ATM technology is intended to support a wide variety of services and applications. Each type of application, whether it be voice, video or data, has its own special data transmission performance requirements. These requirements are expressed in a set of parameters called Quality of Service (QoS) which includes such factors as permissible transmission delay and loss of information. An important task of the ATM network is to be able to provide the appropriate QoS requirements for each application it is carrying. Traffic Management is provided on the ATM network to protect the network and the end-system from congestion in order to achieve network performance objectives. An additional role is to promote the efficient use of network resources. TM allows the devices to transmit over a broader bandwidth for longer periods of time, making the network more efficient.
Service categories
The 7200 Card's ATM data transmission Traffic Management is based on the Available Bit Rate (ABR) or Unspecified Bit Rate (UBR) "service categories". ABR allows a flow control mechanism to be established which supports data "feedback" to control the source data transmission rate in response to changing ATM layer congestion. This feedback is conveyed to the source through specific control cells called Resource Management Cells, or RM-cells. An end-system that adapts its traffic in accordance with the feedback will experience a low cell loss ratio and obtain a fair share of the available bandwidth according to a network specific allocation policy. No numeric commitment is made about cell transfer delay. The ABR service does not require bounding the delay or the delay variation experienced by a given connection.
The UBR service category is inherently open-loop. UBR is not subject to a specific traffic contract but may be subject to a local policy in individual switches and end-systems. The UBR service category is intended for non-real-time applications, i.e. those not requiring tightly constrained delay and delay variation.
ABR flow control occurs between a sending end-system (source), such as a 7200, and a receiving end-system (destination). Sources and destinations are connected via bi-directional connections. For a bi-directional ABR connection, each connection termination point is both a source and a destination. For the sake of simplicity, only the information flow from the source to the destination with its associated RM-cell flows is considered. The forward direction is the direction from the source to the destination, and the backward direction is the direction from the destination to the source. Figure 13-1 shows the data flow and the RM-cell flow over a VCC connection between the two stations. Corresponding to the forward data flow from the source to the destination, there is a control loop consisting of two RM-cell flows, one in the forward direction and one in the backward direction.
Figure 13-1 Example of a source to destination ABR control loop
A source generates forward RM-cells which are turned around by the destination and sent back to the source as backward RM-cells. These backward RM-cells carry feedback information provided by the network elements and/or the destination back to the source. A network element may:
This section contains some of the TM concepts in greater detail.
The Explicit Forward Congestion Indication (EFCI) is a congestion notification mechanism that is set in the cell header by a network element in an pre-congested or congested state. A network element in an impending congested state or a congested state may set an EFCI in the cell header so that this indication may be examined by the destination end-system. For example, the end-system may use this indication to implement a protocol that adaptively lowers the cell rate of the connection during congestion or impending congestion. A network element that is not in a congested state or an impending congested state will not modify the value of this indication. An impending congested state is the state when a network element is operating around its engineered capacity level.
For some service categories the end system may generate traffic flows of cells with Cell Loss Priority (CLP) marking. A congested network element may selectively discard cells explicitly identified as belonging to a non-compliant ATM connection and/or those cells with their CLP bit set. This is to protect cells without their CLP bit set from being discarded for as long as possible.
In implicit rate control (binary mode) the EFCI bit in the cell header is checked. If there has been congestion on the forward path (recognized at the destination end station by the EFCI bits of the incoming data cells), the Congestion Indication (CI) field within the backward RM cell is marked (set to one). The source end station receives the backward RM cell and acts upon it. If the CI field is set or if RM cells are not returned, the sending rate is reduced. When the source end station receives a backward RM cell with the CI bit cleared it may increase the sending rate on that particular VC.
Explicit rate control enhances the implicit rate control by adding the explicit rate (ER) field to the RM cell. In this field the source end station indicates the rate at which it would like to transmit. If an explicit rate switch exists in the virtual connection (VC) route it may reduce the ER field in the backward RM-cells in case of congestion. In this case it indicates explicit rate for the source end station. The source end station, upon receiving the backward RM-cells, adjusts its sending rate according to the ER field.
The traffic management standard as defined by the ATM forum supports VC routes with both explicit and implicit switches. The standard specifies how to handle lost RM-cells as well as fairness between competing ABR connections and enhancements for better link utilization.
The following describes the available traffic and congestion control functions:
The 7200 Card implements Traffic Management that adheres to the emerging ATM Forum standards. The Traffic Management makes use of RM-cells for flow control to allow large networks to be created around ATM technology without having cell loss due to congestion.
The 7200 Card Traffic Management hardware module contains an ASIC and local memory for traffic management tables. It performs processing on ATM cells emerging from or received by the ZipChip. This provides traffic rate shaping, additional statistics and flexibility in ATM network configuration.
The 7200 Card checks connected devices for Traffic Management modules and only activates its own TM module if a corresponding one is found in the connected device.
RM-cells are used by the network to control the rate flow. The RM cell format is as defined in the TM specification. These cells are sent at regular intervals from the 7200 Card and make their way through the network. In the event that there is congestion, the congestion bit of the RM cell is set by the destination end station. The RM cell is then returned to the source 7200 Card. In case the RM-cell is not returned to the source 7200 Card or the CI bit is set, the traffic rate is automatically decreased. A received RM-cell with the CI bit cleared enables the sending 7200 Card to increase its sending rate.
No RM-cells are sent on point-to-multipoint connection. Multicast rates are set in advance to avoid flooding the source with backward RM-cells.
Traffic Management in the 7200 Card operates with both EFCI based and Explicit Rate (ER) based flow control methods.
The data transmission rate is controlled by means of rate tables called profiles. Depending on the information received in the returning RM cells the data transmission rate is indexed up or down in the table. Each table has an upper and lower rate limit according to need. A set of 32 tables is pre-loaded into the system and is designed to cover all requirements of the network. The profiles are identified by their numbers. Please see the manufacturer for the characteristics of each profile.
A dual-rate mechanism is used which depends on whether a single VC is in use or multiple VCs are in use. For a single VC the full rate of 155 Mbps is used; for multiple VCs the maximum rate is set to one-half the full rate.
In the 7200 Card there are two types of Traffic Management: static and dynamic. The static TM is essentially the dual-rate mechanism described in the previous section. The dynamic TM is the ABR feedback system described in the previous sections. The dynamic TM can be enabled and disabled by the LMA commands. When the dynamic TM is disabled, the static TM continues to operate.
The TM system works independently for each ATM connection. For certain remote clients (LECs) it is desirable to set up special TM characteristics which are used when communicating with it. For this purpose the 7200 Card maintains a database which holds the characteristics of each such connection including:
The database contents can be displayed on the Management terminal and entries can be added and removed.
Figure 13-2 below shows a network scenario, where two 7200 Cards, denoted by source station and destination station are found with an ATM cloud between them. The shaded area between the stations represents a VCC connection over which a data flow and an RM-cell flow takes place as shown. The traffic management is performed as follows:
1 . A forward RM cell is sent at regular intervals on each VC from the source end station.
2 . The EFCI bit is set in data cells by the ATM switch when congestion is found along the VC path.
3 . The destination end station recognizes the EFCI bit and the backward RM cell congestion indicator bit is set.
4 . The backward RM cell is returned to the source end station through the ATM switch.
5 . VC traffic flow is altered based on information in the RM cell.
Figure 13-2 Traffic Management control steps
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