PWE3 Working Group Yaakov (Jonathan) Stein Internet Draft Ronen Shaashoua draft-anavi-tdmoip-05.txt Ron Insler Expires: September 2003 Motty (Mordechai) Anavi RAD Data Communications March 2003 TDM over IP draft-anavi-tdmoip-05.txt Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. TDMoIP [PAGE 1] TDM over IP March, 2003 Abstract This document describes methods for transporting time division multiplexed (TDM) digital voice and data signals over Pseudo- wires. It is a revision of the document draft-anavi-tdmoip-04. 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. Table of Contents 1. Introduction .................................................2 2. TDMoIP Encapsulation .........................................3 3. Encapsulation Details for Specific PSNs ......................6 4. TDMoIP Payload types ........................................10 5. Raw Payload (FORMID=1000) ...................................10 6. AAL1 Format Payload (FORMID=11XX) ...........................11 7. ATM PW Compatibility Mode (FORMID=0000) .....................14 8. AAL2 Format Payload (FORMID=1001) ...........................15 9. HDLC Format Payload (FORMID=1111) ...........................17 10. OAM Signaling ..............................................18 11. Implementation Issues ......................................21 12. Security Considerations ....................................23 13. IANA Considerations ........................................23 14. Normative References .......................................23 15. Informative References .....................................24 16. Acknowledgments ............................................25 17. Contact Information ........................................25 1. Introduction Telephony traffic is conventionally carried over connection- oriented synchronous or plesiochronous networks (which will be loosely called TDM networks herein). With the proliferation of packet-switched networks (PSNs), telephony carriers desire integration of TDM services into a unified PSN infrastructure. This integration requires emulation of TDM circuits within the PSN, a function that can be carried out using Pseudo-Wires (PWs), as described in the PWE3 requirements [PWE-REQ] and architecture [PWE-ARCH] documents. This emulation must ensure QoS and voice quality similar to those of existing TDM networks as well as preserving signaling features. Stein et al. [PAGE 2] TDM over IP March, 2003 In this document we describe a protocol for tunneling TDM traffic through PSNs using PWs. This protocol can support various TDM traffic types, including n*64K, unstructured T1/E1, structured T1/E1 with and without CAS signaling, T3/E3, and TDM in AAL1 and AAL2 networks. The precise requirements of the emulation for each of these types are described in the TDM requirements document [TDM-REQ] and in section 11, below. The protocol as herein described is agnostic to the underlying PSN, which may be UDP over IPv4 or IPv6, MPLS, L2TPv3 over IP, or pure Ethernet. Implementation specifics for particular PSNs are discussed in section 3. Although the protocol should be more generally called TDMoPW and its specific implementations TDMoIP, TDMoMPLS, etc. we will use the nomenclature TDMoIP for reasons of consistency with previous versions of this draft. 2. TDMoIP Encapsulation 2.1 Layering Model The protocol-layering model used by TDMoIP is shown in the figure, where the order is that of the physical packet, so that higher layers appear lower in the diagram. +---------------------------+ | PSN-specific layers | +---------------------------+ | RTP(optional) | +---------------------------+ | control word | +---------------------------+ | payload | +---------------------------+ 2.2 PSN-specific layers The PSN-specific layers contain all necessary infrastructure, and may consist of UDP/IP, MPLS, L2TPv3 over IP, or layer 2 Ethernet. The PSN is assumed to be reliable enough and of sufficient bandwidth to enable transport of the required TDM data. TDMoIP edge devices may handle more than one circuit bundle at a time. A circuit bundle is defined as a stream of bits that have originated from a single physical interface or from interfaces that share a common clock, which are transmitted from a single TDMoIP source device to a single TDMoIP destination device. For example, bundles may comprise some number of 64 Kbps timeslots originating from a single E1, or an entire T3 or E3. Circuit bundles are uni-direction streams, but are universally coupled with bundles in the opposite direction to form a bi-directional connection. Stein et al. [PAGE 3] TDM over IP March, 2003 If a TDMoIP edge device is required to handle multiple circuit bundles, then it is the responsibility of the PSN-specific layers to provide a circuit bundle identifier (CBID) in order to enable differentiation between these circuits. These layers will be more fully discussed in section 3. 2.3 RTP If timing information needs to be explicitly transferred over the PSN then RTP MUST be used for this purpose. When the TDMoIP edges have sufficiently accurate local clocks or can derive a sufficiently accurate timing source without explicit timestamps, its use is optional. If RTP is used, the header of following figure, as defined in [RTP], MUST appear. 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |RTV|P|X| CC |M| PT | RTP sequence number | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Timestamp | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | SSRC Identifier | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ RTV (2 bits) the RTP Version number MUST be set to RTV=010 P (1 bit) the RTP padding indicator MUST be set to P=0 X (1 bit) the RTP extension MUST be set to X=0 CC (4 bits) the RTP CSRC count MUST be set to CC=0000 M (1 bit) the RTP marker indicator MUST be set to M=0 PT (7 bits) the RTP Payload Type identifies the RTP stream as carrying a TDMoIP format payload, and MUST be allocated from the range of reserved for dynamic values. RTP Sequence Number (16 bits) is defined separately for each circuit bundle and increments by one for each TDMoIP packet sent for that circuit bundle. It MAY be used by the receiver to detect packet loss and to restore packet sequence. The initial value of the sequence number SHOULD be random (unpredictable) for security purposes. RTP Timestamp (32 bits) The RTP timestamp indicates the precise sampling instant of the first octet in the TDM payload. It MUST be derived from a clock whose resolution and accuracy are sufficient for the required jitter and wander attenuation. Stein et al. [PAGE 4] TDM over IP March, 2003 SSRC Identifier (32 bits) the RTP synchronization source identifier uniquely identifies the circuit bundle's timing source. It is chosen randomly for independent timing sources as described in [RTP]. RTP's main drawback is its large overhead (12 bytes). For this reason TDMoIP allows the RTP header to be omitted when timing information need not be explicitly transferred over the network. 