Telecommunications Concepts

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TelecommunicationsConcepts

• Chapter 3.2
• Packet Switched
• Store and Forward Networks

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Contents

• Store and Forward Networks
• X25
• Frame Relay
• ATM

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Contents

• Store and Forward Networks
• X25
• Frame Relay
• ATM

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Point to Point Networks
Normal Operation Mode Store Forward Commonly
used in Wide-Area Networks The Network itself has
considerable storage capacity
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The 3 lower OSI layersFor Store Forward
Networks
Internet
3
Network
2
Data Link Control
1
Physical
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Contents

• Store and Forward Networks
• X25
• Frame Relay
• ATM

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X25 and related standards

• Standardized, multivendor interface for packet
  switched networks.
• Initially published by CCITT in 1974
• Major revisions in 1976, 1978, 1980, 1984, 1988.
• Provides common procedures between a DTE and a
  packet switched DCE for
• Establishing a connection to the network
• Exchanging data with another DTE
• releasing the connection
• Can be used for direct DTE-DTE connection

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X25 and related standards
Virtual Circuit
DTE
DTE
X25
X25
Packet switched data network
The standard does not specify the internal
operation of the packet switched data network.
It is however possible, and common practice, to
use also the X25 protocols between nodes inside
the data network
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X25 Applications Public Packet Switched Data
Networks
Main purposes Connect terminals to
mainframe Interconnect mainframes
X75
Example Belgacom's DCS
PSTN/ ISDN
PAD
PAD
public PAD
private PAD
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X25 Applications Private Packet Switched Data
Networks
Example old Banksys network
PSTN /ISDN
Private PAD
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X25 in the world

• Public networks
• Very successful in France (Transpac)
• Quite successful throughout Europe
• Marginal in the rest of the world
• Private networks
• dominant technology
• Seventies proprietary networks
• Eighties in Europe X25
• Nineties IP

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X25 Reference

• Uyless Black
• X25 and related protocols
• IEEE Computer Society press, 1991.
• ISBN 0-8186-8976-5
• ISBN 0-8186-5976-9 (microfiche)

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Contents

• Store and Forward Networks
• X25
• Frame Relay
• ATM

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Frame Relay
Origin
Developed to allow fast packet switching in ISDN
channels.
Simple protocols allow very high data rates.
Frame Relay has replaced X25 in high-speed
packet switching independently from ISDN
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Frame Relay

• Facts
• Transmission errors have been significantly
  reduced
• Most high-speed links are seldom switched
• Design consequences
• Simplify error handling
• Separate connection management and data
  transmission
• Main Frame Relay characteristics
• Layer 2
• Error detection but no correction
• Permanent virtual circuits through layer 2
  entities
• Layer 3 empty on data transmission protocol
  stack

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Switching in Frame Relay
Any transport or internet protocol
switching node
Data protocol
Signaling protocol (possibly manual)
Network management
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Frame RelayError Correction

• All frames have Frame Check Sequence. Bad frames
  are not relayed
• Upper layers have to handle the missing frames
• A transport layer designed for a connectionless
  network service can handle such missing frames.

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Frame Relay Flow Control
Basic mechanism Frame Discarding
Transport protocol
Frame Relay layer
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Frame Relay Flow Controladditional mechanisms

• At set-up, committed rate is negotiated (CIR)
• Frames in excess of committed rate have Discard
  Eligible bit set.
• Frames with DE bit set discarded first.
• Users can voluntarily set the DE bit.
• Congestion notification bits in each frame to
  warn upper layers.

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References

• Philip Smith
• Frame Relay
• Principles and Applications
• Addison-Wesley, 1993
• ISBN 0-201-62400-1

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Contents

• Store and Forward Networks
• X25
• Frame Relay
• ATM

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Broadband ISDN Dominant ideas in the early 80's

• Convergence of telecommunication networks
• Telephone
• Cable TV
• Data
• Dominant application Video On Demand
• High Definition TV 155 Mb/s
• Four different programs per home

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Broadband ISDN
A single, universal, communications network
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Broadband ISDNDesign Specifications

• Dominated by the HDTV requirements
• 600 Mb/s throughput to every home
• Fiber to the home
• Simple protocols implemented in hardware
• Guaranteed Quality Of Service
• Connection oriented protocol
• General purpose network
• HDTV, LDTV, Voice, Data
• Different service classes
• Very low data-rate applications (meter reading)
• Multiplexing of very different data-rates
• Low multiplexing overhead

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Asynchronous Transfer Mode

• Designed for broadband ISDN
• Dissociates completely data transfer and
  signaling
• Provides virtual circuits at the physical layer,
  supporting directly the application layers
• Uses very small, fixed size packets, called
  "cells
• Unique protocol for a universal BISDN network.
• Used in the interface between network and
  end-user.
• Used inside the network between nodes

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ATM technology

• Data encapsulated in small (53 bytes) cells
• Long low priority packet can not block service of
  high priority packet
• Very different throughputs can be multiplexed
• Header decoding and cell handling simple
• Hardwired switches
• Very high throughputs possible
• Communication by Virtual Circuits
• Established through separate signaling network
• Addressing etc handled by signaling network
• QOS negotiation through signaling network

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Virtual Path Virtual Channel
Virtual Path
Transmission Path
Virtual Channel
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ATM in the OSI modelwith ad-hoc signaling
Any Application Protocol
switch
ATM
Signaling protocol
Network management
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ATM Interfaces
UNI
NNI
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ATM cell format
Header
Data(48 bytes)
VPI
GFC
VPI
VCI
VCI
PT
PT
CLP
CLP
HEC
HEC
User Node Interface
Network Node Interface
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ATM Adaptation Layer Service Classes
Constant
Variable
Connection
Connectionless
Type of Service
Bit Rate
Bit Rate
Oriented Data
Oriented Data
Class A
Class B
Class C
Class D
Timing coordination
Required
Not Required
Bit Rate
Constant
Variable
Connection Mode
Connection Oriented
Connectionless
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ATM Adaptation Layer Data encapsulation
Higher layer PDU
pad
AAL
ATM header
pad
AAL
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Broadband ISDN The situation in the 90's

• HDTV
• Broadcasters not willing to invest in HDTV
• Public prefers diversity over technical quality
• Video on demand can't compete with video rental
• Digital Signal Processing
• Video compressed into 1.5 Mb/s
• XDSL allows up to 6 Mb/s over copper local loop
• Internet
• Explosive success of cheap, low quality but very
  diversified universal communications network

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ATMThe situation in the 90's

• Huge research investments by vendors
• Data transfer protocols mature
• Signaling immature target market ???
• No push from Telco's
• Privatization restricts long term developments
• New operators compete with low cost technology
  (Voice over IP, )
• Corporate backbones / high throughput LAN's
• Only possibility to recover quickly some of the
  huge investments made in ATM research

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ATM bridges for Ethernet
ATM switch
X
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ATM bridges for Ethernet
WAN with ATM over SDH
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References

• M.De Prycker
• Asynchronous Transfer Mode
• Solutions for broadband ISDN.
• Ellis Horwood 1993.
• Web sites
• http//www.atmforum.com
• Official web site of ATM forum
• http//www.atm25.com/ATM_Reference.html
• Links to many sites on ATM