ATM Switching: An Overview

1. ATM Switching: An Overview Carey Williamson Department of Computer Science University of Calgary

2. Introduction • ATM switches are central to the operation of ATM networks • Lots of research on ATM switch design in the last 10 years • This presentation: an overview of the basic components and functionality common to all ATM switching systems

3. ATM Switch Components • Input ports • Output ports • Switch fabric • Buffering • VPI / VCI table • Control unit

4. Switch Fabric Input Ports VPI VCI Control Unit Generic ATM Switch Output Ports

5. Switch Ports • Provide the physical connection to other ATM entities • Dedicated point-to-point links • Each port (or card) is for a particular type of interface and particular data rate (e.g., OC-3, DS-3, JPEG, Ethernet,T1,...) • N inputs, N outputs

6. Switch Fabric • Defines the interconnection between the input ports and the output ports • Distinguishing characteristic of different types of ATM switches (e.g., shared memory, banyan, Benes, hypercube, ...) • Affects performance (cell delay, cell loss)

7. Buffering • Some switches are bufferless • Some switches are buffered • Buffers provide physical storage (i.e., memory) to hold cells that cannot yet be sent on their desired output ports • Uses RAM, DRAM, or FIFO...

8. Buffering (Cont’d) • Buffering can be located at the input ports, at the output ports, internal to the switch fabric, or any combination • Buffers are usually limited in size: • e.g., 100 cells per port • e.g., 4096 cells per port • Buffer size and location affects performance

9. VPI / VCI Table • Physical memory that is used to keep track of how to map cells of different VC’s from input ports to output ports • Translation table: INPUT OUTPUT Port Port VPI VCI VPI VCI 1 4 15 1 5 12 3 27 104 2 2 33

10. VPI / VCI Table (Cont’d) • Entries initialized during call setup • Lookup done for each data cell • Determines “routing tag” to be used for switch fabric

11. Control Unit • Controls entire operation of switch • Cell switching • Signalling (UNI and NNI) • Call Admission Control (CAC) • Usage Parameter Control (UPC) • Accounting statistics

12. ATM Switch Functionality • Cell switching • also known as “label switching” or “label multiplexing” • cell with VPI = a and VCI = b on input port c is sent out with VPI = d and VCI = e on output port f • uses VPI / VCI table • simple fast harware switching (for data cells)

13. ATM Switch Functionality • Signalling • participate in UNI signalling protocol (edge switches) • participate in NNI signalling protocol (core switches) • connection setup and teardown • update VPI / VCI table

14. ATM Switch Functionality • Call Admission Control (CAC) • makes decision to accept / reject new call • considers traffic descriptor (TD) and QOS requested by the new call (e.g., PCR, SCR, CLR, CDV) • considers available resources, and (potential) impact on the QOS of existing calls • many different approaches proposed

15. ATM Switch Functionality • Usage Parameter Control (UPC) • mechanism to monitor incoming traffic flows on a VPI / VCI basis • check for conformance to traffic descriptor • e.g., leaky bucket, dual leaky bucket • can provide “traffic shaping” • tags or discards non-conforming cells

16. ATM Switch Functionality • Accounting • statistics accumulation • simple counters (e.g., per port) • statistics are reported to network management station (e.g., Newbridge 4602)

17. Summary • ATM switches form the very heart of ATM networks • Basic components and functionality are common to all switches • Switches from different vendors differ in the details (e.g., switch fabric, buffering, control software, etc.)