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TRANSCRIPT
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ASYNCHRONOUS TRANSFER MODE
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Protocol Architecture
Similarities between ATM and packet switching Transfer of data in discrete chunks Multiple logical connections over single physical
interface
In ATM flow on each logical connection is in fixed sized packets called cells
Minimal error and flow control Reduced overhead
Data rates (physical layer) 25.6Mbps to 622.08Mbps
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Protocol Architecture (diagram)
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Reference Model Planes
User plane Provides for user information transfer
Control plane Call and connection control
Management plane Plane management
whole system functions Layer management
Resources and parameters in protocol entities
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ATM Logical Connections
Virtual channel connections (VCC)Analogous to virtual circuit in X.25Basic unit of switchingBetween two end usersFull duplexFixed size cells
Frame relay : variable-length packets called frames ATM : fixed-length packets called cells Data, user-network exchange (control) and network-network
exchange (network management and routing)Virtual path connection (VPC)
Bundle of VCC with same end points
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ATM Connection Relationships
VC (Virtual Channels) all cells belonging to a single message follow the
same VC and remain in their original order until they reach dest. (as lanes of a highway).
VP (Virtual Path) a combination of VCs that bundled together because
part of their paths are the same (as highway itself!)
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Advantages of Virtual Paths
Simplified network architectureIncreased network performance and
reliabilityReduced processingShort connection setup timeEnhanced network services
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VP/VC Characteristics
Quality of serviceSwitched and semi-permanent channel
connectionsCall sequence integrityTraffic parameter negotiation and usage
monitoring
VPC only Virtual channel identifier restriction within VPC
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Control Signaling - VCC
Done on separate connectionSemi-permanent VCCMeta-signaling channel
Used as permanent control signal channel
User to network signaling virtual channel For control signaling Used to set up VCCs to carry user data
User to user signaling virtual channel Within pre-established VPC Used by two end users without network intervention to
establish and release user to user VCC
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ATM Cells
Fixed size5 octet(bytes) header48 octet(bytes) information field
Total size of packet = 53 octet (bytes)
Small cells reduce queuing delay for high priority cells
Small cells can be switched more efficientlyEasier to implement switching of small cells
in hardware
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ATM Cell Format
VPI
VCI
HEC
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Header Format
Generic flow control Only at user to network interface Controls flow only at this point
Virtual path identifierVirtual channel identifierPayload type
e.g. user info or network management
Cell loss priorityHeader error control
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Generic Flow Control (GFC)
Control traffic flow at user to network interface (UNI) to alleviate short term overload
Two sets of procedures Uncontrolled transmission Controlled transmission
Every connection either subject to flow control or not
Subject to flow control May be one group (A) default May be two groups (A and B)
Flow control is from subscriber to network Controlled by network side
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Header Error Control
8 bit error control fieldCalculated on remaining 32 bits of headerAllows some error correction
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HEC Operation at Receiver
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Effect of Error in
Cell Header
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Impact of Random Bit Errors on HEC Performance
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Transmission of ATM Cells
622.08Mbps155.52Mbps51.84Mbps25.6MbpsCell Based physical layerSDH based physical layer
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Cell Based Physical Layer
No framing imposedContinuous stream of 53 octet cellsCell delineation based on header error
control field
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Cell Delineation State Diagram
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SDH Based Physical Layer
Imposes structure on ATM streame.g. for 155.52MbpsUse STM-1 (STS-3) frameCan carry ATM and STM payloadsSpecific connections can be circuit switched
using SDH channelSDH multiplexing techniques can combine
several ATM streams
(Synch dig hierc)
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ATM Service Categories
Real time Constant bit rate (CBR) Real time variable bit rate (rt-VBR)
Non-real time Non-real time variable bit rate (nrt-VBR) Available bit rate (ABR) Unspecified bit rate (UBR) Guaranteed frame rate (GFR)
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CBR
Fixed data rate continuously availableTight upper bound on delayUncompressed audio and video
Video conferencing Interactive audio A/V distribution and retrieval
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rt-VBR
Time sensitive application Tightly constrained delay and delay variation
rt-VBR applications transmit at a rate that varies with time
e.g. compressed video Produces varying sized image frames Original (uncompressed) frame rate constant So compressed data rate varies
Can statistically multiplex connections
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nrt-VBR
May be able to characterize expected traffic flow
Improve QoS in loss and delayEnd system specifies:
Peak cell rate Sustainable or average rate Measure of how bursty traffic is
e.g. Airline reservations, banking transactions
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UBR
May be additional capacity over and above that used by CBR and VBR traffic Not all resources dedicated Bursty nature of VBR
For application that can tolerate some cell loss or variable delays e.g. TCP based traffic
Cells forwarded on FIFO basisBest efforts service
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ABR
Application specifies peak cell rate (PCR) and minimum cell rate (MCR)
Resources allocated to give at least MCRSpare capacity shared among all ARB sourcese.g. LAN interconnection
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Guaranteed Frame Rate (GFR)
Designed to support IP backbone subnetworksBetter service than UBR for frame based traffic
Including IP and EthernetOptimize handling of frame based traffic passing
from LAN through router to ATM backbone Used by enterprise, carrier and ISP networks Consolidation and extension of IP over WAN
ABR difficult to implement between routers over ATM network
GFR better alternative for traffic originating on Ethernet Network aware of frame/packet boundaries When congested, all cells from frame discarded Guaranteed minimum capacity Additional frames carried of not congested
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ATM Bit Rate Services
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ATM standard defines 3 layers
Layers and functions:
1. AAL (Application adaptation layer) Accept transmissions from upper-layer
services and maps them into ATM cells.
2. ATM Layer Providing routing, traffic management,
switching, and multiplexing services.
3. Physical layer
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Segmentation and Reassembly PDU
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AAL Type 1
Supports app. that transfer info at constant bit rates ,eg. video and voice
If data is long divided into 47 bytes long data + 1 byte(8 bits) header
Passed to ATM layer = 48-byte data unit
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Example of calculation
An AAL1 layer receives data at 2Mbps. How many cells are created per second by the ATM layer?
Solution: 2M / 8 bits = 250,000 bytes per second 250,000 byte / 47 bytes = 5319.14 = 5320 ATM cells
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AAL Type 2
Support variable bit-rate app.
If data is long divided into 45 bytes long data + 1 byte(8 bits) header
+ 2 bytes trailer
Passed to ATM layer = 48-byte data unit
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AAL Type 3/4
Support connection-oriented and connectionless data services
Limitation : receive data packet <= 65,535 bytes Data divided into 44-byte long data
+ 4-byte header (at beginning of message) + padding (if necessary )= 0-43 bytes+ 4-byte trailer (at end of message)
Passed to ATM layer 1. First, 4-byte(16 bits) header 2. In the middle = 44-byte data unit (until finished)3. Padding (if necessary )= 0-43 bytes4. Finally, 4-byte trailer
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AAL Type 5
Support app. where transmissions are not route through multiple nodes @ multiplexed with other transmission Sequencing & error correction mechanisms are
unnecessary overhead (in front)
Limitation : receive data packet <= 65,535 bytes
Data divided into 48-byte long data + padding (if necessary )= 0-47 bytes+ 8-byte trailer (at end of message)
Passed to ATM layer 1. First, 44-byte data unit (until finished)2. Padding (if necessary )= 0-47 bytes3. Finally, 8-byte trailer