EDUSAT SESSION FOR COMPUTER NETWORKSI CS64

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EDUSAT SESSION FOR COMPUTER NETWORKS-I
(CS64) Date 30.05.2006 Session
VIII TopicATM Faculty Anita Kanavalli MSRIT
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ATM Layer

• Virtual circuits are identified by specific
  virtual channel identifiers (VCIs).
• A collection of virtual channels that all have
  the same endpoints is called a virtual path
  connection (VPC)
• VPCs are specified by virtual path identifiers
  (VPIs)

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ATM Layer

• Virtual connections established
• VCI and VPI assignments are made dynamically by
  ATM end nodes and switches at the time data are
  to be transmitted
• VCI is not of interest to e.g. public switches
  they would only use the VPI

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Example network
PBX
Videoconferencing
File Transfer Status
VCI 1 (File Transfer)
VCI 3 (Voice Service)
LAN Switch
VCI 2 (Videoconferencing)
VCI 4 (Data Service)
VPI 1
VPI 2
VPI 2
VPI 1
VCI 4
VCI 1
VCI 3
VCI 2
modified after Gallo Hancock (2002)
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ATM Layer

• Advantages of ATM VC approach
• QoS performance guarantee for connection mapped
  to VC (bandwidth, delay, delay jitter)
• Drawbacks of ATM VC approach
• Inefficient support of datagram traffic
• one PVC between each source/dest pair) does not
  scale (N2 connections needed)
• SVC introduces call setup latency, processing
  overhead for short lived connections

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ATM Adaptation Layer

• ATM Adaptation Layer (AAL) adapts upper layers
  (IP or native ATM applications) to ATM layer
  below
• AAL present only in end systems, not in switches
• AAL layer segment (header/trailer fields, data)
  fragmented across multiple ATM cells
• analogy TCP segment in many IP packets

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ATM Adaptation Layer

• Different versions of AAL layers, depending on
  ATM service class
• AAL1 for CBR (Constant Bit Rate) services, e.g.
  circuit emulation
• AAL2 for VBR (Variable Bit Rate) services, e.g.,
  MPEG video
• AAL5 for data (eg, IP datagrams)

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ATM Adaptation Layer
AAL PDU
ATM cell
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ATM Adaptation Layer

• AAL is divided into two sublayers
• Segmentation and reassembly-SAR
• Convergence sublayer-CS
• CS subdivided into Common part-CPCS
• Service specific part-SSCS

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AAL1

• Support services that require constant rate
• Example pcm voice call
• Its PDU contains clock recovery and seq numbering

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AAL1 Process
User Data
47
CS PDU
471
SAR PDU
485
ATM Cell
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AAL2

• Support services that require dynamic bit rate
• Example video

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AAL2 Process
User Data
243
CPCS PDU
471
SAR PDU
548
ATM Cell
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AAL3/4

• Support services with burst data
• Type 4 for connectionless
• AAL 3/4 allows long messages from multiple users
  to be simultaneously multiplexed and interleaved
  in the same ATM VC
• Can operate in two modes message and stream

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AAL3/4 Process
User Data
H(4)InfoPADT(4)
CPCS PDU PAD message to multiple of 4 bytes
H(2)44PT(2)
SAR PDU
548
ATM Cell
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AAL5

• AAL5 low overhead AAL used to carry IP datagrams
• 4 byte cyclic redundancy check
• PAD ensures payload multiple of 48bytes
• large AAL5 data unit to be fragmented into
  48-byte ATM cells

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ATM Signalling

• ATM addressing
• UNI signalling
• PNNI signalling
• PNNI Routing

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ATM n/w or link layer?

• Vision end-to-end transport ATM from desktop
  to desktop
• ATM is a network technology
• Reality used to connect IP backbone routers
• IP over ATM
• ATM as switched link layer, connecting IP routers

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ATM n/w or link layer?
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IP over ATM

• Classic IP only
• 3 networks (e.g., LAN segments)
• MAC (802.3) and IP addresses

Ethernet LANs
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IP over ATM

• IP over ATM
• replace network (e.g., LAN segment) with ATM
  network
• ATM addresses, IP addresses

ATM network
Ethernet LANs
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IP over ATM

• Issues
• IP datagrams into ATM AAL5 PDUs
• from IP addresses to ATM addresses
• just like IP addresses to 802.3 MAC addresses!

ATM network
Ethernet LANs
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Datagram journey in IP over ATM N/w

• at Source Host
• IP layer finds mapping between IP, ATM dest
  address (using ARP)
• passes datagram to AAL5
• AAL5 encapsulates data, segments to cells, passes
  to ATM layer
• ATM network moves cell along VC to destination
• at Destination Host
• AAL5 reassembles cells into original datagram
• if CRC OK, datgram is passed to IP

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ARP in ATM n/w

• ATM network needs destination ATM address
• just like Ethernet needs destination Ethernet
  address
• IP/ATM address translation done by ATM ARP
  (Address Resolution Protocol)
• ARP server in ATM network performs broadcast of
  ATM ARP translation request to all connected ATM
  devices
• hosts can register their ATM addresses with
  server to avoid lookup