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Telecommunications
Concepts
Chapter 3.2
Packet Switched
Store and Forward Networks
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Contents
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Store and Forward Networks
X25
Frame Relay
ATM
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Contents
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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 layers
For Store & Forward Networks
Internet
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Network
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Data Link Control
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Physical
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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
Packet switched
data network
X25
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
Example :
Belgacom's DCS
PSTN/
ISDN
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PAD
public PAD
X75
PAD
private PAD
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X25 Applications :
Private Packet Switched Data Networks
Example:
old Banksys network
PSTN
/ISDN
Public
PAD
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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
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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
Network
management
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Signaling protocol
(possibly manual)
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Frame Relay
Error 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
Risks:
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Unfairness
Congestion Collapse
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Frame Relay Flow Control
additional 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
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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 ISDN
Design 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 enduser.
– 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
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Virtual Channel
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ATM in the OSI model
with ad-hoc signaling
Any Application Protocol
switch
Network
management
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ATM
Signaling protocol
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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
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PT
CLP
CLP
HEC
HEC
User Node Interface
Network Node Interface
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ATM Adaptation Layer
Service Classes
Type of
Service
Timing
coordination
Bit Rate
Connection
Mode
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Constant Variable
Connection
Connectionless
Bit Rate
Bit Rate
Oriented Data
Oriented Data
Class A
Class B
Class C
Class D
Required
Constant
Not Required
Variable
Connection Oriented
Connectionless
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ATM Adaptation Layer
Data encapsulation
Higher layer PDU
pad AAL
ATM
header
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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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ATM
The 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
X
X
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ATM bridges for Ethernet
X
X
X
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.ht
ml
» Links to many sites on ATM
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