All about ATM: ATM Terminology Explained

1. All about ATM: ATM Terminology Explained Carey Williamson Department of Computer Science University of Calgary

2. Introduction • ATM networking is filled with a lingo of its very own, many of which are acronyms, and many of which are quite fundamental to an understanding of what is going on in an ATM network • Examples: VCI, VPI, PVC, SVC, AAL, CBR, VBR, ABR, PCR, SCR, QOS, CDV

3. Review • ATM: Asynchronous Transfer Mode • ATM is a statistical multiplexing technique for high speed integrated services networks, based on the fast packet switching of small fixed size (53 byte) packets called cells • ATM is a connection-oriented low-layer networking concept

4. ATM is Connection-Oriented • An end-to-end path called a virtual channel must be set up in advance, using an ATM signalling (control) protocol, before any data cells can be sent • All cells of a virtual channel travel on the same path • Cells arrive in the order that they were sent • Switches must maintain state about the virtual channels passing through them

5. Definitions • Virtual Channel (VC) • a connection between two communicating ATM entities (e.g., host-switch, switch-switch) • set up at time of call arrival • provides a certain grade of service (negotiated at time of call arrival) • cell sequence is preserved

6. Definitions (Cont’d) • Virtual Channel Identifier (VCI) • the label associated with a VC • 16-bit integer in UNI ATM cell format • carried in ATM cell header for identification • note that VCI’s are locally significant only (i.e., assigned on a per link basis by the ATM devices at either end of that link)

7. Definitions (Cont’d) • Virtual Path (VP) • a group of virtual channels (VC’s) all travelling between the same two points in ATM network • used by the network to simplify provisioning, resource management, providing different grades of service, etc. • “bundles up” traffic heading to same destination

8. Definitions (Cont’d) • Virtual Path Identifier (VPI) • the label associated with a VP • 8-bit integer in UNI ATM cell format • carried in ATM cell header for identification • part of two level addressing scheme in ATM • note that VPI’s are locally significant only (i.e., assigned on a per link basis by the ATM devices at either end of that link)

9. Definitions (Cont’d) • VP switch: an ATM switch that deals only with the VPI’s in cell headers (e.g., a core switch in middle of a large ATM network) • VC switch: an ATM switch that deals only with the VCI’s in cell headers (e.g., access switch at the edge of an ATM network) • VP/VC switch: an ATM switch that deals with both VPI’s and VCI’s in cell switching

10. Definitions (Cont’d) • Permanent Virtual Channel (PVC) • a virtual channel connection (virtual channel) that is set up on a long term basis (e.g., hours, days, months, years) by a human operator • involves statically configuring the “routing table” in ATM equipment • done as part of network provisioning in current ATM network testbeds • supported by all ATM switch vendors

11. Definitions (Cont’d) • Switched Virtual Channel (SVC) • a virtual channel connection (virtual channel) that is set up by the ATM signalling protocol between two communicating ATM entities • set up on an as needed basis, and torn down when complete • short term basis (e.g., seconds, minutes) • involves dynamically configuring the “routing table” in ATM equipment • supported by very few ATM switch vendors

12. Definitions (Cont’d) • ATM Adaptation Layer (AAL) • a protocol for converting between higher layer protocol data units (PDU’s), such as TCP packets, IP packets, or JPEG images, and ATM cells for actual transmission • defines procedures for segmentation and reassembly (SAR) • segmentation: packets to cells (done by sender) • reassembly: cells to packets (done by receiver)

13. Definitions (Cont’d) • Traffic Descriptors • a numerical specification of the statistical characteristics of an ATM traffic flow • used by ATM switches at time of call setup • specifies Peak Cell Rate (PCR), Sustained Cell Rate (SCR), Maximum Burst Size (MBS), etc. • different calls can specify different values for their traffic descriptor • e.g., voice: PCR = SCR = 1000 cells/sec • e.g., data: PCR = 10,000 cells/sec, SCR = 1000

14. Definitions (Cont’d) • Quality of Service (QOS) • a specification of the desired (or acceptable) grade of service required for a traffic flow • some traffic is delay-sensitive (e.g., voice) • some traffic is loss-sensitive (e.g., data) • some traffic is both (e.g., compressed video) • some traffic is neither (e.g., LAN emulation) • QOS requested at time of call setup • ATM network tries to provide requested QOS

15. Definitions (Cont’d) • QOS Parameters • the parameters that can be specified as part of the QOS request at time of call setup • examples: cell loss ratio (CLR), mean cell delay, maximum tolerable cell delay, cell delay variation (CDV) • different calls can specify different values for their QOS parameters • e.g., voice: delay < 50 msec, CLR < 0.001 • e.g., data: delay < 1 sec, CLR < 0.000001

16. Definitions (Cont’d) • QOS Classes • generic service classes for ATM traffic • used to help simplify the management and support of QOS requirements in ATM networks • currently there are five proposed classes: CBR, rt-VBR, nrt-VBR, ABR, and UBR • class specification determines the order of service for cells of different VCI’s

17. Definitions (Cont’d) • Constant Bit Rate (CBR) • simplest type of traffic: constant bit rate (e.g., voice traffic, T1 circuit emulation) • this is the highest priority class because of the delay-sensitive (i.e., time-dependent) nature of the traffic carried • suitable for periodic (isochronous) traffic • need to specify only PCR (which equals SCR) • supported by some ATM switch vendors

18. Definitions (Cont’d) • Variable Bit Rate (VBR) • more complicated type of traffic: the bit rate varies with time (e.g., compressed video) • peak bit rate (i.e., short term) may be much higher than the mean bit rate (i.e., long term) • must specify PCR, SCR, and burstiness • next highest priority class(es) • versions: real-time (rt) and non-real-time (nrt) • supported by some ATM switch vendors

19. Definitions (Cont’d) • Available Bit Rate (ABR) • unpredictable type of traffic: traffic is willing to use as much or as little bandwidth as is available (e.g., Internet traffic, LAN emulation) • usually variable bit rate, delay-insensitive • referred to as “elastic traffic” (e.g., ftp) • next lowest priority class • supported by few ATM switch vendors • still under discussion by ATM Forum

20. Definitions (Cont’d) • Unspecified Bit Rate (UBR) • the “bottom feeder” in the ATM food chain • Minimum Cell Rate (MCR) is zero; guaranteed nothing, but may get more than this if there happens to be extra bandwidth available • lowest priority class • much discussion at ATM Forum • may become Guaranteed Frame Rate (GFR)?

21. Definitions (Cont’d) • Call Admission Control (CAC) • a control function in ATM switches that makes the decision on whether or not to accept a newly incoming call • considers traffic descriptor (TD) and quality of service (QOS) parameters, as well as impact on the QOS of existing calls in the network • can be statistical or deterministic • still an active research topic

22. Definitions (Cont’d) • Usage Parameter Control (UPC) • a control function performed in switches to “police” ATM traffic flows • monitors traffic on a VCI basis, to make sure that it conforms to the declared traffic descriptor (TD) • in case of violation, can tag cells with a violation tag (CLP = 1), discard cells, or abort the connection

23. Definitions (Cont’d) • Cell Loss Priority (CLP) • a single bit in ATM cell headers for denoting the cell loss priority (e.g., violation cells) • in the event of congestion, drop tagged cells before dropping any untagged cells • not to be confused with “delay priority” (i.e., the service classes)

24. Summary • ATM has much confusing terminology, and an endless (growing) list of TLA’s (three letter acronyms) • Get used to it! • Soon you will be speaking it too!!!