2.4 TDMoIP Control Word The 32-bit control word MUST appear in every TDMoIP packet. Its format is given in the following figure. 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |FORMID |L|R| Z |0 0| Length | Sequence Number | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ FORMID Format identifier (4 bits) The following values are presently defined: 1000 raw 1100 AAL1 unstructured 1101 AAL1 structured 1110 AAL1 structured w/ CAS 0000 ATM PW compatibility mode 1001 AAL2 1111 HDLC The payload format for each of these cases will be described in sections 5, 6, 7, 8 and 9 below. L Local Loss of Sync failure (1 bit) The L bit being set indicates that the source has detected or has been informed of a TDM physical layer fault impacting the data to be transmitted. This bit can be used to indicate Physical layer LOS that should trigger AIS generation at the far end. When the L bit is set the contents of the packet may not be meaningful, and the payload size MAY be reduced in order to conserve bandwidth. Once set, if the TDM fault is rectified the L bit MUST be cleared. R Remote Receive failure (1 bit) The R bit being set indicates that the source is not receiving packets at its TDMoIP receive port, indicating failure of that direction of the bi-directional connection. This indication can be used to signal congestion or other network related faults. Receiving remote failure indication MAY trigger fall-back mechanisms for congestion avoidance. The R bit MUST be set after a preconfigured number of consecutive packets are not received, and MUST be cleared once packets are once again received. Z (2 bits) These bits indicate an extended header format and MUST be set to zero. Stein et al. [PAGE 5] TDM over IP March, 2003 Length (6 bits) is used to indicate the length of the TDMoIP packet (control word and payload), in case padding is employed to meet minimum transmission unit requirements of the PSN. It MUST be used if the total packet length (including PSN, optional RTP, control word, and payload) is less than 64 bytes, and SHOULD be set to zero otherwise. Sequence number (16 bits) The TDMoIP sequence number MUST be present when the RTP header is not used and fulfills the same function as the RTP sequence number. In addition, since the basic clock rate for each circuit bundle is constant, the sequence number may be used as an approximate timestamp. The initial value of the sequence number SHOULD be random (unpredictable) for security purposes, and the value is incremented modulo 2^16 separately for each circuit bundle. When both RTP and the control word sequence numbers are used, they SHOULD be identical. 3. Encapsulation Details for Specific PSNs 3.1 UDP/IP The UDP/IP header as described in [UDP] and [IP] is prefixed to the TDMoIP data. The TDMoIP packet structure is as follows: 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | IPVER | IHL | IP TOS | Total Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Identification |Flags| Fragment Offset | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Time to Live | Protocol | IP Header Checksum | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Source IP Address | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Destination IP Address | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | VER | Circuit Bundle Number | Destination Port Number | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | UDP Length | UDP Checksum | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |RTV|P|X| CC |M| PT | RTP Sequence Number | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Timestamp | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | SSRC identifier | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |FORMID |L|R| Z |0 0| Length | Sequence Number | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | TDMoIP Payload | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Stein et al. [PAGE 6] TDM over IP March, 2003 The first five rows are the IP header, the sixth and seventh rows are the UDP header. Rows 8 through 10 are the optional RTP header. Row 11 is the TDMoIP control word. IPVER (4 bits) is the IP version number, e.g. for IPv4 IPVER=4. IHL (4 bits) is the length in 32-bit words of the IP header, IHL=5. IP TOS (8 bits) is the IP type of service. Total Length (16 bits) is the length in octets of header and data. Identification (16 bits) is the IP fragmentation identification field. Flags (3 bits) are the IP control flags and MUST be set to Flags=010 to avoid fragmentation. Fragment Offset (13 bits) indicates where in the datagram the fragment belongs and is not used for TDMoIP. Time to Live (8 bits) is the IP time to live field. Datagrams with zero in this field are to be discarded. Protocol (8 bits) MUST be set to 0x11 to signify UDP. IP Header Checksum (16 bits) is a checksum for the IP header. Source IP Address (32 bits) is the IP address of the source. Destination IP Address (32 bits) is the IP address of the destination. VER (3 bits) is the TDMoIP version number. The original version (VER=000) was experimental and should no longer be used. Presently VER=001 when RTP is not used, and VER=011 when RTP is used. Circuit Bundle Number (13 bits) This field is usually dedicated to the Source Port Number, but here identifies the unique data stream emanating from a given trunk and sharing a common destination. This nonstandard use of a UDP port number is similar to RTP/RTCP's use of port numbers to uniquely identify sessions, and the common practice (sanctioned in H.225) of randomly allocating port numbers for VoIP sessions. Here placing the circuit bundle identifier in the UDP header