TDMoIP Format
TDMoIP operates by segmenting, adapting and encapsulating the TDM traffic at PSN ingress and performing the inverse operations at PSN egress. Adaptation denotes mechanisms that modify the payload to enable its proper restoration at the PSN egress. By using proper adaptation, the TDM signaling and timing can be recovered, and a certain amount of packet loss can be accommodated. Encapsulation signifies placing the adapted payload into packets of the format required by the underlying PSN technology. For the MPLS case, ITU-T Recommendation Y.1413 contains a complete description of the packet format.
In all cases a TDMoIP packet commences with PSN headers. These are the standard headers used by the PSN technology, e.g. the 20-byte header of UDP/IP, or the label-stack of MPLS. After these headers come the "PW label", a four-byte MPLS-like label that serves as to demultiplex different TDM PWs. After the PSN header comes the four-byte TDMoIP "control word". The control word contains a 16-bit packet sequence number (needed to detect packet re-ordering and packet loss), payload length, and flags indicating defect conditions.
After the control word comes the TDMoIP payload. For structure-agnostic transport (SAToP) this is simply a predetermined number of TDM octets, while for the structure-locked format the payload is an integer number of TDM frames. For structure-indication and structure-reassembly TDMoIP draws on proven adaptation mechanisms originally developed for ATM. A side benefit of this choice of payload types is simplified interworking with circuit emulation services carried over ATM networks. For statically allocated, constant bit-rate (CBR) TDM links, TDMoIP employs ATM adaptation layer 1 (AAL1). This mechanism, defined in ITU-T standard I.363.1 and ATM Forum specification atm-vtoa-0078, was developed for carrying CBR services over ATM. AAL1 operates by segmenting the continuous stream of TDM data into small 48-byte cells and inserting sequencing, timing, error recovery, and synchronization information into them. TDMoIP allows concatenation of any number of AAL1 cells into a packet (note that these are AAL1 cells and not ATM cells, i.e. they do not include the five-byte "cell tax"). By allowing multiple cells per packet, TDMoIP facilitates flexible tradeoffs of buffering delay (which decreases with fewer cells per packet) for bandwidth efficiency (which increases with more cells per packet, due to the per packet overhead). For dynamically allocated TDM links, whether the information rate varies due to activation of time slots or due to voice activity detection, TDMoIP employs ATM adaptation layer 2 (AAL2). This mechanism, defined in ITU-T standard I.363.2, was developed for carrying variable bit rate (VBR) services over ATM. AAL2 operates by buffering each TDM time slot into short minicells, inserting the time slot identifier and length indication, sequencing, and then sending this minicell only if it carries valid information. TDMoIP concatenates the minicells from all active time slots into a single packet. For time slots carrying high-level data link control (HDLC) data, such as data for common channel signaling (CCS), TDMoIP has a special adaptation that encapsulates stretches of non-idle data.
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