MPOA Operations Multi-Protocol over ATM MPOA
Figure 9-18 Unstructured CES-IWF protocol architecture.
Figure 9-19 AAL1 format for CES.
Figure 9-20 AAL1 SDT structure: a single frame, b multiframe with CAS.
There are two methods used for recovering the source clock in the asynchronous clocking operation: Synchronous Residual Time Stamp SRTS and Adaptive Clock Recovery ACR.
In the SRTS method, it is assumed that both the receiver and sender CES-IS entities are synchronized to a common reference clock from the network. The sender CES-IS entity measures the reference clock cycles with respect to the N
service clock cycles. The difference between the service clock and reference clock cycles is then transmitted to the receiver as a 4-bit Residual Time Stamp RTS in the CSI bit in SAR-PDU headers corresponding to odd sequence count values of 1,
3, 5 and 7. The receiver CES-IS entity uses the RTS information and the reference clock from the network to synchronize to the service clock of the sender.
In the ACR method, the receiver uses a buffer to synchronize to the service clock of the sender. The receiver writes the received data into the buffer and then reads the data from the buffer using a local clock. The fill level of the buffer is used to
control the local clock. The local clock control mechanism tries to maintain the fill level at around the medium position. The ACR method is typically used when the common reference clock PRS is not available.
As part of the VTOA implementation, the ATM Forum described a specification AF-VTOA-0089 for ATM trunking using AAL1 for narrowband telephony services. It is possible to connect two narrowband telephony networks through ATM
trunking as shown in Figure 9-21. This specification addresses IWF issues between the ATM network and narrowband networks in terms of signaling and transport of 64 kbits narrowband channels.
Figure 9-21 ATM trunking for narrowband networks.
In terms of signaling, ATM trunking IWF supports Digital Subscriber Signaling System Number 1 DSS1 for narrowband ISDN, Private Subscriber Signaling System Number 1 PSS1 for PBX signaling and Channel Associated signaling CAS
methods. The CAS signaling described here uses EM CAS with DTMF signaling. The ATM trunking IWF uses Digital Subscriber Signaling System Number 2 DSS2 for broadband ISDN between two IWF entities connected over an ATM
network. The ATM trunking IWF assumes that the same signaling method is used at both ends of a trunk connection. In other words, no signaling protocol conversion is allowed between the two narrowband networks connected through ATM
trunks.
It is assumed that the narrowband networks are connected to the ATM network through E1 or DS1 physical interfaces carrying 64 kbits channels. These 64 kbits channels may carry speech and other voice-band services such as fax and
modem services. The ATM trunking IWF uses AAL1 to carry the 64 kbits channel content through the ATM network.
The IWF for ATM trunking supports two types of mapping for 64 kbits narrowband channels. It can either map each 64 kbits narrowband channel to a separate VCC one-to-one mapping or map multiple 64 kbits narrowband channels to a
single VCC many-to-one. It also uses a separate VCC for IWF to IWF signaling using DDS2 or Network Management NM. This is illustrated in Figure 9-22.