rather than the application area enables fast switching. The available circuit bundle numbers are 1-8063; 0 is invalid; 8191 (1FFF) is used for OAM control messages (see section 10); and the 127 ports 8064-8190 are reserved. Stein et al. [PAGE 7] TDM over IP March, 2003 Destination Port Number (16 bits) MUST be set to 0x085E (2142), the user port number which has been assigned to TDMoIP by the Internet Assigned Numbers Authority (IANA). UDP Length (16 bits) is the length in octets of UDP header and data. UDP Checksum (16 bits) is the checksum of UDP/IP header and data. If not computed it must be set to zero. 3.2 MPLS The MPLS header as described in [MPLS] is prefixed to the TDMoIP data. The packet structure (as seen at the edges) is as follows: 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Outer Label | EXP |S| TTL | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Inner Label = CBID | EXP |S| TTL | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |FORMID |L|R| Z |0 0| Length | Sequence Number | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | PAYLOAD | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The first two rows depicted above are the MPLS header; the third is the TDMoIP control word. Outer Label (20 bits) is the MPLS label which identifies the MPLS LSP used to tunnel the TDM packets through the MPLS network. It is also known as the tunnel label or the transport label. The label number can be assigned either by manual provisioning or via the MPLS control protocol. While transiting the MPLS network there can be zero, one or more outer label rows. For label stack usage see [MPLS]. EXP (3 bits) experimental field S (1 bit) stacking bit where 1 indicates stack bottom S=0 for all outer labels TTL (8 bits) MPLS Time to live Inner Label (20 bits) the MPLS inner label (also known as the PW label or the interworking label), contains the circuit bundle identifier used to multiplex multiple circuit bundles within the same tunnel. Valid values are as in subsection 3.1 supra. Note that the inner label is always be at the bottom of the MPLS label stack, and hence its stacking bit is set. Stein et al. [PAGE 8] TDM over IP March, 2003 3.3 L2TPv3 If L2TP is used over IPv4 without UDP the L2TPv3 header defined in [L2TPv3] is prefixed to the TDMoIP data. 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Session ID = CBID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | cookie 1 (optional) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | cookie 2 (optional) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |FORMID |L|R| Z |0 0| Length | Sequence Number | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | PAYLOAD | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Session ID (32 bits) is the locally significant L2TP session identifier, and contains the circuit bundle identifier used to multiplex multiple circuit bundles within the same tunnel. Valid values are as in subsection 3.1 supra. Cookie (32 or 64 bits) is an optional field that contains a randomly selected value that can be used to validate association of the received frame with the expected circuit bundle. 3.4 Ethernet The TDMoIP packet described in the previous subsections will frequenctly be further encapsulated in an Ethernet frame by prefixing the Ethernet preamble, destination and source MAC addresses, optional VLAN header, etc. and appending the four octet frame check sequence after the TDMoIP frame. TDMoIP implementations MUST be able to receive both industry standard (DIX) Ethernet and IEEE 802.3 CSMA/CD frames and SHOULD transmit Ethernet frames. Ethernet encapsulation introduces restrictions on both minimum and maximum packet size. Whenever the entire TDMoIP packet is less than 64 bytes, zero padding is introduced and the true length indicated by using the Length field in the control word. In order to avoid fragmentation the TDMoIP packet must be restricted to the maximum payload size. For example, the length of the Ethernet payload for a non-RTP AAL2 adapted E1 trunk with 31 channels is 8*4 + 31*47 = 1489 octets. This falls below the maximal permitted payload size of 1500 bytes. The direct use of layer 2 Ethernet frames without IP or MPLS layers is for further study. Stein et al. [PAGE 9] TDM over IP March, 2003 4. TDMoIP Payload types TDMoIP is a trunking application, i.e. it transports entire trunks containing multiple voice and/or data streams. Trunking can be carried out at two levels - circuit emulation and loop emulation. In circuit emulation the entire TDM trunk is transferred across the network as a whole without separation into individual timeslots, while in loop emulation the individual timeslots are identified and transported, albeit while preserving the trunk integrity. At present we define five different payload types, namely raw, AAL1, ATM-PW compatibility mode, AAL2, and HDLC. Raw encapsulation is only advisable for high quality networks carrying unstructured TDM traffic. AAL1 is used for circuit emulation, while AAL2 is used for loop emulation. AAL1 is thus best for unstructured trunks, or for structured trunks with relatively constant usage. AAL1 also must be used for structured transport when some of the timeslots carry data. AAL2 is used to conserve bandwidth for structured voice trunks in which usage is highly variable. The HDLC mode is mainly for efficient transport of trunk associated CCS signaling. The AAL family of protocols is a natural choice for trunking applications. Although originally developed to adapt various types of application data to the rigid format of ATM, the mechanisms are general solutions to the problem of transporting constant or variable bandwidth data streams over a packet network. In addition, since the AAL mechanisms are extensively used within and on the edge of the telephony system, they were specifically designed for audio, non-audio data and telephony signaling. Finally, simple service interworking with legacy TDM networks is a major design goal of TDMoIP. Hence, payload types were chosen in order to simplify interworking with the existing infrastructure, including AAL1 and AAL2 networks. 