B1-channel
|B1|B2|x |x |x |x |x |x | B2: VAD for B2-channel
+--+--+--+--+--+--+--+--+ x: Reserved
Figure 5 Mask of B-channels with VAD applied in an ISDN BRI
The payload format for each AAL2 PDU in a TDMoIP packet will have the
format shown at Figure 6.
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0 1 2 3 4 5 6 7
--- +-+-+-+-+-+-+-+-+
VAD-mask (Optional) |V|V|x|x|x|x|x|x|
--- +-+-+-+-+-+-+-+-+
| Octet 1 |
Bi-frame structure #1 | . . . |
| Octet N |
--- +-+-+-+-+-+-+-+-+
| Octet 1 |
Bi-frame structure #2 | . . . |
| Octet N |
--- +-+-+-+-+-+-+-+-+
| Octet 1 |
. . . | . . . |
| Octet N |
--- +-+-+-+-+-+-+-+-+
| Octet 1 |
Bi-frame structure #m | . . . |
| Octet N |
--- +-+-+-+-+-+-+-+-+
Figure 6 Payload format for the AAL2 PDUs associated to an ISDN BRI
6.2. AAL2 TDMoIP payload for ISDN PRIs
Each structure carries the in-use channels of one E1 or T1 PRI
physical frame (single-frame structure) with 1 octect per in-use D or
B-channel. Figure 7 shows an example of these structure.
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
| B1 |
+---------------+
| B15 |
+-+-+-+-+-+-+-+-+
| D16 |
+---------------+
| B17 |
+-+-+-+-+-+-+-+-+
| B31 |
+---------------+
Figure 7 Example of the single-frame structure in a E1 ISDN PRI with
the in-use channels "B1, B15, D16, B17, B31"
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As the number of in-use channels may exceed the capabilities of the
AAL2 UUI field, its function will be better performed by a channel-
mask at the beginning of the AAL2 payload. This mask will be 4 bytes
long for E1 PRIs and 3 bytes long for T1 PRIs. For E1 PRIs, the first
mask bit will always be unset. Figure 8 shows an example.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0|1|0|0|0|0|0|0|0|0|0|0|0|0|0|1|1|1|0|0|0|0|0|0|0|0|0|0|0|0|0|1|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 8 Example of the channel-mask for an E1 ISDN PRI with the
channels B1, B15, D16, B17 and B31 in use
Optionally, AAL2 payload may have a VAD-mask to indicate the channels
where VAD is applied. When the voice call in channel Bi uses VAD, the
bit in position 'i' is set. Figure 9 shows an example. Both PEs may
negotiate the use of VAD using the sub-AVP "TDMoIP AAL2 Options" [RFC
5287].
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0|1|0|0|0|0|0|0|0|0|0|0|0|0|0|1|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|1|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 9 Example of VAD-mask for an E1 ISDN PRI with VAD on channels
"B1, B15, B31"
The payload format for each AAL2 PDU in a TDMoIP packet would have
the format shown at Figure 10 for E1 ISDN PRIs. The payload format
for T1 ISDN PRIs will follow the similar approach.
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0 1 2 3 4 5 6 7
--- +-+-+-+-+-+-+-+-+
| Octet 1 |
Channel-mask | Octet 2 |
| Octet 3 |
| Octet 4 |
--- +-+-+-+-+-+-+-+-+
| Octet 1 |
VAD-mask (Optional) | Octet 2 |
| Octet 3 |
| Octet 4 |
--- +-+-+-+-+-+-+-+-+
| Octet 1 |
Single-frame structure #1 | . . . |
| Octet N |
--- +-+-+-+-+-+-+-+-+
| Octet 1 |
Single-frame structure #2 | . . . |
| Octet N |
--- +-+-+-+-+-+-+-+-+
| Octet 1 |
| . . . |
| Octet N |
--- +-+-+-+-+-+-+-+-+
| Octet 1 |
Single-frame structure #m | . . . |
| Octet N |
--- +-+-+-+-+-+-+-+-+
Figure 10 Payload format for the AAL2 PDUs associated with an E1 ISDN
PRI
7. Security Considerations
This document does not introduce any new security considerations
above those present for PWs in general.
8. IANA Considerations
This document requires no IANA actions.
9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
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9.2. Informative References
[I.430] ITU-T Recommandation I.430. "Basic user-network interface
- Layer 1 specification", November 1995.
[I.431] ITU-T Recommandation I.431. "Primary rate user-network
interface - Layer 1 specification", March 1993.
[RFC2719] Ong, L., Rytina, I., Garcia, M., Schwarzbauer, H., Coene,
L., Lin, H., Juhasz, I., Holdrege, M., and C. Sharp,
"Framework Architecture for Signaling Transport",
November 1999.
[RFC4197] Riegel, M., "Requirements for Edge-to-Edge Emulation of
Time Division Multiplexed (TDM) Circuits over Packet
Switching Networks", October 2005.
[RFC4553] Vainshtein, A., Ed., and YJ. Stein, Ed., "Structure-
Agnostic Time Division Multiplexing (TDM) over Packet
(SAToP)", June 2006.
[RFC5086] Vainshtein, A., Ed., Sasson, I., Metz, E., Frost, T., and
P. Pate, "Structure-Aware Time Division Multiplexed (TDM)
Circuit Emulation Service over Packet Switched Network
(CESoPSN)", December 2007.
[RFC5087] Y(J). Stein, Shashoua, R., Insler, R., and M. Anavi,
"Time Division Multiplexing over IP (TDMoIP)", December
2007.
[RFC5287] Vainshtein, A. and Y(J). Stein, "Control Protocol
Extensions for the Setup of Time-Division Multiplexing
(TDM) Pseudowires in MPLS Networks", August 2008.
Acknowledgments
Funding for the RFC Editor function is currently provided by the
Internet Society.
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Authors' Addresses
Javier Munoz-Calle
Universidad de Sevilla
Escuela Superior de Ingenieros
Isla de la Cartuja. Avd. de los Descubrimientos, s/n
Departamento de Ingenieria Telematica
Sevilla 41092
SPAIN
Phone: +34 954 48 73 85
Email: javi@trajano.us.es
Juan M. Vozmediano
Universidad de Sevilla
Escuela Superior de Ingenieros
Isla de la Cartuja. Avd. de los Descubrimientos, s/n
Departamento de Ingenieria Telematica
Sevilla 41092
SPAIN
Phone: +34 954 48 73 85
Email: jvt@trajano.us.es
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