The ATM forum defined CES in January 1997 as af-vtoa-0078.000. Today, Circuit Emulation represents a stable and reliable standard, widely implemented by ATM equipment suppliers.
When using circuit emulation, the ATM network provides a transparent transport mechanism for structured G.703/4 links. Voice is encoded into these links as in a normal TDM network using PCM, ADPCM, or other encoding and compression mechanisms.
The network ensures that the delivered circuit is reconstructed exactly as received. CES is a full duplex mechanism, and it presents the voice equipment with an apparent leased circuit. This approach is valuable because no change to an existing TDM or PBX network is required. A circuit-emulated link can carry any type or mixture of data/voice/video traffic.
CES uses the ATM AAL1 adaptation mechanism to segment the incoming E1 or T1 traffic into ATM cells with the necessary timing information to ensure that the circuit can be correctly reassembled at the destination.
The advantage to CES is the simplicity of implementation; the ATM network is used to provide virtual replacements for physical links in an existing network. CES also provides an ideal stepping-stone from legacy TDM networks to full ATM-enabled broadband solutions.
However, CES exhibits two limitations:
• CES is unable to provide statistical multiplexing— The ATM network does not differentiate between idle and active timeslots; idle traffic/time-slots are carried. CES voice transport consumes about 10 percent more bandwidth than would be required to transfer the same voice traffic over leased circuits.
• CES is often implemented as a point-to-point service— CES provides the transport of the contents of one network physical interface to a single other physical network interface. This can prevent the implementation of some network topologies, and can result in increased network cost. This is because a physical interface must be provided for traffic destined to each remote destination.
The restrictions of simple CES resulted in the development of a new standard by the ATM Forum: DBCES.
This standard was ratified in July 1997 as af-vtoa-0085.000, and is implemented by many member companies in their equipment.
The objective of this standard is to enable dynamic bandwidth utilization by detecting which time slots of a TDM trunk are active and which are inactive.
When an inactive state is detected in a specific time slot, the time slot is dropped from the next ATM CES data structure and the bandwidth is reused for other services.
DBCES can use any method of time slot activity detection. The specific implementation and method(s) chosen by individual vendors for activity detection is not defined, and various companies adopt differing strategies.
Cisco implements DBCES with the dynamic rate queue mechanism. A rate queue defines the speed at which individual virtual circuits transmit data to the remote end. Rate queues can be configured as permanent, dynamic (allow the software to set up rate queues), or some combination of permanent and dynamic. The software dynamically creates rate queues when a virtual circuit (VC) is created with a peak rate that does not match a user-configured rate queue. The Cisco IOS software automatically creates rate queues as necessary when you create a VC. If traffic shaping is not configured on the VC, the peak rate of the VC is set to the UBR at the maximum peak rate that the physical layer interface module (PLIM) allows. A rate queue is then dynamically created for the peak rate of that VC.
The most common implementation mechanisms are the monitoring of A and B (on-hook/off-hook) bits in the channel associated signaling, and the detection of idle codes within the payload of the voice channel.
DBCES can operate in either PVC or SVC ATM network configurations. The active time slots are transmitted using the standardized CES service.
In operation, the transmitting system assigns sufficient bandwidth to support the DBCES function when all the provisioned time slots are active. When some of the time slots become inactive, the transmitting system dynamically stops transmitting the inactive time slots, resulting in fewer queued cells for transmission.
The queuing system in the ATM switch can then take the bandwidth not used by the DBCES function and temporarily assign it to another service. This capability provides bandwidth for unspecified bit rate (UBR)-type services during times of lighter voice load, increasing the effective bandwidth utilization of the network.
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