5. Raw Payload (FORMID=1000) For transport of unstructured TDM over networks where the packet delay variation and packet loss are expected to be very low, the payload can be encapsulated without adaptation. Arbitrary constant length payloads MAY be placed as-is in the payload field, with no bit or byte alignment implied. By constraining the payload size to be a constant for a given flow, a certain resilience to packet loss is attained. When a packet is determined to have been lost, the egress edge device MAY send the proper number of a preconfigured byte to the TDM interface. This ensures that the TDM timing will be maintained, although the TDM data will be corrupted. Stein et al. [PAGE 10] TDM over IP March, 2003 6. AAL1 Format Payload (FORMID=11XX) For the prevalent case for which the timeslot allocation is static and no activity detection is performed, the payload can be efficiently encoded using constant bit rate AAL1 adaptation. The AAL1 format is described in [AAL1] and its use for circuit emulation over ATM in [CES]. We will herein briefly describe the use of AAL1 in the context of TDMoIP; the reader will find the full description in the normative references. In AAL1 mode the TDMoIP payload consists of between one and thirty 48-octet subframes. The number of subframes, which can be inferred by the receiving side from the total packet length as specified in the PSN header, is pre-configured and typically chosen according to latency and bandwidth constraints. Using a single subframe reduces latency to a minimum, but incurs the highest overhead, while using, for example, eight subframes reduces the overhead percentage while increasing the latency by a factor of eight. +-------------+-----------------+ |control word |48-octet subframe| +-------------+-----------------+ Single TDMoIP-AAL1 subframe per TDMoIP frame +-------------+-----------------+ +-----------------+ |control word |48-octet subframe|---|48-octet subframe| +-------------+-----------------+ +-----------------+ Multiple TDMoIP-AAL1 subframes per TDMoIP frame The first octet of each 48-octet AAL1 subframe consists of an error protected three-bit sequence number. 1 2 3 4 5 6 7 8 +-+-+-+-+-+-+-+-+----------------------- |C| SN | CRC |P| 47 octets of payload +-+-+-+-+-+-+-+-+----------------------- where C (1 bit) convergence sublayer indication, its use here is limited to indication of the existence of a pointer (see below) C=0 means no pointer, C=1 means a pointer is present. SN (3 bits) The AAL1 sequence number increments from subframe to subframe. CRC (3 bits) is a 3 bit error cyclic redundancy code on C and SN P (1 bit) even byte parity Stein et al. [PAGE 11] TDM over IP March, 2003 As can be readily inferred this octet can only take on eight different values, and incrementing the sequence number forms an eight subframe sequence number cycle, the importance of which will become clear shortly. The structure of the remaining 47 octets in the TDMoIP-AAL1 subframe depends on the subframe type, of which there are three, corresponding to the three types of AAL1 circuit emulation service defined in [CES]. These are known as namely unstructured circuit emulation, structured circuit emulation and structured circuit emulation with CAS. The simplest subframe is the unstructured one which is used for transparent transfer of whole trunks (T1,E1,T3,E3). The 47 octets after the sequence number octet contain 376 bits from the TDM bit stream. No frame synchronization is supplied or implied, and framing is the sole responsibility of the end-user equipment. Hence the unstructured mode can be used for leased lines which carry data rather than N*64 Kbps timeslots, and even for trunks with nonstandard frame synchronization. For the T1 case the raw frame consists of 193 bits, and hence 1 183/193 T1 frames fit into each TDMoIP-AAL1 subframe. The E1 frame consists of 256 bits, and so 1 15/32 E1 frames fit into each subframe. When the TDM trunk is segmented into timeslots according to [G704], and it is desired to transport N*64 Kbps circuit where N is only a fraction of the full E1 or T1, it is advantageous to use one of the structured AAL1 circuit emulation services. Structured AAL1 views the data not merely as a bit stream, but as a circuit bundle of timeslots. Furthermore, when CAS signaling is used it can be formatted such that it can be readily detected and manipulated. In the structured circuit emulation mode without CAS, N octets from the N timeslots to be transported are first arranged in order of timeslot number. Thus if timeslots 2, 3, 5, 7 and 11 are to be transported the corresponding five octets are placed in the subframe immediately after the sequence number octet. This placement is repeated until all 47 octets in the subframe are taken; octet 1 2 3 4 5 6 7 8 9 10 --- 41 42 43 44 45 46 47 timeslot 2 3 5 7 11 2 3 5 7 11 --- 2 3 5 7 11 2 3 the next subframe commences where the present subframe left off octet 1 2 3 4 5 6 7 8 9 10 --- 41 42 43 44 45 46 47 timeslot 5 7 11 2 3 5 7 11 2 3 --- 5 7 11 2 3 5 7 and so forth. The set of timeslots 2,3,5,7,11 is called a structure and the point where one structure ends and the next commences is a structure boundary. Stein et al. [PAGE 12] TDM over IP March, 2003 The problem with this arrangement is the lack of explicit indication of the octet identities. As can be seen in the above example, each TDMoIP-AAL1 subframe starts with a different timeslot, so a single lost packet will result in misidentifying timeslots from that point onwards, without possibility of recovery. The solution to this deficiency is the periodic introduction of a pointer to the next structure boundary. This pointer need not be used too frequently, as the timeslot identification are uniquely inferable unless packets are lost. The particular method used in AAL1 is to insert a pointer once every sequence number cycle of length eight subframes. The pointer is seven bits and protected by an even parity MSB, and so occupies a single octet. Since seven bits are sufficient to represent offsets larger than 47, we can limit the placement of the pointer octet to subframes with even sequence number. Unlike usual TDMoIP- AAL1 subframes with 47 octets available for payload, subframes which contain a pointer, called P-format subframes, have the following format. 