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NE20E-S V800R010C10SPC500 Feature Description - QoS 01

This is NE20E-S V800R010C10SPC500 Feature Description - QoS
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Huawei uses machine translation combined with human proofreading to translate this document to different languages in order to help you better understand the content of this document. Note: Even the most advanced machine translation cannot match the quality of professional translators. Huawei shall not bear any responsibility for translation accuracy and it is recommended that you refer to the English document (a link for which has been provided).
Overview of ATM QoS

Overview of ATM QoS

What Is ATM

Asynchronous transfer mode (ATM) is a cell-based data transfer technique in which channel demand determines packet allocation.

ATM integrates the features of circuit switching and packet switching. On one hand, ATM is connection-oriented and any ATM user needs to communicate with another ATM user over an established connection. On the other hand, ATM sends data in fixed-sized cells and multiple ATM connections can share bandwidth resources.

Statistical Multiplexing of ATM

ATM uses statistical multiplexing to achieve the best utilization of network resources. Statistical multiplexing dynamically allocates network resources to different services based on the statistical features of these services, so that network resources are best utilized. An ATM network can transmit multiple types of services, such as data, voice, and video services, at different rates and provides QoS guarantee for real-time services.

Figure 10-1 Statistical multiplexing of ATM

As shown in Figure 10-1, the data of users D, C, and A is allocated to transmission lines based on their arrival sequence. Because user B does not send any data, user B does not occupy bandwidth resources. In this sense, an ATM connection is a virtual connection.


On an ATM network, the source and destination must communicate over an established connection. The establishment of an ATM connection is similar to the establishment of a telephone call connection. An ATM connection is a virtual circuit connection (VCC) uniquely identified by a virtual path identifier (VPI)/virtual channel identifier (VCI) pair.

From the perspective of routing, a VPI/VCI pair functions in a similar way as an IP address. Multiple VPI/VCI pairs uniquely identify a multi-segment connection. When a switching node receives an ATM cell, the switching node searches its local VPI/VCI mapping table and replaces the incoming VPI/VCI pair carried in the cell with the corresponding outgoing VPI/VCI pair.

Figure 10-2 VCC

ATM switching includes VP switching and VC switching. A VP switching node changes only the VPI value of the VPI/VCI pair, whereas a VC switching node changes both the VPI and VCI values of the VPI/VCI pair. A VP can be viewed as a large pipe with VCs as its small pipes. Figure 10-3 shows the relationships between VPs and VCs.

Figure 10-3 Relationships between VPs and VCs

The multiplexing, switching, and transmission of ATM cells are performed on VCs.


ATM supports two types of VCs:

  • Switched virtual circuits (SVCs): are established by ATM user terminals using signaling. SVCs function in a similar way as user lines on a telephone network. An ATM network establishes an SVC for two users to communicate only after one of the two users initiates a communication request. After the communication is complete, the SVC is released by the signaling. SVCs can appropriately utilize network resources to reduce communication costs.

  • Permanent virtual circuits (PVCs): are statically configured by administrators. A PVC cannot be automatically released after the communication is complete. PVCs function in a similar way as leased lines on a telephone network. Users connected over PVCs can communicate even if network resources are insufficient. PVCs apply to scenarios with high communication requirements.

Nowadays, most ATM networks use PVCs to transmit data.

Importance of Congestion Management for an ATM Network

Compared with congestion management on a circuit or packet switched network, congestion management on an ATM network has the following characteristics:

  • Important: ATM is an asynchronous data transfer technique that uses statistical multiplexing to dynamically allocate network resources to different services. Statistical multiplexing improves network resource usage and allocation flexibility, but also increases network congestion risks.
  • Difficult:
    • An ATM network transmits a variety of services. The data traffic features of these services are hard to control.
    • An ATM network transmits services at high rates. If traffic congestion occurs on a certain connection, the congestion rapidly spreads to other connections.

Congestion management is a daunting task faced by ATM networks.

Basic Principles of Congestion Management

The International Telecommunication Union-Telecommunication Standardization Sector (ITU-T) develops a set of congestion control mechanisms to satisfy the congestion management requirements of ATM networks. The basic principles of these mechanisms are to prevent traffic congestion from occurring by appropriately managing network resources.

Congestion management measures can be classified into two types:

  • Preventive measures (traffic control): are designed to prevent traffic congestion from occurring. These measures include traffic contract parameter determination, call admission control, and traffic parameter control.
  • Responsive measures (congestion control): are designed to minimize the impact of traffic congestion after traffic congestion occurs. These measures include selectively dropping ATM cells and reporting congestion indications.

