Introduction: ATM service categories
Voice, video, and file transfer have different requirements for rate, delay, and cell loss. ATM separates these requirements through service categories. Each category describes a traffic profile and the objectives for a connection.
QoS parameters include Cell Loss Ratio (CLR), maximum Cell Transfer Delay (maxCTD), and Cell Delay Variation (CDV). CDVT describes permitted cell clumping in the traffic profile and is distinct from CDV.
ATM is a connection-oriented technology. A virtual connection is established before transmission, and the application negotiates a traffic contract with the network. During setup, Connection Admission Control (CAC) evaluates whether the required resources are available. After activation, Usage or Network Parameter Control (UPC/NPC) compares arriving cells with the contract and applies the defined treatment to non-conforming traffic. The contract specifies traffic and QoS parameters. For cells that conform to the negotiated values, the network declares corresponding performance objectives.
ATM service categories
The ATM Forum defined six service categories. Each category describes a traffic profile, contract parameters, and expected QoS. ITU-T Recommendation I.371 describes corresponding ATM transfer capabilities, using DBR and SBR for the mechanisms called CBR and VBR by the Forum. The two naming systems are therefore not interchangeable in every detail.
1. CBR (Constant Bit Rate): fixed bit rate
CBR serves connections that require a fixed amount of resources throughout a session. PCR describes the contracted cell rate, while Cell Delay Variation Tolerance (CDVT) limits the permitted cell clumping. The category supports tightly constrained delay and delay variation.
- Traffic profile: The source may emit cells at PCR at any time during the connection. Resource reservation is included in network capacity planning.
- Application Examples: Uncompressed voice (like a traditional telephone line), real-time video, circuit emulation services.
- Key QoS Parameter: .
2. rt-VBR (real-time Variable Bit Rate): variable real-time rate
rt-VBR serves real-time applications that generate bursty traffic with a variable rate. PCR, SCR, MBS, and CDVT describe the source profile and permitted clumping. Delay and delay variation must remain within tightly constrained limits.
- Traffic profile: The source rate changes over time. PCR, SCR, and MBS describe peak and average rates together with the permitted burst size.
- Application Examples: Compressed interactive video (e.g., video conferencing), real-time multimedia distribution.
- Key QoS Parameters: PCR, , Maximum Burst Size (MBS).
3. nrt-VBR (non-real-time Variable Bit Rate): variable non-real-time rate
nrt-VBR serves non-real-time applications with bursty traffic. PCR, SCR, MBS, and CDVT describe the source profile, and a low cell loss objective applies to cells that conform to the traffic contract. This category defines no delay bounds.
- Traffic profile: Connections may be statistically multiplexed. The cell-loss objective applies to traffic within the negotiated contract.
- Application Examples: Frame Relay interworking, banking transactions, booking systems.
4. ABR (Available Bit Rate): available bit rate
ABR controls the source rate through feedback carried in Resource Management (RM) cells. The source specifies PCR and MCR, updates its sending rate from RM information, and changes the allowed rate with network conditions. The category does not bound connection delay.
- Control mechanism: The source adapts its cell flow to RM-cell information. The network may allocate capacity above MCR according to its local fairness policy.
- Application Examples: Data transfers, email, any application that benefits from dynamic bandwidth adjustment.
- Key QoS Parameter: .
5. UBR (Unspecified Bit Rate): unspecified bit rate
UBR serves non-real-time traffic without negotiated QoS commitments. The network forwards cells within available resources and may discard them during congestion. Congestion control may be handled at a higher layer.
- Traffic profile: Rate and delay depend on current load and the service policy of network elements.
- Application Examples: Standard web browsing, non-critical file transfers (where higher-layer protocols like TCP handle reliability).
6. GFR (Guaranteed Frame Rate): guaranteed frame rate
GFR serves non-real-time applications that require a minimum rate and can use additional capacity. The guarantee applies to complete conforming AAL5 PDUs composed of CLP=0 cells. Traffic exceeding MCR or the permitted MBS may be delivered within the currently available capacity.
- Traffic profile: Cells are grouped into AAL5 frames, and GFR processing recognizes frame boundaries. During conformance checking, cells in one frame have the same CLP value. GFR.1 preserves CLP, while GFR.2 allows the network to tag cells under congestion. During congestion, the network may discard a complete PDU while preserving frame boundaries.
- Application Examples: LAN interworking and transfers of frames with variable traffic intensity.
- Key Parameters: PCR, , MBS, and Maximum Frame Size (MFS).
ATM OAM cells
ATM uses special OAM (Operation, Administration, and Maintenance) cells to manage connections and monitor faults and performance. OAM cells have the ATM cell format, but carry diagnostic and control information instead of user data. ITU-T Recommendation I.610 defines F4 flows for the Virtual Path (VP) level and F5 flows for the Virtual Channel (VC) level. F4 uses the same VPI as user VPC cells, with VCI 3 for a segment and VCI 4 for an end-to-end flow. F5 uses the same VPI/VCI values as the user VCC, with PTI values 100 and 101 distinguishing segment and end-to-end flows. Each flow can cover the complete connection or a selected segment.
Key OAM Functions
- Fault Management: Used to detect and report failures.
- : Sent toward the receiver by the element that detects a defect.
- : Sent toward the source after an AIS state is declared.
- Performance Management: Allows for checking the quality of a connection. FPM and BR cells support measurement and reporting. OAM cells can be used for:
- Verifying cell sequence integrity.
- Monitoring for errored cells.
- Performing loopback tests, where a cell is sent to a specific point and echoed back to measure delay and check for errors.
- Measuring lost cells.