EDUSAT SESSION FOR COMPUTER NETWORKSI CS64

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EDUSAT SESSION FOR COMPUTER NETWORKS-I
(CS64) Date 29.05.2006 Session
VII TopicATM Faculty Anita Kanavalli MSRIT
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Why ATM

• All digital telephone network
• Need for data applications like computer
  communications and facsimile
• Nonvoice applications like videoconferencing need
  to be included in the future networks
• Circuit switching not suitable but packet
  switching would accommodate this
• ISDN were the first effort to address this

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ISDN Overview

• There are many types of WAN technologies that can
  be used to solve the problems of users who need
  network access from remote locations.
• ISDN has been specifically designed to solve the
  low bandwidth problems that small offices or
  dial-in users have with traditional telephone
  dial-in services. 
• Telephone companies developed ISDN with the
  intention of creating a totally digital network
  whilst making use of the existing telephone
  wiring system.
• ISDN works very much like a telephone - When you
  make a data call with ISDN, the WAN link is
  brought up for the duration of the call and is
  taken down when the call is completed

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ISDN Overview

• ISDN allows digital signals to be transmitted
  over existing telephone wiring.
• This became possible when the telephone company
  switches were upgraded to handle digital signals.
  
• ISDN is generally viewed as an alternative to
  leased lines, which can be used for telecommuting
  and networking small and remote offices into
  LANs.

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ISDN Overview

• ISDN's ability to bring digital connectivity to
  local sites has many benefits, including the
  following
• ISDN can carry a variety of user traffic signals
  including
• digital video, packet-switched data, and
  telephone network services.
• ISDN offers much faster call setup than modem
  connections because it uses out-of-band (D, or
  delta, channel) signaling.
• For example, some ISDN calls can be setup in less
  than one second.

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ISDN Overview

• ISDN provides a faster data transfer rate than
  modems by using the bearer channel (B channel of
  64kbps).
• With multiple B channels, ISDN offers users more
  bandwidth on WANs than some leased lines.
• For example, if you were to use two B channels,
  the bandwidth capability is 128Kbps because each
  B channel handles 64Kbps.
• ISDN can provide a clear data path over which to
  negotiate PPP links.

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ISDN Overview
Components Considerations ISDN Router Multiple
remote users at the same location
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ISDN Overview
BRI (Basic Rate Interface) Connection from the
ISDN office to the user location provides for
access to three channels. The channels are two
64Kb B-channels and one 16Kb D-channel The
B-channels and the D-channel provide the user
with access to the circuit switched network
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ISDN Overview
PRI (Primary Rate Interface) ISDN Primary Rate
Interface service provides digital access via a
T1 line. A T1 line provides a 1.544 bandwidth.
This bandwidth is divided into 24 64Kb channels.
The ISDN PRI service uses 23 B channel access and
uses the 24th (D) channel for signaling purposes
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Third Generation Lans -ATM

• ATM is a high-speed switching network
  architecture
• ATM can be used to carry data, voice, and video
• separately or simultaneously over same network
  path
• ATM has a robust quality of service (QoS)
• can provide seamless interconnectivity between
  LANs and WANs
• supports a wide range of data rates
• 25 to 155 Mbps over copper
• 100 to 622 Mbps and higher over fiber
• common implementation is 155-Mbps ATM

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ATM Cells

• ATM cells are always 53 bytes long
• partitioned into
• 5 byte header ? contains addressing information
• 48 byte payload ? contains user data

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BISDN Ref Model

• There are three planes
• User Plane
• Control Plane
• Management Plane

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User Plane

• ATM is specified via a three-layer reference
  model
• Physical layer (OSIs physical layer)
• ATM layer (generally OSIs data link layer)
• ATM adaptation layer (AAL) (generally OSIs
  higher-level layers (transport, session, and
  application)

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Physical Layer

• Physical layer (2 sublayers)
• Physical medium PM (lower sublayer)
• definition for the medium
• the bit-timing capabilities.
• Transmission convergence (TC) (upper sublayer)
• makes sure that valid cells are being created and
  transmitted
• involves breaking off individual cells from the
  data stream of the higher layer (the ATM layer)
• checking the cells header
• Encoding the bit values

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ATM Layer

• ATM layer
• service-independent layer
• creates cell headers and trailers
• defines virtual channels and paths and gives them
  unique identifiers
• cells are multiplexed or demultiplexed.
• ATM layer creates the cells and uses the physical
  layer to transmit them.

