Data Transport Networks Wide Area Networks

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Data Transport Networks- Wide Area Networks

• Rong Wang
• CGS3285
• Spring2004

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RECOMMENDED READING

• From textbooks
• Chapter 18 and 19 of Data Communications and
  Networking, 3rd Edition, Behrouz A. Forouzan
  (ISBN 0-07-251584-8)
• Chapter 8 of Data Communications From Basics to
  Broadband, 3rd Edition by William J. Beyda (ISBN
  0-13-096139-6)

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Wide Area Networks

• Switched communication networks
• Circuit switching and telephone network
• Packet switching principles
• X.25 and Frame Relay
• ATM

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SWITCHED COMMUNICATION NETWORKS

• Long distance transmission is typically done over
  a network of switched nodes
• Nodes not concerned with content of data
• End devices are stations
• Computer, terminal, phone, etc.
• A collection of nodes and connections is a
  communications network
• Data routed by being switched from node to node

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NODES

• Nodes may connect to other nodes only, or to
  stations and other nodes
• Node to node links usually multiplexed
• Network is usually partially connected
• Some redundant connections are desirable for
  reliability
• Two different switching technologies
• Circuit switching
• Packet switching

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SWITCHING
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SIMPLE SWITCHED NETWORK
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CIRCUIT SWITCHING NETWORKS

• Dedicated communication path between two stations
• Three phases
• Circuit Establishment
• Data Transfer
• Circuit Disconnect
• Must have switching capacity and channel capacity
  to establish connection
• Switches must have intelligence to work out
  routing

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CIRCUIT SWITCHING - APPLICATIONS

• Inefficient
• Channel capacity dedicated for duration of
  connection
• If no data, capacity wasted
• Set up (connection) takes time
• Once connected, transfer is transparent
• Developed for voice traffic (phone)

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PUBLIC CIRCUIT SWITCHED NETWORK
Circuit switch
Also Central office
A telephone system
Also Toll office
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TELECOMMS COMPONENTS

• Subscriber
• Devices attached to network (e.g, telephone)
• Subscriber line
• Local Loop or subscriber loop
• Connection to network
• Few km up to few tens of km
• Exchange
• Switching centers, (e.g., tandem office)
• End office - supports subscribers (e.g. central
  office)
• Trunks
• Branches between exchanges (e.g. interoffice
  trunk, intertoll trunk)
• Multiplexed (TDM or FDM)

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PACKET SWITCHING PRINCIPLES

• Circuit switching designed for voice
• Resources dedicated to a particular call
• Disadvantages of circuit switching
• Much of the time a data connection is idle
• Data rate is fixed
• Both ends must operate at the same rate

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BASIC OPERATION

• Messages/Files broken down into data packets
• aka, datagrams, frames, packets
• Typically 1000 octets
• Each packet encapsulated with header and error
  control information (e.g., checksum, CRC)
• Control Info
• Header contains source destination addresses
• Packets passed to network which routes to
  destination address
• Routing taken care of by packet network
• Packets are received, stored briefly (buffered)
  and past on to the next node Store and forward
• Network includes series of nodes
• Packets can follow different routes to destination

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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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DATAGRAM DIAGRAM

• Often used in network layer
• Each packet (referred to datagram) is treated
  independently of all others
• Does not need to call setup
• This approach can cause datagrams of a
  transmission to arrive at their destination out
  of order
• Switching at the network layer in the Internet is
  done using the datagram approach to packet
  switching

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VIRTUAL CIRCUIT

• Often used in data link layer
• Preplanned route established before any packets
  sent
• Call request and call accept packets establish
  connection (handshake)
• Each packet contains a virtual circuit identifier
  (VCI) 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 IDENTIFIER

• A small number that only has switch scope
• Used by a frame between two switches
• When a frame arrives at a switch, it has one VCI
• When a frame leaves, it has another VCI

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VCI PHASES

• To communicate, a source and destination need to
  go through three phases
• Setup
• Data transfer
• Connection tear down

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SWITCH AND TABLE
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SOURCE-TO-DESTINATION TRANSFER
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SETUP PHASE SETUP REQUEST

• A setup request is sent from source to
  destination, following entries can be determined
• Imcoming port
• Outgoing port
• Incoming VCI

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SETUP PHASE SETUP ACKNOWLEDGEMENT

• A special frame, called the acknowledgment frame,
  can complete the entries in the switching tables

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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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ADVANTAGES OF PACKET SWITCHING

