ATM and PSN Integration

The ATM technology was designed to resolve all network communication problems. However, because ATM is too well developed, its complex architecture causes difficulties in ATM system development, configuration, management, and fault location. The ATM technology does not have a chance to display its super performance with a pure ATM network.

In late 1990s, the Internet and IP technology gain an overwhelming competitive edge over ATM with their simplicity and flexibility. Because ATM has a great advantage in providing guaranteed service transmission quality, technologies that integrate ATM with the packet switched network (PSN) appear.

ATMoPSN

ATMoPSN uses the pseudo wire emulation edge-to-edge (PWE3) technology to transparently transmit ATM services over a PSN.

Figure 10-10 ATMoPSN networking model

Figure 10-11 shows the ATMoPSN packet structure.

Figure 10-11 ATM PWE3 encapsulation

One ATMoPSN packet contains only one ATM cell.

PSNoATM

NOTE:

For the convenience of description, the IPoA is called PSNoATM technologies.

• IPoA

  With the IP-oriented development trend of core networks and the popularity of the Ethernet technology among access layer devices, ATM networks are widely used to carry IP and Ethernet services. As bearer networks for IP and Ethernet services, ATM networks provide high-speed PPP connections, high network performance, and good QoS guarantee.

  When an IP packet is transmitted over an ATM network, the IP packet must be adapted to the AAL so that the packet can be fragmented into cells at the source and be reassembled at the destination.

  Figure 10-12 IPoA
  
  

  IPoA encapsulates IP packets before transmitting IP packets over an ATM network.

  Figure 10-13 Encapsulation of IP
  
  

ATMoPSN QoS

Figure 10-14 shows a typical ATMoPSN networking model. Figure 10-15 shows the ATM PWE3 packet structure.

Figure 10-14 ATMoPSN networking model

Figure 10-15 ATM PWE3 packet structure

ATM cells are encapsulated with Multiprotocol Label Switching (MPLS) labels when being transmitted over a PSN. MPLS QoS can ensure the end-to-end QoS of services transmitted over a PW.

To retain ATM QoS during the transmission of ATM cells over a PSN, ATM QoS parameters must be mapped to MPLS EXP values. Huawei routers implement this function using ATM traffic classification:

• When ATM cells enter a PSN, the ingress node maps CLP values carried in ATM cells to routers' internal service classes and drop priorities (colors) based on the ATM service type.
• When ATM cells leave a PSN, the egress node maps the internal service classes and colors of ATM cells to the original CLP values carried in ATM cells.
  Figure 10-16 Priority mapping for ATMoPSN QoS
  
  

ATM traffic classification used in ATMoPSN QoS is forced traffic classification.

• ATM Forced Traffic Classification

  Forced traffic classification is implemented on the upstream interfaces of edge routers on an ATM network. Forced traffic classification forcibly specifies the service classes and colors of IP packets transmitted over a PVC, through an interface (main interface or sub-interface), or over a PVP, irrespective of the ATM service type and CLP values. Then, the ATM network applies QoS policies on the downstream interface of edge routers based on the specified service classes and colors.

PSNoATM QoS

NOTE:

For the convenience of description, the IPoA is all called PSNoATM technologies.

Besides the traffic control and congestion control mechanisms offered by ATM, Huawei routers provide the following ATM QoS mechanisms:

• ATM Traffic Classification

  Before transmitting IP, Ethernet, or PPP services, an ATM network must use ATM traffic classification to map the IP priorities to ATM priorities.

  PSNoATM supports the following types of ATM traffic classification:

  • ATM Forced Traffic Classification

    ATM forced traffic classification in PSNoATM is similar to that in ATMoPSN.

  • ATM MF Classification

    ATM Multiple Field (MF) classification in PSNoATM is similar to complex traffic classification in IP QoS. The only difference is that the traffic policies used in ATM complex traffic classification must be configured on ATM interfaces (including ATM sub-interfaces) or VE interfaces.

    For more information about complex traffic classification in IP QoS, see the section MF Classification.

• ATM Traffic Shaping

  ATM traffic shaping enables cells to be sent at a relatively even rate by adjusting the traffic characteristics of cells transmitted over a VCC or VPC.

  ATM traffic shaping uses the following methods:

  • Reducing the peak cell rate
  • Limiting the burst traffic size
  • Adjusting the cell transmission interval
  • Queuing cells

  ATM traffic shaping is similar to IP QoS traffic shaping. The differences are:

  • IP QoS traffic shaping applies to IP packets, whereas ATM traffic shaping apply to ATM cells.

  • IP QoS traffic shaping uses token bucket algorithms, whereas ATM traffic shaping uses leaky bucket algorithms.

    Leaky bucket algorithms forcibly limit traffic rates, whereas token bucket algorithms allow burst traffic transmission while enabling existing traffic to be evenly transmitted.

  Although ATM traffic shaping can be implemented on any parts of an ATM network, it is used on the egress of an ATM network.

• ATM PVC Congestion Management

  In ATM PVC congestion management, packets exceeding the PVC bandwidth are not dropped. Instead, these packets are cached, and transmitted when the PVC is idle, using queuing mechanisms.

  A PVC supports eight queues. The first queue uses the strict priority (SP) scheduling algorithm and is called the priority queuing (PQ) queue. The other queues use the weighted fair queuing (WFQ) algorithm and are called WFQ queues. Currently, an ATM PVC supports only one-level queue scheduling. The queue scheduling mechanism used by a PQ queue on an ATM PVC is similar to the queue scheduling mechanism used by a PQ queue in IP QoS. For more information, see Queues and Congestion Management.

  Huawei routers allow you to adjust the internal service classes of UBR services transmitted over a PVC or PVP to guarantee the quality of high-priority services.