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ATM Adaptation Layer

• ATM adaptation layer (AAL) (2 sublayers)
• Segmentation and reassembly SAR (lower sublayer)
• packages variable size packets into fixed-size
  cells at the transmitting end
• repackages the cells at the receiving end
• responsible for finding and dealing with cells
  that are out of order or lost
• convergence sublayer CS (upper sublayer)
• provides the interface for the various services
  (e.g. data, voice, and video).
• users connect to CS through service access points
• (SAPs).

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ATM Architecture
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ATM

• Supports two type of connections
• Point to point
• Point to multipoint
• In terms of duration ATM provides PVCs permanent
  virtual connections
• SVCs switched virtual connections

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ATM

• PVCs provisioned by operator
• SVCs by user demand using signaling procedures
• Components of signaling include
• User network interfaceUNI
• Network network interface NNI
• Broad band intercarrier interface B-ICI

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ATM

• Control plane has same three layers of user plane
• Control plane supports signaling and network
  applications

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ATM Physical Layer

• Two pieces (sublayers) of physical layer
• Transmission Convergence Sublayer (TCS) adapts
  ATM layer above to PMD sublayer below
• Physical Medium Dependent depends on physical
  medium being used
• TCS Functions
• Header checksum generation 8 bits CRC
• Cell delineation
• With unstructured PMD sublayer, transmission of
  idle cells when no data cells to send

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ATM Physical Layer

• Physical Medium Dependent (PMD) sublayer
• SONET/SDH transmission frame structure (like a
  container carrying bits)
• bit synchronization
• bandwidth partitions (TDM)
• several speeds OC1 51.84 Mbps OC3 155.52
  Mbps OC12 622.08 Mbps
• TI/T3 transmission frame structure (old
  telephone hierarchy) 1.5 Mbps/ 45 Mbps
• unstructured just cells (busy/idle)

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ATM Layer

• Service transport cells across ATM network
• analogous to IP network layer
• very different services than IP network layer

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ATM Layer
Guarantees ?
Network Architecture Internet ATM ATM ATM ATM
Service Model best effort CBR VBR ABR UBR
Congestion feedback no (inferred via
loss) no congestion no congestion yes no
Bandwidth none constant rate guaranteed rate gua
ranteed minimum none
Loss no yes yes no no
Order no yes yes yes yes
Timing no yes yes no no
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ATM Layer -Cell

• 5-byte ATM cell header
• 48-byte payload
• Why? small payload -gt short cell-creation delay
  for digitized voice
• halfway between 32 and 64 (compromise!)

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ATM Layer -Cell
Cell header
Cell format
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ATM Layer -Cell

• VCI virtual channel ID
• will change from link to link thru net
• PT Payload type (e.g. RM cell versus data cell)
• CLP Cell Loss Priority bit
• CLP 1 implies low priority cell, can be
  discarded if congestion
• HEC Header Error Checksum
• cyclic redundancy check
• VPI GFC

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ATM Layer

• VC transport cells carried on VC from source to
  dest
• call setup, teardown for each call before data
  can flow
• each packet carries VC identifier (not
  destination ID)
• every switch on source-dest path maintain state
  for each passing connection
• link,switch resources (bandwidth, buffers) may be
  allocated to VC to get circuit-like perf.
• Permanent VCs (PVCs)
• long lasting connections
• typically permanent route between to IP
  routers
• Switched VCs (SVC)
• dynamically set up on per-call basis

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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