• Easy access
• Dedicated access or dial up ports
• Protocol conversion performed by PADs
• Packet Assemblers/Disassemblers
• Flexible Virtual Circuits
• Variable destinations (changing VCs like circuit
  switching)
• Single fixed destination (simulates dedicated
  connection)
• Reliable
• Flexible routing
• Error checking
• Efficient
• Single node to node link can be shared by many
  packets over time
• Packets queued and transmitted as fast as
  possible
• High speed at lower price (per packet charges may
  apply)
• Shared resources and physical transmission media

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ADVANTAGES OF PACKET SWITCHING (contd)

• 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
• Ease of maintenance administered by PPSN
  providers

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CIRCUIT VS PACKET SWITCHING

• Performance
• Propagation delay
• Time it takes a signal to propagate from one node
  to the next
• Transmission time
• The time it takes for a transmitter to send out a
  block of data
• Node delay
• The time it takes for a node to perform the
  necessary processing as it switches data

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Comparison of Communication Switching Techniques
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X.25 PACKET SWITCHING

• 1976
• X.25 one of the earliest PPSNs (Public Packet
  Switching Networks)
• Interface between host and packet switched
  network
• Generally, X.25 refers to the interface from
  synchronous DTE into the network (DCE)
• Packet level (OSI Network Layer 3)
• User data and control info are combined into
  packets here. Addressing info included in packet
  header
• Frame level (OSI Data Link Layer 2)
• Uses LAP-B protocol (Link Access
  Procedure-Balanced). Similar to SDLC. Packet
  error detection/correction
• Physical level (OSI Physical Layer 1)
• RS232C (also known as V.24) synchronous, full
  duplex
• X.28 defines interface from async DTE to network
  PAD

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FRAME RELAY NETWORKS

• 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 NETWORKS

• Relies on digital links between users and nodes
• Therefore assumes near error-free transmission
• Error checking not performed by nodes
• Must be performed end-to-end by source and
  destination DTE in higher protocol layers
• Operates only at Layers 1 and 2 of OSI model
• Link Access Protocols (LAPD, LAPF)
• Nodes merely relay frames (packets)
• No need to open packets and inspect content for
  errors
• Route may vary based on network status
• Result is faster transmission of packets due to
  lower processing time at each node

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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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ATM NETWORKS

• Asynchronous Transfer Mode
• ATM Forum, founded 1991
• Carries all types of traffic
• Data Fax
• Real Time Voice Video
• Fixed sized data packages, called "cells
• 53 octets (bytes) - (5 byte Header / 48 byte
  Payload)
• Very high speed connections
• Up to T3 (45 Mbps)
• Fiber is preferred transmission media

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ATM NETWORKS (cont.)

• Asynchronous related to arrival rate of next
  cell, not physical transmission of bits
• To make data stream more predictable, use idle
  bits or idle cells
• Data inserted into continuous stream as needed.
• Supports two header types
• User-Network Interface-UNI
• Network-Network Interface - NNI
• Headers contain one of two identifier types
• Virtual Channel Identifier - VCI
• Virtual Path Identifier - VPI

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TP, VPs and VCs

• Connection between two endpoints is accomplished
  through transmission paths (TPs), virtual paths
  (VPs), and virtual circuits (VCs).
• TP
• Physical connection (write, cable, statellite,
  and so on)
• VP
• Provide a connection or a set of connection
  between two switches
• VC
• A single message path between source and
  destination

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EXAMPLE OF VPS AND VCS

• Note that a virtual connection is defined by a
  pair of numbers the VPI and the VCI.

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CONNECTION IDENTIFIERS
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AN ATM CELL
An ATM cell
Header format
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ATM ROUTING

• Cells are self routing
• Virtual channel/path determined during call setup
• Same channel/path for all cells
• Routing tables in each node in path updated with
  next node address
• When cell reaches a node
• Node retrieves channel/path identifier from cell
  header
• Looks up identifier routing table to get next
  node in path
• Sends cell out port associated with next node
• May modify header along the way if necessary
• Routing method and high speed physical links
  allow use with real time, isochronous data
• Cells arrive at destination in order of sending
• Cells arrive at destination at rate comparable to
  sending

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ROUTING WITH A SWITCH
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LAYERS OF ATM

• ATM Adaptation Layer
• breaks information into 48 octet blocks
• ATM Layer
• adds 5 octet header with routing information
• Physical layer
• groups multiple cells into a payload envelope,
  adds framing and maintenance information and
  sends data