ATM%20Switching:%20An%20Overview

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
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 (Contd)

• 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 VCs from input ports
  to output ports
• Translation table

INPUT
OUTPUT
Port
Port
VPI
VPI
VCI
VCI
1
4
15
1
5
12
3
27
104
2
2
33
10
VPI / VCI Table (Contd)

• 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.)