Wide-Area Networks - CCIE Routing and Switching Exam Certification Guide - page 230

SSCOP provides the reliable transport of signaling messages between peer entities. Signaling
requests are encapsulated into SSCOP frames and are carried across the ATM network in AAL5
packets. These signals are carried over a well-known PVC. The ATM Forum PVC for UNI
signaling uses the VCI of 5.

Signaling requests support the following message types:
•

Call setup

•

Call processing

•

Status

•

Call release

ATM Traffic Management
ATM connections support various service classes. The different service classes in turn support
various network services. For example, video transmission uses a constant bit rate (CBR) class
of service (CoS). Compressed voice service uses a real-time variable bit rate (RT-VBR). Data
might use a non-real-time variable bit rate (NRT-VBR) type of traffic, or if the data device
supports flow control, an available bit rate (ABR) traffic type. Low priority data might use an
unspecified bit rate (UBR) traffic type to save money.

Each traffic type uses a different set of QoS parameters. These QoS parameters determine how
the traffic is treated at the ingress of the network, through the core of the network, and at the
egress.

When an ATM cell enters an ATM network, it is subjected to ingress policing. Ingress policing
is referred to as usage parameter control (UPC) . The purpose of UPC is to prevent the originating
device from sending cells faster than the VC is configured to transport them. UPC prevents the
network from becoming congested.

Figure 5-21 shows a model that describes how traffic policing works. It is important to note that
the leaky bucket is simply a model. The ATM interface does not actually contain tiny little
buckets.

When a cell enters an ATM interface where policing is enabled, a token representing the cell is
placed into the bucket. The tokens leak out of the bucket at a specified rate. For example, in
CBR traffic the leak rate of the bucket is referred to as the peak cell rate (PCR) .

As long at the cell arrival rate (AR) is equal to or less than the leak rate of the bucket, it never
fills up and all cells are said to be compliant. If the AR is greater than the leak rate, the bucket
starts to fill up. The number of tokens the bucket can hold before it is full is specified as a time
period, and in a single bucket model, this is referred to as the cell delay variation tolerance
(CDVT) .