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 This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. 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Code Components extracted from this document must Munoz-Calle Expires July 10, 2013 [Page 1] Internet-Draft Concentration of ISDN Access Gateways January 2013 include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. 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. Munoz-Calle Expires July 10, 2013 [Page 2] Internet-Draft Concentration of ISDN Access Gateways January 2013 +---+ +-----+ +---+ +-------+ +------+ |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. Munoz-Calle Expires July 10, 2013 [Page 3] Internet-Draft Concentration of ISDN Access Gateways January 2013 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. Munoz-Calle Expires July 10, 2013 [Page 4] Internet-Draft Concentration of ISDN Access Gateways January 2013 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. Munoz-Calle Expires July 10, 2013 [Page 5] Internet-Draft Concentration of ISDN Access Gateways January 2013 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: Munoz-Calle Expires July 10, 2013 [Page 6] Internet-Draft Concentration of ISDN Access Gateways January 2013 |======================================================| | | | 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. Munoz-Calle Expires July 10, 2013 [Page 7] Internet-Draft Concentration of ISDN Access Gateways January 2013 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" Munoz-Calle Expires July 10, 2013 [Page 8] Internet-Draft Concentration of ISDN Access Gateways January 2013 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. Munoz-Calle Expires July 10, 2013 [Page 9] Internet-Draft Concentration of ISDN Access Gateways January 2013 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. Munoz-Calle Expires July 10, 2013 [Page 10] Internet-Draft Concentration of ISDN Access Gateways January 2013 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. Munoz-Calle Expires July 10, 2013 [Page 11] Internet-Draft Concentration of ISDN Access Gateways January 2013 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 Munoz-Calle Expires July 10, 2013 [Page 12]