0 1 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+----------------------- |C| SN | CRC |P|E| pointer | 46 octets of payload +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+----------------------- where C (1 bit) convergence sublayer indication, C=1 for P-format subframes SN (3 bits) is an even AAL1 sequence number, CRC (3 bits) is a 3 bit error cyclic redundancy code on C and SN P (1 bit) even byte parity LSB for sequence number octet E (1 bit) even byte parity MSB for pointer octet pointer (7 bits) pointer to next structure boundary Since P-format subframes have 46 octets of payload and the next subframe has 47 octets, viewed as a single entity the pointer needs to indicate one of 93 octets. If P=0 it is understood that the structure commences with the following octet (i.e. the first octet in the payload belongs to the lowest numbered timeslot). P=93 means that the last octet of the second subframe is the final octet of the structure, and the following subframe commences with a new structure. The special value P=127 indicates that there is no structure boundary to be indicated (needed when extremely large structures are being transported). Stein et al. [PAGE 13] TDM over IP March, 2003 The P-format subframe is always placed at the first possible position in the sequence number cycle that a structure boundary occurs, and can only occur once per cycle. The only difference between the structured circuit emulation format and structured circuit emulation with CAS is the definition of the structure. Whereas in structured circuit emulation the structure is composed of the N timeslots, in structured circuit emulation with CAS the structure encompasses the superframe consisting of multiple repetitions of the N timeslots and then the CAS signaling bits. The CAS bits are tightly packed into octets and the final octet is padded with zeros if required. For example, for E1 trunks the CAS signaling bits are updated once per superframe of 16 frames. Hence the structure for N*64 derived from an E1 with CAS signaling consists of 16 repetitions of N octets, followed by N sets of the four ABCD bits, and finally four zero bits if N is odd. For example, the structure for timeslots 2,3 and 5 will be as follows 2 3 5 2 3 5 2 3 5 2 3 5 2 3 5 2 3 5 2 3 5 2 3 5 2 3 5 2 3 5 2 3 5 2 3 5 2 3 5 2 3 5 2 3 5 2 3 5 [ABCD2 ABCD3] [ABCD5 0000] Similarly for T1 ESF trunks the superframe is 24 frames, and the structure consists of 24 repetitions of N octets, followed by the ABCD bits as before. For the T1 case the signaling bits will in general appear twice, in their regular (bit-robbed) positions and at the end of the structure. 7. ATM PW Compatibility Mode (FORMID=0000) Since TDM traffic can be carried over ATM circuit emulation services using AAL1, the protocols described in [ATM-ENCAP] may be used to indirectly transport TDM over pseudo-wires, see [DAVARI]. In such a case the TDM is first converted into an AAL1 ATM flow according to [AAL1,CES], and thereafter this ATM flow is encapsulated as described in [ATM-ENCAP]. The TDMoIP control word with an all zero FORMID is compatible with the control word of [ATM-ENCAP] for the mandatory N:1 mode. The N:1 mode concatenates ATM cells including their cell headers, with the exception of the HEC. Hence, a valid and locally unique VPI/VCI must be allocated to the TDM bundle before this mode can be utilized. The format of the control word and payload are as follows: Stein et al. [PAGE 14] TDM over IP March, 2003 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 0 0 0|L|R|0 0|0 0| Length | Sequence Number | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | VPI | VCI | PTI |C| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ATM AAL1 Payload (48 bytes) | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | VPI | VCI | PTI |C| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ATM AAL1 Payload (48 bytes) | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ L and R bits are described in section 2.4 above VPI (12 bits), VCI (16 bits) are the ATM labels taken from the ATM cell header PTI (3 bits) is the ATM payload type identifier copied from the ATM header and indicates congestion as well as differentiating between cells containing user data and those used for maintenance C (1 bit) is the cell loss priority field copied from the ATM header, with C=1 indicating lower priority. When using N:1 mode with N greater than one, MPLS OAM signaling MUST be employed to signal local and remote defects. While ATM PW compatibility mode enables utilization of network devices designed according to [ATM-ENCAP] and facilitates service interworking with existing ATM circuit emulation systems, it has higher overhead (an additional 4 bytes per 48 byte cell) and its use impedes exploitation of some features of the intrinsic AAL1 mode. For example, due to the separation of the TDM processing from the edge devices, access to timing related information may be lost, resulting in jitter and wander attenuation inferior to that obtainable via the intrinsic AAL1 mode. Packet interpolation (see section 11.3, infra) and TDM alarm handling (see section 10, infra) may also suffer as compared with FORMID=11XX modes. 8. AAL2 Format Payload (FORMID=1001) When timeslots are dynamically allocated, or silence can be detected for bandwidth conservation, or congestion avoidance mechanisms are required (see [TDMoIP-LE]), the payload can be efficiently encoded using variable bit rate AAL2 adaptation. The variable bit rate AAL2 format is described in [AAL2] and its use for loop emulation over ATM is explained in [SSCS,LES]. Stein et al. [PAGE 15] TDM over IP March, 2003 For TDMoIP the AAL2 streams are not be segmented into ATM cells, rather the AAL2 payloads belonging to all timeslots are concatenated, and a single packet sent over the network. The basic AAL2-CPS packet is : | Octet 1 | Octet 2 | Octet 3 | 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+------------ | CID | LI | UUI | HEC | PAYLOAD +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+------------ CID (8 bits) channel identifier is a unique identifier for the bundle. The values below 8 are reserved and so there are 248 possible channels. The mapping of CID values to trunk timeslots is outside the scope of the TDMoIP protocol and must be configured during installation or via network management. LI (6 bits) length indicator is one less than the length of the payload in octets. (Note that the payload is limited to 64 octets.) UUI (5 bits) user-to-user indication is the higher layer (application) identifier and counter. For voice data the UUI will always be in the range 0-15, and SHOULD be incremented modulo 16 each time a channel buffer is sent. The receiver MAY monitor this sequence. UUI is set to 24 for CAS signaling packets. HEC (5 bits) the header error control Payload - voice A block of length indicated by LI of voice samples are placed as- is into the AAL2 packet. Payload - CAS signaling For CAS signaling the payload is formatted as a type 3 packet (in the notation of [AAL2]) in order to ensure error protection. The signaling is sent with the same CID as the corresponding voice channel. Signaling is sent whenever the state of the ABCD bits changes, and is sent with triple redundancy, i.e. sent three times spaced 5 milliseconds apart. In addition, the entire set of the signaling bits is sent periodically to ensure reliability. