Transcript 694-4-13-2000

Asynchronous Transfer Mode (ATM)
•
ATM is a specific asynchronous packet-oriented information, multiplexing and
switching transfer model standard, originally devised for digital voice and video
transmission, which is
– Based on 53-byte fixed-length cells.
– Each cell consists of a 48 byte information field and a 5 byte header, which is
mainly used to determine the virtual channel and to perform the appropriate
routing.
– Cell sequence integrity is preserved per virtual channel. Thus all cells belonging
to a virtual channel must be delivered in their original order.
– Original primary rate: 155.52 Mbps. Additional rate: 622.08 Mbps
•
ATM is connection-oriented.
– Header values including virtual path/circuit numbers are assigned to each
section of a connection for the complete duration of the connection.
•
•
The information field of ATM cells is carried transparently through the
network. No processing like error control is performed on it inside the network.
All services (voice, video, data, ) can be transported via ATM, including
connectionless services.
– To accommodate various services an appropriate adaptation layer is provided to
fit information of all services into ATM cells and to provide service specific
functions (e.g. clock recovery, cell loss recovery, ...).
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Synchronous Vs. Asynchronous
Data Transmission
Synchronous Transmission in a T1 Line
Asynchronous Transmission in an ATM Line
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Virtual Circuits
• When a virtual circuit is established:
– The route is chosen from beginning to end (circuit setup needed).
– Routers or switches along the circuit create table entries used to
route data transmitted on the virtual circuit.
– Permanent virtual circuits - Switched virtual circuits
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ATM Cells & Switches
ATM Cell Format
Fixed cell size = 53 bytes
Cell Duration: ~ 2.7 msecfor 155.52 Mbps ATMs
An ATM switch
Input
side
~ 700 nsec for 622.08 Mbps ATMs
Output
side
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ATM Layer Headers
4 bits
8 bits
16 bits
3 bits
1
8 bits
ATM layer header at User-Network Interface UNI
12 bits
16 bits
3 bits
1
8 bits
HEC:
ATM layer header at Network-Network Interface NNI
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Values of PTI ATM Header Field
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Layers of
The ATM
Model
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Functions of ATM Layers/Sublayers
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The Network Layer In ATM Networks
• The ATM layer handles the functions of the network
layer:
– Moving cells from source to destination in order.
– Routing algorithms within ATM switches, global
addressing.
• Connection-oriented without acknowledgments.
• The basic element is the unidirectional virtual circuit or
channel with fixed-size cells.
• Two possible interfaces:
– UNI (User-Network Interface): Boundary between an
ATM network and host.
– NNI (Network-Network Interface): Between two ATM
switches (or routers).
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ATM Network Connection Setup/Release
Connection Setup
Connection Release
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ATM Layer Messages Used To
Establish/Release Connections
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ATM Virtual Path Re-routing Example
Rerouting a virtual path re-routes all of its virtual circuits
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ATM Routing Example
Possible routes through the Omaha ATM switch
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ATM Routing Example: Table Entries
Table entries corresponding to routes through the Omaha ATM switch
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•
ATM Switch Functions
The main function of an ATM switch is to relay user data cells from input ports to
the appropriate output ports. The switch processes only user data cell headers and
the payload is carried transparently.
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Establishment and control of the VP/VC connections.
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–
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Unlike user data cells, information in signaling or control cells payload is not transparent to
the network.
The switch identifies signaling cells, and even generates some itself.
Connection Admission Control (CAC) carries out the major signaling functions required.
Signaling/control information may not pass through the cell switch fabric, and instead is
exchanged through a separate signaling network.
Network management functions, concerned with monitoring the controlling the
network to ensure its correct and efficient operation.
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–
–
•
As soon as the cell comes in through the input port, Virtual Path/Channel Identifiers
(VPI/VCI) information is extracted from the cell and used to route the cells to the
appropriate output port.
This function can be divided into three functional blocks: the input module at the input
port, the cell switch fabric (or switch matrix) that performs the actual routing, and the
output modules at the output ports.
Fault management functions,
Performance management functions,
Configuration management functions.
