ATM

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Chapter 13

• ATM

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Asynchronous Transfer Mode (ATM)

• High-speed, connection-oriented switching and
  multiplexing technology
• Capable of transmitting voice, video, and data
• Used to connect LANs to other LANs
• Offers opportunity to combine LAN and WAN
  technologies
• Provides high performance for bursty data traffic
• Suitable for delay sensitive voice and video
  traffic

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Market Segments for ATM Carrier Services

• Four market segments
• Segment 1 - ATM provides high-speed backbone to
  connect carriers (telcos) together
• Segment 2 - PC is equipped with ATM adapter to
  connect directly to ATM backbone
• Segment 3 - Very sophisticated ATM switch
  connects users in a building backbone to a public
  ATM network
• Segment 4 - ATM switches provide ports to connect
  each desktop to the ATM network within a building

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ATM Carrier Markets
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Fast Packet Standards Services

• Clear distinction between Frame Relay and Cell
  Relay services and technology used by them
• ISDN and Frame Relay are classified as frame
  services
• ATM and SMDS are categorized as cell services
• Primary difference between Frame and Cell Relay
  services
• Frame Relay uses variable length frames
• Cell Relay uses a fixed length cell

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Frame-based Implementations

• LAPD for relaying traffic
• Q.931
• Specifies control signaling
• How connections are set up between user and
  network
• Q.922
• Stipulates protocols must encapsulate data units
  within a Q.922 Annex A frame

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Cell-based Implementations

• ATM and SMDS use Cell Relay
• Broadband-Integrated Services Digital Network
  (B-ISDN)
• Developed to handle voice, video and data
  applications within same transmission
• Provide for on-demand, reserved or permanent
  services between service provider and organization

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ATM Benefits

• High capacity bandwidth
• Dedicated or switched circuits
• Low latency
• Multimedia, data, voice, video, and image
  transmission
• Uniform packet size offers predictable traffic
  flow
• Guaranteed delivery of time-sensitive traffic

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ATM Traffic Technology

• Designed to provide bandwidth on demand
• Users request and receive as much bandwidth as
  needed dynamically
• Handles all traffic types through a fast-packet
  switching technique
• Connection-oriented technique designed to
  transport both connection and connectionless
• Boundary operations of a network are
  connection-oriented

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ATM Interfaces

• User to Network Interface (UNI)
• Connection exists between user equipment and ATM
  equipment
• Network-to-Network Interface (NNI)
• All traffic travels between ATM devices within
  same network
• Intercarrier Interface (ICI)
• Used to send traffic across intermediate networks
  
• Data Exchange Interface (DXI)
• When non ATM equipment is used, transmits packets
  rather than cells to ATM interface

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ATM Service Categories
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ATM Virtual Paths Channels

• Virtual Channel (VC)
• Fixed pathway or route between two points
• Virtual Path (VP)
• Group of VCs tells switch how to forward ATM cell
  through ATM Network
• Virtual Path Identifier (VPI)
• Used to define route established in the ATM
  Switch
• Virtual Channel Identifier (VCI)
• Identifies a channel within a Virtual Path (VP)

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ATM VCI VPI Swapping

• Logical ID Swapping
• Each ATM switch is responsible for changing
  logical ID of one link to a different logical ID
  on another link as a cell passes through it
• ATM switch changes cell header VPI/VCI fields to
  reflect a new VPI and VCI for outgoing cell
• Virtual path is predefined in the switch and the
  logical IDs from end station to end station use
  that path identifier
• Virtual path is set up dynamically with call set
  up procedures when the ATM cell reaches the switch

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Lookup Tables

• Cell forwarding process is dependent on two
  lookup tables within the ATM switch
• Virtual Path (VP) table maintains a record of the
  VPs on each link.
• Virtual channel (VC) table maintains the output
  VP and VC to be used to send the cell

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ATM Cell Structure Addressing

• Uses numerical addressing similar to that used
  for telephone numbers.
• It uses E.164 addresses for public ATM (B-ISDN)
  networks
• ATM Forum has extended ATM addressing to include
  private networks
• Uses an overlay model where ATM layer is
  responsible for mapping network layer (IP)
  addresses to ATM addresses  

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ATM Network Layers

• ATM Network is designed as a series of layers
• First layer contains bulk of the transmission and
  is known as adaptation layer
• 48-byte payload divides data into different types
  in the adaptation layer
• The ATM layer contains five bytes referred to as
  overhead.
• Overhead is cell header information that directs
  the transmission.
• Physical layer attaches electrical elements and
  the network interfaces

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ATM Cell Header Octets
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Address Formats for ATM Networks
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Fields in ATM Address Format

• Fields are divided into two sections network and
  user
• Fields with network prefixes include fields
  needed by network side of UNI
• Fields include AFI, DCC, High-Order Domain
  Specific Part, End System Identifier, Selector,
  ICD and E.164.

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Address Fields to ATM Network

• ATM address is constructed as a 20-byte string
• Includes fields for country code, an
  administrative authority, a routing domain, area
  identifier, end-system identifier, NSAP,
  international code, and ISDN telephone number
• First byte identifies format of the 20 byte ATM
  address
• Three possible values for this AFI field, (1) DCC
  is hex 39, (2) E-164 is hex45 and (3) ICD is hex
  47.
• AFI-specific fields of authority that ATM devices
  belong to is named in bytes 2-9

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Routing Domains

• Routing Domains (RD)
• Used for traffic management to provide for an
  effective way to allocate bandwidth capacity
• Routing domains are defined in lookup tables in
  ATM switch
• Routing domains in example RD1, RD2 and RD3.
• Each routing domain is unique within ATM network
  and is specified in bytes 10 and 11.
• Areas
• Each identifies a unique area within a routing
  domain that ATM device resides in.
• Area is specified in bytes 12 and 13 of ATM
  address

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RoutingDomains
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ATM Reference Model Layers
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ATM Backbone Switch

• Core element of an ATM network
• Centralizes traffic flow and provides interfaces
  at various speeds
• Switch control point card implements cell
  switching capabilities
• Switch can handle aggregate capacity of 3 Gbps.
• Threshold levels are provided so time-critical
  traffic can be mixed with delay-tolerant traffic.

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ATM Backbone Switch
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ATM Routing Switch Fabric

• Used to switch cells within a switch
• Switch fabric is self-routing
• Can manage flow of ATM cells through the switch.
• Binary (0s 1s) algorithm is used at each
  element to determine output queue
• 3-bit tag is used to send cell to appropriate
  output port
• Several output queues are located at each port of
  switch
• Buffer management is provided at each output
  queue to hold traffic temporarily

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ATMRoutingSwitch
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ATM Cell Switching Transfer Mode

• ATM switch uses a table to manage cell transfer
  through switch
• Translation table includes incoming port number
  on the switch, virtual connection ID, outgoing
  port, and virtual connection ID.
• Switch interprets cell headers and consults table
  in order to determine how to forward cell through
  ATM network

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ATM Multiplexing

• ATM uses asynchronous transmission
• Has its own multiplexing technique called
  asynchronous transfer mode multiplexing.
• Each user or device can use time slots on demand
  as these become available.
• Multiplexing Example
• User (or device 3) is assigned to every fourth
  time slot
• User or device 2 requires lots of bandwidth,
  remaining available time slots can be used

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ATM Multiplexing Example
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Quote of the Day

• I fear all we have done is to awaken a sleeping
  giant and fill him with a terrible resolve.
• Isoroku Yamamoto