Figure 9-13 MPOA control and data flows.
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Copyr ight © CRC Pr ess LLC
by Abhijit S. Pandya; Ercan Sen CRC Press, CRC Press LLC
ISBN: 0849331390 Pub Date: 110198
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IV. Voice and Telephony over ATM VTOA
At the present, voice communication still dominates telecommunication activity around the world. In other words, voice communication is still the preferred or only choice for communication. As a recognition of the fact, ATM Forum issued a
series of specifications for VTOA application in 1997. The main purpose of these specifications is to allow interworking capability between the ATM-based voice services and the traditional narrowband telephony networks. Each of these
specifications addresses a specific aspect of voice and telephony interworking. There are two key aspects of this interworking. The first aspect deals with the end-users subscribers. The second aspect deals with carrying traditional
narrowband voice circuits over ATM networks.
The specification AF-VTOA-0078 describes the Circuit Emulation Service CES Interoperability. CES sets the foundation of interworking at the physical layer between ATM and narrowband networks. It describes the protocols to carry PDH
circuits DS1E1, DS3E3 and J2 over ATM networks using an AAL1 adaptation layer. These PDH circuit types due to the evolutionary nature of the telecommunication networks have evolved to be used for both voice and data applications. Hence,
this specification is applicable to both voice and data applications.
A. Service Types
This specification describes two basic CES types: Structured and Unstructured. It further specifies two sub classes with respect to signaling: Structured with Channel Associated Signaling CAS and Structured without CAS. The Structured
without CAS is also referred to as a basic structure. These various CES modes are summarized in Figure 9-14.
1. Structured CES
The structured mode is also called Nx64 since a specific physical layer interface DS1, E1, etc. can be divided into one or more logical channels composed of N times 64Kbps time-slots. The range of N is 1-24 for DS1, 1-31 for E1 and 1-96 for J2.
Hence, it is possible to carry multiple logical channels over a single physical interface. Each Nx64 channel is carried as a separate VCC across the ATM network. For example, through PBX equipment, several voice channels and data channels
can be carried over a single DS1 interface. The Nx64 mode is modeled based on the Fractional T1 DS1 concept. The logical Nx64 mode describes the CES function without specifying any particular PDH interface. Most recently, there has
been an attempt to define structured DS3E3 in terms of N times T1E1 by extending the concept of Fractional T1 to Fractional T3 DS3.
Figure 9-14 CES taxonomy.
The basic function of the CES Interworking Function CES-IWF is to convert PDH time-slots into AAL1 ATM cells at the ingress of the ATM network and do the reverse conversion, i.e., convert AAL1 cells to time-slots at the egress of the ATM
network. This interworking operation is shown in Figure 9-15. Within the ATM network, each Nx64 logical channel is set up as a separate VCC. ATM CES interworking offers very flexible mapping of Nx64 channels. For example, multiple Nx64
channels from different physical interfaces can be consolidated into a single physical PDH interface at the egress point.
Another useful aspect of CES is the ability to transfer only the used time-slots of a PDH interface at the ingress point. For example, if a DS1 interface has only 4 of its time-slot assigned for Nx64 traffic, only these 4 time-slots are carried over the
ATM network. This is a significant advantage particularly for low usage PDH interfaces since we do not waste any ATM bandwidth for the unused time-slots. This characteristic of CES is illustrated in Figure 9-16. Also note that the mapping of
the used time-slots onto egress PDH interface can be different than the input PDH assignments. This flexibility is very useful when consolidating multiple ingress PDH interfaces into one egress PDH interface.
In the Structured CES mode, each Nx64 channel is assigned to a dedicated AAL1 instance and a corresponding VCC. The protocol architecture of Structured CES is illustrated in Figure 9-17. As shown in Figure 9-17, in the ingress direction, the
mapping function maps an Nx64 channel from the PDH physical layer time slots to a particular AAL1 instance. In the egress direction, the mapping functions map each AAL1 instance to the designated time slots of the PDH physical layer.
2. Unstructured CES
The unstructured CES mode uses a wire-level emulation. In other words, the ATM CES transports the whole content of the PDH interface transparently without interpreting its format bit by bit. This type of CES service provides end-to-end
transparent transport function through the ATM network. In this case, a fixed amount of ATM network bandwidth is allocated at the line speed of the PDH interface, i.e., 1.5 Mbps for a DS1 connection. The Unstructured CES is similar to
leased lines. The specifics such as the frame format, synchronization and alarming are left to the end-user equipment. For example, DS1-based end-user equipment may choose to use the standard framing formats such as Super Frame SF and
Extended Super Frame ESF or a non-standard frame format. It may choose to synchronize to the network or run asynchronously with respect to the network. It is up to the end-user equipment to handle alarms and Facility Data Link
FDL. The protocol architecture of unstructured CES is shown in Figure 9-18.