Connection admission control, usage/network parameter control and congestion
control, usually handled by input modules.
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A Generic ATM Switching Architecture
CAC
SM
IM
ATM/
SONET
Lines
IM
:
.
OM
Cell
Switch
Fabric
IM
=
=
=
=
:
.
ATM/
SONET
Lines
OM
Input Side
IM
OM
CAC
SM
OM
Output Side
Input Module
Switch Interface
Output Module
Connection Admission Control
Switch Management
}
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ATM Switch Interface
Input Modules
– The input module first terminates the incoming signal (in
case of a SONET signal) and extracts the ATM cell stream:
• Signal conversion and recovery.
• Processing SONET overhead, and cell delineation and rate
decoupling.
– For each ATM cell the following functions should be
performed:
• Error checking the header using the Header Error Control (HEC)
field.
• Validation and translation of VPI/VCI values.
• Determination of the destination output port.
• Passing signaling cells to CAC and OAM cells to Switch
Management
• Addition of an internal tag containing internal routing and
performance monitoring information for use only within the
switch.
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ATM Over SONET Example
•
The 53 bytes ATM cells are mapped into STS-3c or OC-3 frame payload as
shown:
90 Columns (bytes)
9
rows
STS-3c
or OC-3
Frame
ATM cells
Scrambled ATM Cell Payload
ATM cells
Cell continued
in next frame
3 bytes line overhead
•
•
+
1 byte path overhead per row
An STS-3c frame has a payload capacity of 3 * (90 - 3 - 1) columns * 9 rows or 2340 bytes.
Because of the STS-3c payload capacity is not an integer multiple of the ATM cell, a cell is
allowed to cross the frame boundary.
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ATM Switch Interface
Output Modules
Prepare ATM cell streams for physical transmission by:
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–
–
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Removing and processing the internal tag.
Possible translation of VPI/VCI values.
HEC field generation.
Possible mixing of cells from CAC and Switch
Management with outgoing cell streams.
– Cell rate decoupling.
– Mapping cells to SONET payloads and generation of
SONET overhead.
– Conversion of the digital bit stream to an optical signal.
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Connection Admission Control (CAC)
Establishes, modifies and terminates virtual path/channel
connections. Responsible for:
– Signaling ATM Adaptation Layer (AAL) functions to
interpret or generate signaling cells.
– Interface with a signaling network.
– Negotiation of traffic contracts with users requesting new
VPCs/VCCs.
– Renegotiation with users to change established
VPCs/VCCs.
– Allocation of switch resources for VPCs/VCCs, including
route selection.
– Admission/rejection decisions for requested VPCs/VCCs
generation of usage/network parameter control
(UPC/NPC) parameters.
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The Cell Switch Fabric
• The cell switch fabric is primarily responsible for
transferring cells between the other functional blocks
(includes data cells and possibly signaling and management
cells as well). Other possible functions include:
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Cell buffering and queuing.
Traffic concentration and multiplexing
Redundancy for fault tolerance
Multicasting or broadcasting
Cell scheduling based on delay priorities
Congestion monitoring.
• Fabric Connection Types:
– Fully Interconnected fabrics.
– Fabrics using Multistage interconnection networks
(MINs).
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Fully Interconnected ATM Switch Fabric
– Independent paths exist between all N2 possible pairs of
inputs and outputs.
– Broadcast all incoming cells on separate buses to all outputs.
– Address filters pass the appropriate cells to the output
queues.
Input
Side
Output Side
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Fully Interconnected ATM Switch Fabric:
The Knockout Switch
Output Side
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An ATM Switch Fabric Using MINs:
Batcher-Banyan
Input Side
Output Side
Switching fabric of a Batcher-Banyan ATM switch
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Batcher-Banyan ATM Switch
Cell Routing Example
Four cells being routed through an 8-input Batcher-Banyan switch
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ATM Switch Queuing Modes
An ATM switch with input side queuing
An ATM switch with output side queuing
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Running TCP/IP Over An ATM Subnet
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ATM LANs
LAN
Emulation
Server
ATM LAN Emulation
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