Traffic Control

On an ATM network, traffic control is implemented based on service types and quality.

The ATM Forum classifies service types as constant bit rate (CBR), real-time variable bit rate (RT-VBR), non-real-time variable bit rate (NRT-VBR), available bit rate (ABR), and unspecified bit rate (UBR) based on service rates. These service types will be described in details in the section ATM Service Types.

Besides determining service types based on service rates, the ATM Forum defines QoS and traffic parameters to measure ATM service quality. Before an ATM connection is established, QoS and traffic parameters are negotiated between an ATM user and the ATM network or between two ATM networks. These negotiated parameters form a traffic contract.

Traffic Contract Parameters

The traffic contract used by ATM networks is similar to the service level agreement (SLA) used by IP networks.

Figure 10-4 ATM traffic contract

An ATM traffic contract includes:

  • Traffic parameters

    Some traffic parameters describe the traffic characteristics of services. These parameters are called source traffic parameters. These parameters include:

    • PCR(Peak Cell Rate): indicates the maximum allowable rate at which cells can be transmitted along an ATM connection. Cells exceeding the PCR will be dropped by the ATM ingress or marked as droppable. Cells marked as droppable will be dropped by any node that encounters traffic congestion. For CBR services, the PCR represents the constant bandwidth provided by a VC.
    • Sustainable cell rate (SCR): indicates the average allowable, long-term cell transfer rate on a specific ATM connection. The SCR is specific to VBR services.
    • Minimum cell rate (MCR): indicates the minimum allowable rate at which cells can be transmitted along an ATM connection.
    • Maximum burst size (MBS): indicates the maximum allowable burst size of cells that can be transmitted contiguously on a particular ATM connection. The MBS is specific to VBR services. The burst size indicates the ratio of the peak bit rate to the average bit rate. The larger the burst size, the larger the rate variation of the service.

    Some traffic parameters describe the characteristics of services in relation to time. For example, the cell delay variance tolerance (CDVT) indicates the maximum cell delay variance (CDV) allowed between two terminals. These parameters apply to real-time services.

  • QoS parameters

    • peak-to-peak CDV (peak-to-peak Cell Delay Variance): indicates the difference between the maximum and minimum cell transfer delay (CTD) experienced during the connection.

    • Maximum cell transfer delay (MCTD): indicates the maximum CTD. The CTD indicates the delay experienced by a cell between the time it takes for the first bit of the cell to be transmitted by the source and the last bit of the cell to be received by the destination. The MCTD is an important parameter for CBR services. If the transmission duration of some cells is too long, the destination will regard these cells as lost or delayed. The destination drops delayed cells even if these cells have been reassembled into packets. Large CTD will affect the quality of voice services.

    • Cell loss rate (CLR): indicates the percentage of cells that are lost on the network due to errors or congestion and are not received by the destination. Cells may fail to reach the destination in the following situations:

      1. The destination is incorrect.

      2. ATM nodes experience severe congestion.

      3. The burst size of the traffic sent by the source exceeds the specifications in the traffic contract.

      4. The CTD of cells exceeds the MCTD.


When a terminal initiates a call request, the terminal includes the characteristics of the traffic to be sent to the network and service quality requirements in the call request.

After the network receives a call request, the network starts the call admission control (CAC) function to detect the distribution of network resources. Then, the network determines whether the available network resources can meet the service quality requirements.

If the available network resources can meet requirements, the network accepts the call request and establishes a new VC. In this situation, the service quality of existing connections remains unchanged. Otherwise, the network rejects the call request.

Traffic Parameter Control

After a VC is established, the characteristics of traffic transmitted over the VC may go beyond the characteristics determined by the network using the CAC function. In this situation, network service quality may be affected if traffic parameters are not appropriately controlled. To solve the preceding problem, a traffic monitoring and control mechanism is deployed on user-to-network interfaces (UNIs) and network-to-network interfaces (NNIs) to ensure that the characteristics of the incoming traffic bound for each VC conform to the negotiated characteristics specified in the traffic contract. The traffic monitoring and control process is called traffic parameter control.

A main traffic parameter control measure is to mark the traffic that exceeds negotiated specifications. Marked traffic will be dropped first if congestion occurs. The service quality of marked traffic cannot be ensured.

Congestion Control

When an ATM network detects congestion, the network starts congestion control by selectively dropping cells of minor importance and reporting forward and feedback congestion indications. If the preceding measures cannot put congestion under control, the ATM network releases the congested connection or reroute traffic.