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |RED| timestamp | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | RES | ABCD | type | CRC +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ CRC (cont) | PAD | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Stein et al. [PAGE 16] TDM over IP March, 2003 RED (2 bits) is the triple redundancy counter. For the first packet it takes the value 00, for the second 01 and for the third 10. RED=11 means non-redundant information and is used for periodic refresh of the CAS information. Timestamp (14 bits) The timestamp is the same for all three redundant transmissions. RES (4 bits) is reserved and MUST be set to zero ABCD (4 bits) are the CAS signaling bits type (6 bits) for CAS signaling this is 000011 CRC-10 (10 bits) is a 10 bit CRC error detection code PAD (8 bits) is set to zero. [PWE-ARCH] denotes as Native Service Processing (NSP) functions all processing of the TDM data before its use as payload. As discussed in [TDMoIP-LE] arbitrary NSP functions MAY be performed before the timeslot is placed in the AAL2 loop emulation payload. This includes testing for on-hook/off-hook status, voice activity detection, speech compression, fax/modem relay, etc. 9. HDLC Format Payload (FORMID=1111) The motivation for handling HDLC in TDMoIP is to efficiently transport CCS (common channel signaling such as SS7) which is embedded in the TDM stream. This mechanism is not intended for general HDLC payloads. The HDLC format is intended to operate in port mode, transparently passing all HDLC data and control messages over the PW. In order to transport HDLC the sender monitors flags until a frame is detected. The contents of the frame are collected and the FCS tested. If the FCS is incorrect the frame is discarded, otherwise the frame is sent after initial or final flags and FCS have been discarded and bit unstuffing has been performed. When an TDMoIP- HDLC frame is received its FCS is calculated, and the original HDLC frame reconstituted. This format assumes that the HDLC messages are shorter than the maximum packet size and hence fragmentation is never required. Stein et al. [PAGE 17] TDM over IP March, 2003 10. OAM Signaling Since the TDMoIP PW is not absolutely reliable, it requires a signaling mechanism to provide feedback regarding problems in the communications environment. In addition, such signaling can be used to collect statistics relating to the performance of the underlying PSN [IPPM]. If the underlying PSN has adequate signaling mechanisms then these are to be used. If not, the ICMP-like procedures detailed below SHOULD be followed. All TDMoIP OAM signaling messages MUST use CBID 8191 (1FFF). All PSN layer parameters (for example, IP addresses, TOS, EXP bits, and VLAN ID) MUST remain those of the circuit bundle being investigated. 10.1 Connectivity-Check Messages In most conventional IP applications a server sends some finite amount of information over the network upon explicit request from a client. With TDMoIP the source sends a continuous stream of packets towards the destination without knowing whether the destination device is ready to accept them, leading to flooding of the PSN. The problem may occur when an edge device fails or is disconnected from the PSN, or the PW is broken. After an aging time the destination edge disappears from the routing tables, and intermediate routers may flood the network with the TDMoIP packets in an attempt to find a new path. The solution to this problem is to significantly reduce the number of TDMoIP packets transmitted per second when PW failure is detected, and to return to full rate only when the PW is restored. The detection of failure and restoration is made possible by the periodic exchange of one-way connectivity-check messages, as defined in [CONNECT]. Connectivity is tested by periodically sending OAM messages from the source edge to the destination edge, and having the destination reply to each message. The format of connectivity- check messages is given in subsection 10.3 infra. The connectivity check mechanism can also be useful during setup and configuration. Without OAM signaling one must ensure that the destination edge is ready to receive packets before starting to send them. Since TDMoIP edge devices usually operate full-duplex, both edges must be set up and properly configured simultaneously if flooding is to be avoided. By using the connectivity mechanism a configured edge device waits until it can detect its destination before transmitting at full rate. In addition, errors in configuration can be readily discovered by using the service specific field. Stein et al. [PAGE 18] TDM over IP March, 2003 10.2 Performance Measurements In addition to one way connectivity, the OAM signaling mechanism can be used to request and report on various PSN metrics, such as one way delay, round trip delay, packet delay variation, etc. It can also be used for remote diagnostics, and for unsolicited reporting of potential problems (e.g. dying gasp messages). 10.3 The format of an OAM message packet is depicted in the following figure. Note that PSN-specific layers are identical to those used to carry the TDMoIP data, with the exception that their CBID = 1FFF instead of the usual circuit bundle identifier. 