Traffic Management

Connection Admission Control

Connection Admission Control (CAC) is a procedure used to permit or deny the setup of a connection based on available resources. The CAC algorithm ensures that the Quality of Service of each connection can be maintained when a new connection is admitted. If sufficient resources to support the existing connections and the new connection are not available, the new connection setup is rejected. The cell rate generated by each CES connection is dependent on the configuration.

The algorithm for CBR traffic uses a simple summation of the Peak Cell Rates (PCR) for each CBR connection. If the sum of the PCR of each existing connection plus the PCR of the new connection exceeds 99% of the physical link's bandwidth (cell rate capacity), the connection is rejected. This leaves 1% of the link bandwidth for carrying overhead traffic without causing excessive cell delay variation. No over-allocation of trunk bandwidth is supported.

The nominal cell rate for the OC3 port is 353,207 cells per second (CPS). 99% of this is 349,675 CPS. The basic mode of the DS0 = 171 CPS, and the OAM cell rate = 2; therefore, the cell rate is 348,768 CPS. However, if CES Structured mode is used with partial cell fill, it is possible to generate more cells than the trunk can carry. This is why it is important to perform the CAC check before allowing a new CES connection to be configured to terminate on the trunk.

Traffic Shaping

Traffic shaping is not performed for CES CBR PVCs. The maximum Cell Delay Variation for cells transmitted out the ATM trunk port within the same CBR PVC is expected to be 1.6 milliseconds. The worst case CDV increases as more CES connections are configured within the same group of DS1 ports within a DS3 (1-8, 9-16, 17-24, 25-28), and is also related to the synchronization of the DS1 frame boundaries within these groups.

Traffic shaping is performed for the in-band management PVC. Sustained and Peak cell rates (SCR and PCR) can be configured to shape the traffic on the in-band management PVC to conform to a Variable Bit Rate (VBR) traffic contract.

Traffic Policing

Traffic policing controls the connection usage based on the admitted traffic contract. It ensures fair allocation of network resources as requested for the connection by either tagging excess traffic as discard eligible (discarded in preference to non-tagged traffic during congestion), or discarding the excess traffic. Traffic policing is typically done at the Network side of an ATM User to Network Interface. Since the MGX 8240 is not an ATM switch, it always resides on the user side of the UNI to the ATM network.

Traffic policing is not performed on traffic received from the network on the ATM trunk connection because of the following:

Traffic policing should be done at the ingress point of the network to protect the network. When the MGX 8240 receives cells, it is situated at the egress point in the network.
If a traffic source misbehaves, the common resource of the ATM trunk bandwidth towards the MGX 8240 has already been used. If the trunk bandwidth has been exceeded, the damage is already done.

ATM Connections

All ATM connections are PVCs that terminate on the ATM trunk connection. ATM PVC endpoints are not created directly; they are created automatically when either a CES to ATM connection is created, or an in-band management PVC is created. The destination VPI/VCI specified for the CES connection is the VPI/VCI of the automatically created ATM connection endpoint. First Day of Service configuration of the in-band management VPI/VCI causes the in-band management PVC endpoint to be created. The table below shows information for VPI/VCI.

Identifier	Bits	Range
VPI	2	0 to 3
VCI	12	0 to 4096

The default VPI/VCI assignment is as follows (you may override this):

VPI = The number of the DS3, which is 1, 2, or 3.
VCI = (DS1 port number - 1) x 24 + first DS0 number + 100.

All ATM PVCs terminate at an AAL5 interworking function (for in-band management traffic), or an AAL1 interworking functions (for CES). In either case, both ATM segment and end-to-end OAM cells are terminated. The MGX 8240 detects ATM network failures indicated by reception of AIS or RDI cells. In the case of AIS cells, the MGX 8240 responds with RDI cells. No condition exists under which the CES IWF generates AIS OAM cells. The MGX 8240 responds to ATM segment and end-to-end loopback OAM cells.

All characteristics of the ATM PVC endpoint can be managed via either the CES Connection or the in-band management PVC.

Quality of Service Classes

Two levels of Quality of Service (QoS) are supported on the ATM trunk connection:

Constant Bit Rate (CBR)—this is used to carry CES traffic.
Variable Bit Rate-Non-Real Time (VBR-NRT)—this is used to carry in-band management traffic.

CES Connection Cell Rate

CES connections generate cells at a fixed rate. The rate is primarily affected by the following:

Structure mode (Basic/CAS mode) or unstructured mode.
Bit Rate (number of DS0s) on the circuit.
Number of octets of partial cell fill for the circuit.

Generally, 48 octets of payload per 53-octet cell are forwarded to the network from the MGX 8240. Data will queue in a buffer until the cell is filled and then it is forwarded to the network.

When traffic is entering the buffer slowly (from a few DS0s at a time), it takes longer to fill the cell. This slow traffic condition can introduce unwanted delay into the circuit. To lower the delay, the user can configure the system to support partial cell filling. This allows cells that are only partially filled with data to be forwarded to the network.

Note The partial cell is still 53 octets; however, the cell itself is only partially filled with data. The greater the number of partially filled cells, the greater the bit rate on the ATM connection.

A partial cell fill can only be configured for structured (N x 64) mode. The partial cell fill range is 20 to 47.

Constant Bit Rate

Constant Bit Rate (CBR) is used for user traffic that:

Carries constant bit rate traffic.
Has a fixed timing relationship between samples.
Requires a guarantee of extremely low cell loss.

Variable Bit Rate, Non-Real Time

Variable Bit Rate, non-real time (VBR-nrt) is used for management traffic that:

Carries variable bit rate traffic.
Has no fixed timing relationship between samples.
Requires a guarantee of very low cell loss.
Requires an upper bound on delay.

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