  • Selectively Dropping Cells

    The cell loss priority (CLP) bit of cells transmitted over an ATM network indicates the drop priority of cells. The CLP bit has two values: 0 and 1. If congestion occurs, the ATM network drops cells with the CLP bit as 1 first.

    Figure 10-5 ATM cell structure

    The cells with the CLP bit as 1 may be cells of minor importance sent by users or cells whose CLP bits are changed from 0 to 1 by usage parameter control (UPC) or network parameter control (NPC) due to inconsistency with negotiated specifications. The ATM network drops cells with the CLP bit as 1 to ensure the transmission quality of high-priority cells.

  • Reporting Congestion Indications

    The measure of selectively dropping cells is taken by the ATM node where congestion occurs. In some situations, the entire network needs to work in coordination to deal with congestion. The nodes that experience traffic must be able to spread congestion information to other parts of the network for other nodes to take responsive measures to control congestion. Congestion indication types are classified as explicit forward congestion indication (EFCI) and feedback congestion indication (FCI).

    The EFCI process is as follows:

    1. The source sends a cell with the EFCI bit as 0.
    2. The congested ATM node re-sets the EFCI bit of the cell to 1 before forwarding the cell.
    3. After the destination receives a cell with the EFCI bit as 1, the destination sets the EFCI bit of a cell to 1 before sending the cell to the source.
    4. After the source receives a cell with the EFCI bit as 1 from the destination, the source determines that congestion has occurred on the connection between itself and the destination and lowers the traffic sending rate to relieve traffic congestion.

    FCI applies to ABR services and is implemented using resource management (RM) cells. The FCI process is as follows:

    1. The source injects an RM cell into the cell flow sent to the destination. The header of the RM cell carries the CLP bit (0) and PTI field (110) to detect available bandwidth.
    2. The destination returns the received RM cell to the source. The returned RM cell contains the feedback information added by ATM nodes along the transmission path. The feedback information reflects bandwidth availability.
    3. The source takes responsive measures based on actual situations:
      • If the source does not receive the returned RM cell, the source continuously reduces the traffic sending rate.
      • If the source receives the RM cell and the feedback information contained in the RM cell indicates that the available bandwidth has increased, the source can increase the traffic sending rate.
      • If the source receives the RM cell and the feedback information contained in the RM cell indicates that the available bandwidth has decreased, the source should rapidly reduce the traffic sending rate.

ATM Service Types

  • CBR

    The traffic of constant bit rate (CBR) services is transmitted at a constant bit rate over a VC. CBR services apply to interactive digital audio and video applications that require continuous digital information streams, such as video conference, telephony, and distant education.

    Figure 10-6 CBR service characteristics

    The amount of bandwidth allocated to CBR services is characterized by the PCR. CBR services are tailored for any type of data for which the terminals require predictable responsive time and a static amount of bandwidth continuously available for the lifetime of the connection.

  • VBR

    The source of a variable bit rate (VBR) service sends cells at a variable rate, and the traffic can be regarded as burst traffic. VBR services include:

    • Real-time variable bit rate (RT-VBR) services have strict delay and jitter requirements and are suited for applications with high requirements for real-time communication, such as IP-based voice and video services.
    • non-real-time variable bit rate (NRT-VBR) services are suited for applications that have low requirements for real-time communication and allow burst traffic, such as ticket booking systems and bank transaction systems. NRT-VBR services can guarantee low cell loss for traffic that conforms to the traffic contract but has no restrictions on cell transfer delay.
      Figure 10-7 VBR service characteristics

  • ABR

    Available Bit Rate (ABR) services do not have restrictions on delay or jitter. As a result, ABR services cannot be used for applications that require real-time communication. ABR services use some flow control measures to control network congestion and cell loss, reducing the cell loss rate and guaranteeing bandwidth availability. Typical ABR service applications include LAN emulation services and LAN interworking services.

    Figure 10-8 ABR service characteristics

  • UBR

    Unspecified bit rate (UBR) services do not guarantee bandwidth availability. UBR services are best-effort services used for applications that are tolerant of delay and jitter, such as email and FTP services.

    UBR services do not provide QoS guarantee. The cell loss rate and cell transmission delay of connections cannot be guaranteed. The PCR is optional in CAC and UPC. If a network does not have strict requirements for the PCR, you do not need to use the PCR.

    The difference between ABR and UBR services lies in that when a VC is congested, the ABR service reduces the traffic sending rate whereas the UBR service drops cells.

    Figure 10-9 UBR service characteristics

Updated: 2019-01-03

Document ID: EDOC1100055126

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