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | PSN-specific layers (with CBID=1FFF) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |FORMID |L|R| Z | Length | OAM Sequence Number | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | OAM Msg Type | OAM Msg Code | Service specific information | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Source CBID | Destination CBID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Source Transmit Timestamp | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Destination Receive Timestamp | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Destination Transmit Timestamp | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ FORMID and L, R, and Z are identical to those used for the circuit bundle being tested. Length is the length in bytes of the OAM message packet. OAM Sequence Number (16 bits) is used to uniquely identify the message. Its value is unrelated to the sequence number of the TDMoIP data packets for the circuit bundle in question. It is incremented in query messages, and replicated without change in replies. OAM Msg Type (8 bits) indicates the function of the message. At present the following are defined: 0 for one way connectivity query message 8 for one way connectivity reply message. OAM Msg Code (8 bits) is used to carry information related to the message, and its interpretation depends on the message type. Stein et al. [PAGE 19] TDM over IP March, 2003 For type 0 (connectivity query) messages the following codes are defined: 0 validate connection. 1 do not validate connection for type 8 (connectivity reply) messages the available codes are: 0 _ acknowledge valid query 1 _ invalid query (configuration mismatch). Service specific information (16 bits) is a field that can be used to exchange configuration information between edge devices. If it is not used this field MUST contain zero. Its interpretation depends on the FORMID field. At present the following is defined for AAL1 payloads. 0 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Number of TSs | Number of SFs | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Number of TSs (8 bits) is the number of timeslots being transported, e.g. 24 for full T1. Number of SFs (8 bits) is the number of 48-octet AAL1 subframes per packet, e.g. 8 when packing 8 subframes per packet. Source CBID (16 bits) uniquely identifies the circuit bundle as labeled by the source edge. Destination CBID (16 bits) uniquely identifies the circuit bundle as labeled by the destination edge. Source Transmit Timestamp (32 bits) represents the time the source edge transmitted the query message in units of 100 microseconds. This field and the following ones only appear if delay is being measured. Destination Receive Timestamp (32 bits) represents the time the destination edge received the query message in units of 100 microseconds Destination Transmit Timestamp (32 bits) represents the time the destination edge transmitted the reply message in units of 100 microseconds. Stein et al. [PAGE 20] TDM over IP March, 2003 11. Implementation Issues General requirements for transport of TDM over pseudo-wires are detailed in [TDM-REQ]. In the following subsections we review additional aspects essential to successful TDMoIP implementation. 11.1 Quality of Service TDMoIP does not provide mechanisms to ensure timely delivery or provide other quality-of-service guarantees; hence it is required that the lower-layer services do so. Layer 2 priority can be bestowed upon a TDMoIP stream by using the VLAN priority field, MPLS priority can be provided by using EXP bits, and layer 3 priority is controllable by using TOS. Switches and routers which the TDMoIP stream must traverse should be configured to respect these priorities. 11.2 Timing TDM networks are inherently synchronous; somewhere in the network there will always be at least one extremely accurate primary reference clock, with long-term accuracy of one part in 10E-11. This node, whose accuracy is called "stratum 1", provides reference timing to secondary nodes with lower "stratum 2" accuracy, and these in turn provide reference clock to "stratum 3" nodes. This hierarchy of time synchronization is essential for the proper functioning of the network as a whole; for details see [G823,G824]. The use of time standards less accurate than stratum 4 is NOT RECOMMENDED as it may result in service impairments. Packets in IP networks reach their destination with delay that has a random component, known as jitter. When emulating TDM on a PSN, it is possible to overcome this randomness by using a "jitter buffer" on all incoming data, assuming the proper time reference is available. The problem is that the original TDM time reference information is not disseminated through the PSN. In broadest terms there are two methods of overcoming this difficulty; in one the timing information is provided by some means independent of the PSN, while in the other the timing must be transferred over the PSN. For example, if the entire TDM infrastructure (or at least major portions of it) is replaced by TDMoIP timing information MUST be delivered over the IP network, and the reconstructed TDM stream SHOULD conform to ITU-T recommendations [G823] for E1 and [G824] for T1 trunks. However, TDMoIP is frequently used in a "toll-bypass" scenario, where an IP link connects two existing TDM networks. In such a case both TDMoIP devices MUST receive accurate timing from the TDM networks to which they connect, and MUST use this local timing when outputting to the TDM network. There is no need to carry timing over the IP network and the overhead associated with RTP can be avoided. Stein et al. [PAGE 21] TDM over IP March, 2003 11.3 Jitter and Packet Loss In order to compensate for packet delay variation that exists in any IP network a jitter buffer MUST be provided. The length of this buffer SHOULD be configurable and MAY be dynamic (i.e. grow and shrink in length according to the statistics of the delay variation). In order to handle (infrequent) packet loss and misordering a packet order integrity mechanism MUST be provided. This mechanism MUST track the serial numbers of packets in the jitter buffer and MUST take appropriate action when faults are detected. When missing packet(s) are detected the mechanism MUST output interpolation packet(s) in order to retain TDM timing. Packets with incorrect serial numbers or other detectable header errors MAY be discarded. Packets arriving in incorrect order SHOULD be swapped. Whenever possible, interpolation packets SHOULD ensure that proper synchronization bits are sent to the TDM network. While the insertion of arbitrary interpolation packets may be sufficient to maintain the TDM timing, for voice traffic packet loss can cause in gaps or artifacts that result in choppy, annoying or even unintelligible speech, see [TDM-PLC]. An implementation MAY blindly insert a preconfigured constant value in place of any lost speech samples, and this value SHOULD be chosen to minimize the perceptual effect. Alternatively one MAY replay the previously received packet. Since a TDMoIP packet is usually declared lost following the reception of the next packet, when computational resources are available, implementations SHOULD conceal the packet loss event by estimating the missing sample values. 