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Case Study: ATM (+ MPLS). ATM’s Key Concepts. ATM uses Virtual-Circuit Packet Switching ATM can reserve capacity for a virtual circuit. This is useful for voice and video, which require a minimum level of service

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atm s key concepts
ATM’s Key Concepts
  • ATM uses Virtual-Circuit Packet Switching
    • ATM can reserve capacity for a virtual circuit. This is useful for voice and video, which require a minimum level of service
    • Overhead for setting up a connection is expensive if data transmission is short (e.g., web browsing)
  • ATM packets are small and have a fixed sized
    • Packets in ATM are called cells
    • Small packets are good for voice and video transmissions

Header(5 byte)

Data (48 byte)

Cell is 53 byte long

53 byte cells
53 Byte Cells
  • Why 53 Bytes?A 48 byte payload was the result of a compromise between a 32 byte payload and a 64 byte payload
  • Advantages
    • Low packetization delay for continuous bit rate applications (video, audio)
    • Processing at switches is easier
  • Disadvantages
    • High overhead (5 Bytes per 48)
    • Poor utilization at lower line rates links
atm cells
ATM Cells
  • 4-bit Generic flow control
  • 8/12 bit Virtual Path Identifier
  • 16 bit Virtual Channel Identifier
  • 3 bit Payload Type
  • 1 bit Cell Loss Priority
  • 8 bit Header Error Control
  • 48 byte payload
  • GFC field only in UNI cells

UNI Cell

a packet switch
A Packet Switch

Header

Data

Packet

Packet switch

forwarding with vcs
Forwarding with VCs

Part 1: VC setup from X to E

forwarding with vcs1
Forwarding with VCs

Part 2: Forwarding the packet

2

5

5

3

virtual paths and virtual circuits
Virtual Paths and Virtual Circuits

Link

Virtual ChannelConnection

Virtual Path Connections

VPI identifies virtual path (8 or 12 bits)

VCI identifies virtual channel in a virtual path (16 bits)

vpi vci assignment at atm switches
VPI/VCI assignment at ATM switches

1/24

3/24

7/24

2/17

3/24

1/40

slide12

MPLS domain

push L1

swap L1, L2

pop L2

Eth

L1

IP

Eth

L2

IP

Eth

Eth

IP

IP

MPLS
  • Provides Virtual Circuit Switching to IP networks
    • Design started in 1997 by IETF, RFC 3031 was released in 2001.
  • Goal: Make IP networks faster
  • Approach:
  • Add s small label to packets at the ingress to the network
  • Switch packets based upon these labels
  • Label is removed by the egress router
labels in mpls

IP

header

Data

Labels in MPLS
  • Label
    • Identifier (20 bits) for a Virtual Circuit
    • With Ethernet and IP: a label is contained in a “shim header”
    • A label can be pushed, popped, swapped by MPLS routers

MPLS

“shim header”

Data link header

stacking of mpls labels

Eth

Eth

L1

L1

IP

IP

Eth

L4

L1

IP

Eth

L3

L1

IP

Eth

Eth

IP

IP

Stacking of MPLS labels
  • MPLS networks can be nested
  • Multiple labels can be added
    • Push a label at each new ingress
    • Pop label at each egress
  • Result is a generalization of VPI/VCI switching in ATM

MPLS domain

MPLS domain

push L3

swap L3, L4

pop L4

pop L1

push L1

regular ip forwarding

IP 47.1.1.1

Regular IP Forwarding

47.1

1

IP 47.1.1.1

2

IP 47.1.1.1

1

3

2

IP 47.1.1.1

1

47.2

3

47.3

2

IP destination address unchanged in packet header!

http://www-net.cs.umass.edu/cs653/

mpls label distribution

Request: 47.1

Request: 47.1

Mapping: 0.50

Mapping: 0.40

MPLS Label Distribution

1

47.1

3

3

2

1

1

2

47.3

3

47.2

2

http://www-net.cs.umass.edu/cs653/

label switched path lsp

IP 47.1.1.1

IP 47.1.1.1

Label Switched Path (LSP)

1

47.1

3

3

2

1

1

2

47.3

3

47.2

2

http://www-net.cs.umass.edu/cs653/