Labels, Label Switching - Computer Network - Lecture Slides, Slides of Computer Networks

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COMPUTER

NETWORKS

Lecture-

Review Lecture 18

A single Global Technology

ATM Design and Cells

Connection Oriented Service

VPI/VCI

Labels and Label Switching

 As cells arrive at an ATM switch, their VPI/VCI is modified using a forwarding table which gives the new VPI/VCI for the next leg of the cell’s trip

 The forwarding table is essentially indexed by the incoming cell’s VPI/VCI, and the contents yield the new VPI/VCI

Label Rewriting

 The replacement of the incoming cell’s VPI/VCI with a probably different VPI/VCI is called Rewriting

 ATM is thus called a Label Rewriting, or Label Switching system

 Thus two computers with a connection through an ATM network will likely have different VPI/VCI values for each end of the connection! Docsity.com

Example

Example

 The sending computer, S, uses a VPI/VCI of 3 and sends a cell

 Switch A receives the cell and looks up 3, then rewrites the VPI/VCI as 4, and sends the cell out of its port 2

 Switch B receives the cell and looks up 4, then rewrites the VPI/VCI as 1, and sends the cell out of its port 2

Permanent Virtual Circuits

 ATM can provide customers with virtual circuits that look like traditional leased digital circuits

• Such permanent virtual circuits (PVC) last as long as the customer pays the periodic fee for its use
• The forwarding tables are automatically restored after power or equipment failure
• The forwarding table entries for such permanent VCs are statically configured; the term used by telcos for this is provisioning

Permanent Virtual Circuits

 Provisioning requires two steps:

• Determine a complete path (that is, identify the switches that will be used)
• Choose appropriate VCI/VPI for each step in the path, and configure each adjacent pair of switches (easy, since each switch rewrites the VCI/VPI)

Establishing an SVC

 Computer sends a connection request to the switch to which it is attached

 Software in the switch finds a network path to the destination and sends along the connection request

 Each pair of switches in the path communicate to choose a VPI/VCI for their tables

Establishing an SVC

 Once the connection is accepted by the destination, a message is sent back to the originating computer to indicate the SVC is ready

 If any switch, or the destination computer, does not agree to setting up the VC, an error message is sent back and the SVC is not established

Signaling

 These connection requests and network control messages are also sent in cells, and since the VPI/VCI used in these cells are reserved, there is no confusion between data and control cells

Quality of Service (QoS)

 Networks are more frequently being designed to allow specification of the quality of service required by users

 For example, a typical voice telephone call might give a QoS parameter for througput of 64Kbps and delay less than 500 msec. A user sending video might require throughput of 2Mbps

ATM QoS Specifications

 Constant Bit Rate (CBR) : Used for uncompressed audio and video, since these have predefined maximum data rates

 Variable Bit Rate (VBR) : Used for compressed audio and video, where the data rate depends on the level of compression that can be achieved

ATM QoS Specifications

 Available Bit Rate (ABR) is used for typical data applications (where the data rate may be unknown and bursty), and allows use of whatever bandwidth is available at a given time