ConnectionOriented Networking and ATM

1
Connection-Oriented Networking and ATM

• Department of Computer Science
• Prof. Paulo Martins

2
Introduction

• Previous chapters described
• Local Area Network Technologies
• Wide Area Network Technologies
• This chapter discusses
• ATM networks
• Connection-oriented packet switching

3
A Single, Global Network

• Three types of worldwide communication networks
  (Mid 1980s)
• Telephone network
• Television networks
• Computer networks
• Telecoms industry decision Need to carry
  traffic other than voice. Expand business (voice
  communication was becoming a commodity)

4
Goals

• Universal service
• Support for all users
• Single, Unified Infrastructure
• Service Guarantees
• Support for Low-cost Devices

5
ISDN

• ISDN was the first attempt to satisfy network
  goals
• When ISDN emerged, it was relatively expensive
• ISDN is still available
• ISDN has not gained universal acceptance

6
ATM

• After ISDN, the telecom industry developed ATM, a
  second, more comprehensive technology.
• Asynchronous Transfer Mode (ATM) handles much
  higher speeds and offers many more services.
• The challenge of ATM voice, data and video have
  different sets of requirements.

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Requirements

• Voice and video low delay and low jitter (low
  variance in delay)
• Video substantially higher data rate than audio

8
ATM design

• Large packets introduce large delays this is
  because it takes time to accumulating enough
  samples to fill the packet. (PCM produces 1
  byte/125 µs)
• 1 byte 125 D 4000 x 125
• 4000 - D (?) 0.5 secs
• Half a second is intolerable for users
• Echo cancellation works only when delays are low.

9
ATM Cells

• ATM technology divides all data into small, fixed
  size packets called cells.
• This allow packet switches to operate at high
  speeds and to achieve
• low delay,
• low jitter, and
• echo cancellation

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ATM cells

• Large cells optimal for data (minimize
  overhead)
• Small cells optimal for voice (minimize delay)
• ATM cells are a compromise
• The header is a 10 of the payload area
• With a payload size of 48 octets, the header was
  fixed at 5 octets.
• For data networks, 10 overhead is too high
  (compare to Ethernet, with 1 overhead, i.e. 14
  header/1500 packet size).

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ATM Cells
Figure 14.1 Fields in the 5-octect header found
in an ATM cell. Each line in the diagram
represents one octet
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Cell structure

• VPI and VCI identify the cells destination
• PRIO the priority of the Cell identifies if a
  packet can be discarded when the network becomes
  congested

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Connectionless Service

• Design decisions
• Connection-oriented service
• Connectionless service
• Both
• The TCP/IP protocol
• Include both connectionless and
  connection-oriented services

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Connection-Oriented Service

• Before two computers can communicate, they must
  establish a connection through the network.
• One of the computers requests a connection and
  the second computer must agree to accept the
  connection.

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Connection-Oriented Service

• Requesting the connection is analogous to dialing
  a telephone number
• Accepting the connection is analogous to
  answering the telephone.
• After both computers agree to communicate, the
  underlying network hardware establishes a data
  path called a connection.

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Connection-Oriented Service

• Once the connection has been established, the two
  computers can exchange data.
• There are three phases involved
• Establishing the connection
• Data Transmission
• Closing the connection

17
VPI/VCI

• An ATM connection is known as a Virtual Channel
  (VC)
• VC is divided into two parts
• VPI (8 bits) Virtual Path identifier
  specifies the path through the network
• VCI (16 bits)Virtual Channel identifier
  specifies a single VC within the path
• Because a computer refers to both parts together,
  the identifier is known as VPI/VCI

18
Labels and Label Switching

• An ATM network is made of network switches.
• The simplest ATM network consists of a single ATM
  switch with multiple computers attached.
• An ATM switch change the VPI/VCI in each cell it
  handles

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Label Switching

• Inside each switch is a forwarding table that
  specifies how the hardware will forward cells
• Each entry in the table corresponds to a possible
  VPI/VCI for a given port
• The switch uses the VPI/VCI in an incoming cell
  to locate an entry in its forwarding table
• The switch rewrites the VPI/VCI in the cell
  header with the replacement, and forwards the
  cell.

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Label Switching

• Changing the VPI/VCI is known as label rewriting
  or label switching
• ATM is characterized as a label switching system.

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Label Switching

• Two computers using a given VC usually have
  different VPI/VCI values
• An ATM switch uses label switching to rewrite the
  connection identifier (VPI/VCI) in each cell it
  forwards.

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Labels and Label Switching
Figure 14.2 Illustration of an ATM VC between
two computers
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An Example Trip Through an ATM Network
Figure 14.3 An illustration of three ATM
switches and the forwarding table in each switch
that allows a single VC to span the network. Only
those table entries that correspond to the VC are
shown.
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Items Not Yet Covered in this Class

• An example trip through an ATM Network
• Permanent Virtual Circuits
• Switched Virtual Circuits
• Quality of Service

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ATM Data Transmission And AAL5

• ATM defines a set of adaptation protocols, which
  can be thought of APIs
• ATM Adaptation Layer 5 (AAL5) is used to send
  data
• On the sending side, AAL5
• accepts a block of data (64K octets),
• divides the block into cells,
• transfers the cells across the ATM network.

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ATM Data Transmission And AAL5

• On the receiving side, AAL5
• accepts incoming cells,
• Extracts the data,
• Delivers the original block
• This process is known as segmentation and
  reassembly.

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Critique of ATM

• Expense
• Connection Setup Latency
• Cell Tax
• Specification of service requirements
• Lack of Efficient Broadcast
• Complexity of QoX
• Assumption of Homogeneity

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MultiProtocol Label Switching

• ATM did not become popular
• Proponents continue to devise and promote
  variations of connection-oriented networking
• MPLS is designed to serve in the Internet core
  (instead of attempting to replace the Internet)
• The basic idea is to provide virtual circuits
  among IP routers.

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Summary

• Phone companies created ISDN and ATM
• ATM intended to be universal
• Voice
• Video
• Data
• ATM uses a connection-oriented paradigm
• An application first creates a Virtual channel
  (VC), uses the channel and then terminates it.

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Summary

• The connection is implemented by one or more
  switches.
• ATM has not been widely accepted.
• Some phone companies still use it in their
  backbone networks
• Factors that prevented ATM from becoming more
  prevalent
• Expense
• Complexity
• Lack of interoperability