Date Network Support

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Date Network Support
Content 1. Integrated Services Digital Network
(ISDN) 2. Asymmetric Digital Subscriber Line
(ADSL) 3. X.25 and Frame Relay 4. Asynchronous
Transfer Mode (ATM)
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ISDN User Network Interface

• ISDN allows multiplexing of devices over single
  ISDN line
• Two interfaces
• Basic ISDN Interface
• Primary ISDN Interface

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Basic ISDN Interface (1)

• Digital data exchanged between subscriber and NTE
  - Full Duplex
• Separate physical line for each direction
• Pseudoternary coding scheme
• 1no voltage, 0positive or negative 750mV /-10
• Data rate 192kbps
• Basic access is two 64kbps B channels and one
  16kbps D channel
• This gives 144kbps multiplexed over 192kbps
• Remaining capacity used for framing and sync

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Basic ISDN Interface (2)

• B channel is basic user channel
• Data
• PCM voice
• Separate logical 64kbps connections for different
  destinations
• D channel used for control or data
• LAPD frames
• Each frame 48 bits long
• One frame every 250?s

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Frame Structure
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Primary ISDN

• Point to point
• Typically supporting PBX
• 1.544Mbps
• Based on US DS-1
• Used on T1 services
• 23 B channels plus one D channel
• Line coding is AMI using B8ZS
• 2.048Mbps
• Based on European standards
• 30 B channels plus one D channel
• Line coding is AMI using HDB3

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Bipolar AMI, B8ZS and HDB3
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Primary ISDN Frame Formats
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Conceptual View of ISDN Connection Features
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Asymmetrical Digital Subscriber Line

• ADSL
• Link between subscriber and network
• Local loop
• Uses currently installed twisted pair cable
• Can carry broader spectrum
• 1 MHz or more

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ADSL Design

• Asymmetric
• Greater capacity downstream than upstream
• Frequency division multiplexing
• Lowest 25kHz for voice
• Plain old telephone service (POTS)
• Use echo cancellation or FDM to give two bands
• Use FDM within bands
• Range 5.5km

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ADSL Channel Configuration
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Discrete Multitone

• DMT
• Multiple carrier signals at different frequencies
• Some bits on each channel
• 4kHz subchannels
• Send test signal and use subchannels with better
  signal to noise ratio
• 256 downstream subchannels at 4kHz (60kbps)
• 15.36MHz
• Impairments bring this down to 1.5Mbps to 9Mbps

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DMT Transmitter
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Other Types of xDSL

• High data rate DSL (HDSL)
• Single line DSL (SDSL)
• Very high data rate DSL (VDSL)

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Comparison of xDSL Alternatives
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Use of Packets
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Advantages

• Line efficiency
• Single node to node link can be shared by many
  packets over time
• Packets queued and transmitted as fast as
  possible
• Data rate conversion
• Each station connects to the local node at its
  own speed
• Nodes buffer data if required to equalize rates
• Packets are accepted even when network is busy
• Delivery may slow down
• Priorities can be used

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Switching Technique

• Station breaks long message into packets
• Packets sent one at a time to the network
• Packets handled in two ways
• Datagram
• Virtual circuit

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Datagram

• Each packet treated independently
• Packets can take any practical route
• Packets may arrive out of order
• Packets may go missing
• Up to receiver to re-order packets and recover
  from missing packets

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Virtual Circuit

• Preplanned route established before any packets
  sent
• Call request and call accept packets establish
  connection (handshake)
• Each packet contains a virtual circuit identifier
  instead of destination address
• No routing decisions required for each packet
• Clear request to drop circuit
• Not a dedicated path

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Virtual Circuit vs Datagram

• Virtual circuits
• Network can provide sequencing and error control
• Packets are forwarded more quickly
• No routing decisions to make
• Less reliable
• Loss of a node looses all circuits through that
  node
• Datagram
• No call setup phase
• Better if few packets
• More flexible
• Routing can be used to avoid congested parts of
  the network

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X.25

• 1976
• Interface between host and packet switched
  network
• Almost universal on packet switched networks and
  packet switching in ISDN
• Defines three layers
• Physical
• Link
• Packet

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X.25 - Physical

• Interface between attached station and link to
  node
• Data terminal equipment DTE (user equipment)
• Data circuit terminating equipment DCE (node)
• Reliable transfer across physical link
• Sequence of frames

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X.25 - Link

• Link Access Protocol Balanced (LAPB)
• Subset of HDLC (High-Level Data Link Control)