11.4 Overhead vs. Latency Trade-off TDMoIP is designed to be parsimonious in bandwidth. To assist in achieving this goal we allow merging multiple subframes into a single packet in order to incur a single header. For example, for AAL1 payloads, there are N subframes per packet, where N is a configurable parameter. While higher values of N reduce overhead, they increase the amount of time that passes between ingress of a TDM sample and its transmission over the PSN. This buffering delay must be added to the network propagation delay and all other delays the packet experiences. Since the amount of latency that can be considered acceptable is dependent upon the application, it is essential that there be a method for tuning this trade-off between efficiency and latency. Stein et al. [PAGE 22] TDM over IP March, 2003 12. Security Considerations TDMoIP does not enhance or detract from the security performance of the underlying PSN, rather it relies upon the PSN's mechanisms for encryption, integrity, and authentication whenever required. TDMoIP does not provide protection against malicious users utilizing snooping or packet injection during setup or operation. Circuit bundle identifiers SHOULD be selected in an unpredictable manner rather than sequentially or otherwise in order to deter session hijacking. When using L2TP randomly selected cookies MAY be used to validate circuit bundle origin. Sequence numbers SHOULD be randomly initialized in order to increase the difficulty of decrypting based on packet headers. 13. IANA Considerations When used with UDP/IP the destination port number MUST be set to 0x085E (2142), the user port number which has been assigned by the to TDMoIP. The format identifiers (FORMID) will need to be standardized. 14. Normative References [UDP] RFC 768 (STD0006) User Datagram Protocol (UDP) [IPv4] RFC 791 (STD0005) Internet Protocol (IP) [NTP] RFC 1305 Network Time Protocol (NTP) (Version 3) [RTP] RFC 1889 RTP: Transport Protocol for Real-Time Applications [IPPM] RFC 2330 Framework for IP Performance Metrics [CONNECT] RFC 2678 IPPM Metrics for Measuring Connectivity [DELAY] RFC 2679 A One-way Delay Metric for IPPM [MPLS] RFC 3032 MPLS Label Stack encoding [L2TPv3] draft-ietf-l2tpext-l2tp-base-06.txt (01/03) Layer Two Tunneling Protocol (L2TPv3), J. Lau et al., work in progress [G704] ITU-T Recommendation G.704 (10/98) Synchronous frame structures used at 1544, 6312, 2048, 8448 and 44736 Kbit/s hierarchical levels Stein et al. [PAGE 23] TDM over IP March, 2003 [G823] ITU-T Recommendation G.823 (03/00) The control of jitter and wander within digital networks which are based on the 2048 Kbit/s hierarchy [G824] ITU-T Recommendation G.824 (03/00) The control of jitter and wander within digital networks which are based on the 1544 Kbit/s hierarchy [AAL1] ITU-T Recommendation I.363.1 (08/96) B-ISDN ATM Adaptation Layer (AAL) specification: Type 1 [AAL2] ITU-T Recommendation I.363.2 (11/00) B-ISDN ATM Adaptation Layer (AAL) specification: Type 2 [SSCS] ITU-T Recommendation I.366.2 (02/99) AAL Type 2 service specific convergence sublayer for trunking [CES] ATM forum specification atm-vtoa-0078 (CES 2.0) Circuit Emulation Service Interoperability Specification Ver. 2.0 [LES] ATM forum specification atm-vmoa-0145 (LES) Voice and Multimedia over ATM - Loop Emulation Service Using AAL2 [ATM-ENCAP] draft ietf-pwe3-atm-encap-01.txt (02/03) Encapsulation Methods for Transport of ATM Cells/Frame Over IP and MPLS Networks, L. Martini et al., work in progress 15. Informative References [DAVARI] draft-davari-pwe3-aal12-over-psn-01.txt (02/03) Transport of ATM AAL1 frames over PSN, S. Davari, work in progress [TDM-REQ] draft-riegel-pwe3-tdm-requirements-01.txt (02/03), Requirements for Edge-to-Edge Emulation of TDM Circuits over Packet Switching Networks, M. Riegel et al., work in progress [PWE3-REQ] draft-ietf-pwe3-requirements-04.txt XiPeng Xiao et al, Requirements for Pseudo Wire Emulation Edge-to-Edge (PWE3), Work in progress, December 2002 [PWE3-ARCH] draft-ietf pwe3-arch-02.txt Stewart Bryant et al, PWE3 Architecture, Work in progress, November 2002 [TDM-PLC] draft-stein-pwe3-tdm-packetloss-00.txt (09/02), The Effect of Packet Loss on Voice Quality for TDM over Pseudowires, Y(J) Stein and I. Druker, work in progress [TDMoIP-LE] draft-stein-pwe3-tdmoiple-00.txt (02/03), TDMoIP using Loop Emulation, Y(J) Stein et al., work in progress Stein et al. [PAGE 24] TDM over IP March, 2003 16. Acknowledgments The authors would like to thank Hugo Silberman, Shimon HaLevy, Tuvia Segal, Eyal Ben Saadon, and Eitan Schwartz of RAD Data Communications for their valuable contributions to the technology described herein. 17. Contact Information Yaakov (Jonathan) Stein RAD Data Communications 24 Raoul Wallenberg St., Bldg C Tel-Aviv 69719 ISRAEL Phone: +972 3 645-5389 Email: yaakov_s@rad.com Ronen Shashoua RAD Data Communications 24 Raoul Wallenberg St., Bldg C Tel-Aviv 69719 ISRAEL Phone: +972 3 645-5447 Email: ronen_s@rad.com Ron Insler RAD Data Communications 24 Raoul Wallenberg St., Bldg C Tel-Aviv 69719 ISRAEL Phone: +972 3 645-5445 Email: ron_i@rad.com Motty (Mordechai) Anavi RAD Data Communications 900 Corporate Drive, Mahwah, NJ 07430 USA Phone: +1 201 529-1100 Ext. 213 Email: motty@radusa.com Stein et al. [PAGE 25] TDM over IP March, 2003 Copyright Notice Copyright (C) The Internet Society (2002). All Rights Reserved. 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