X.25 - Packet

• External virtual circuits
• Logical connections (virtual circuits) between
  subscribers

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X.25 Use of Virtual Circuits
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Virtual Circuit Service

• Virtual Call
• Dynamically established virtual circuit
• Permanent virtual circuit
• Fixed network assigned virtual circuit

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Frame Relay

• Designed to be more efficient than X.25
• Developed before ATM
• Larger installed base than ATM
• ATM now of more interest on high speed networks

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Frame Relay Background - X.25

• Call control packets, in band signaling
• Multiplexing of virtual circuits at layer 3
• Layer 2 and 3 include flow and error control
• Considerable overhead
• Not appropriate for modern digital systems with
  high reliability

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Frame Relay - Differences

• Call control carried in separate logical
  connection
• Multiplexing and switching at layer 2
• Eliminates one layer of processing
• No hop by hop error or flow control
• End to end flow and error control (if used) are
  done by higher layer
• Single user data frame sent from source to
  destination and ACK (from higher layer) sent back

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Advantages and Disadvantages

• Lost link by link error and flow control
• Increased reliability makes this less of a
  problem
• Streamlined communications process
• Lower delay
• Higher throughput
• ITU-T recommend frame relay above 2Mbps

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Frame Relay - Virtual Circuits

• Permanent virtual circuits (PVCs)
• Original standard, more commonly used
• Switched virtual circuits (SVCs)
• Getting popular now

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Permanent Virtual Circuits

• Set up by a network operator
• Defined as a connection between two sites
• Fixed path, not to be set up on a call-by-call
  basis
• Pre-configured by the provider or network manager
  with given bandwidth allocated packet-by-packet

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Switched Virtual Circuits

• Available by a call-by-call basis
• User specifies a destination address similar to a
  phone number
• Network dynamically establishes connections based
  on requests by many users
• Network allocates bandwidth based on the users
  request

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Asynchronous Transfer Mode (1)

• Cell-based switching and multiplexing technology
• Asynchronous transmitted cells need not be
  periodic in times as in STM
• General purpose for a wide range of services
  voice, packet data, video, imaging
• Applied to both LAN and private network
  technologies

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Asynchronous Transfer Mode (2)

• Support both constant bit rate (CBR) or variable
  bit rate (VBR)
• Each cell contains addresses information that
  establishes a virtual connection from source to
  destination
• Support both permanent virtual connections or
  switched virtual connections (PVCs or SVCs)
• Support multiple Quality of Service (QoS) classes
  for different applications on delay and loss
  performance

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Multimedia Communications using ATM

• Transmit text, voice, video, data traffic

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ATM Cell

• Fixed size cell 53 octets, 5-octet header and
  48-octet payload (1 octet 8 bits)

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Protocol Architecture

• Similarities between ATM and packet switching
• Transfer of data in discrete chunks
• Multiple logical connections over single physical
  interface
• In ATM flow on each logical connection is in
  fixed sized packets called cells
• Minimal error and flow control
• Reduced overhead
• Data rates (physical layer) 25.6Mbps to 622.08Mbps

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Protocol Architecture (Diagram)
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Reference Model Planes

• User plane
• Provides for user information transfer
• Control plane
• Call and connection control
• Management plane
• Plane management
• whole system functions
• Layer management
• Resources and parameters in protocol entities

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ATM Logical Connections

• Virtual channel connections (VCC)
• Analogous to virtual circuit in X.25
• Basic unit of switching
• Between two end users
• Full duplex
• Fixed size cells
• Data, user-network exchange (control) and
  network-network exchange (network management and
  routing)
• Virtual path connection (VPC)
• Bundle of VCC with same end points

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ATM Connection Relationships
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Advantages of Virtual Paths in ATM

• Simplified network architecture
• Increased network performance and reliability
• Reduced processing
• Short connection setup time
• Enhanced network services

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Summary of X.25, Frame Relay and ATM

• X.25
• Widely used, inexpensive, but relatively slow
• Frame Relay
• Reliable, inexpensive, faster than X.25, able to
  handle heavy traffic in LAN
• ATM
• Fast, high bandwidth, high-capacity multimedia
  capabilities for voice, video and data
  communications

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Main References

• Data and Computer Communications, 6/e, by William
  Stallings, Prentice Hall.
• ISDN and Broadband ISDN with Frame Relay and ATM,
  4/e, by William Stallings, Prentice Hall.
• eBusiness Essentials Technology and Network
  Requirements for Mobile and Online Markets, 2/e,
  by Mark Norris and Steve West, John Wiley Sons.