Broadband ISDN and ATM
발표자 : 박종민
Jmpark@cs.chonbuk.ac.kr
Contents
1. Introduction to B-ISDN and ATM
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The B-ISDN ATM Reference Model
2. Physical Layer in ATM
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Virtual Circuits versus Circuit Switching
Transmission in ATM Networks
ATM Switches ( Knockout, Batcher-Banyan )
3. Data Link Layer in ATM
4. Network Layer in ATM
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Cell Formats
Connection Setup
Routing and Switching
Service Categories
Quality of Service
Congestion Control
ATM LANs
5. ATM AAL Layer Protocols
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Structure of the ATM Adaptation Layer
AAL 1, AAL 2, AAL 3 / 4, AAL 5
Comparison of AAL Protocols
Service Specific Connection-Oriented Protocol(SSCOP)
1. Introduction to B-ISDN & ATM
• B-ISDN(Broadband Integrated Services Digital Network)
– A single physical network integrates variety of services
– Problems
• QoS Requirements for services widely different
– Voice : Real time(low delay jitter), tolerates occasional losses
– Data : Usually no real-time requirements, error-free, guaranteed delivery
– Video : High bandwidth, low delay and jitter
• Traffic characteristics also widely different
• Certain applications require synchronization among multiple traffic
streams
• ATM(Asynchronous Transfer Mode)
– Underlying technology that makes B-ISDN possible
– Transmit all information in small, fixed size packet (ATM cell)
– Cell switching technology
• Flexible and handle both constant(audio, video) and variable(data) rate traffic
• Easier than traditional multiplexing techniques using fiber optics
• Broadcasting(television distribution)
– Connection-oriented : Cell order is guaranteed
– Intended speed : 155 Mbps, 622 Mbps, gigabit speeds later
• 155 Mbps(exact choice) : made for compatibility with AT&T’s SONET
B-ISDN ATM Reference Model
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Physical Layer(deals with physical medium)
– Voltage, bit timing, various other issues.
– Designed to be independent of the transmission medium
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ATM Layer(deals with cells and cell transport)
– Defines the layout of a cell
– Establishment and release of virtual circuits
– Congestion control
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AAL Layer(ATM Adaptation Layer)
– Allows users to send packets larger than a cell
– Segments packets, transmits the cells individually, and reassembles then at the
other end
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User plane
– Deals with data transport, flow control, error correction, and other user functions
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Control plane is concerned with connection management
2. Physical Layer in ATM
• B-ISDN service : Compromise between circuit and packet switching
• Virtual Circuits
– Connection oriented
– Implemented internally with packet switching
– Connection Types
• Permanent virtual circuits (No setup time)
• Switched virtual circuits (Like telephone call)
– Circuit establishment
• The route is chosen from source to destination
• All switch along the way make table entries so they can route any packet on
that virtual circuit
Transmission in ATM Network
• Synchronous transmission mode
• Asynchronous transmission mode
– Not required that the stream of cells coming out of a computer be
continuous(Gaps between the data cells are filled by idle cells)
– Does not standardize the format for transmitting cells
– All ATM links are point-to-point(unidirectional)
– Multicasting : Enter a switch on one line and exit it on multiple lines
ATM Switches
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General Model for an ATM cell switch
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ATM switches are generally synchronous
– Cell arrive on the input lines asynchronously
– Master clock marks the beginning of a cycle
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Common goal
– Switch all cells with as low a discard rate as possible
– Never reorder the cells on a virtual circuit
• Input queueing : head-of-line blocking
• Output queueing
The Knockout Switch
• Output queueing
Batcher-Banyan Switch
3. Data Link Layer in ATM
• TC(Transmission Convergence) sublayer
• Cell Transmission
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takes a sequence of cells
add HEC to each one ( HEC : Header Error Control )
convert the result to bit stream
match the bit stream to the speed of the underlying physical
transmission system by inserting OAM cells as filler
– OAM : Operation And Maintenance
• Cell Reception
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takes an incoming bit stream
locates cell boundaries
verifies the header
processes the OAM cells
passes the data cell up to the ATM layer
• Locating the cell boundaries
– HUNT : Shifting bits into the shift registers one at a time looking for a
valid HEC
– PRESYNCH : shifts in the next 424 bits(5-bytes) without examining them
4. Network Layer in ATM Network
• Connection-oriented layer
– Basic element : virtual circuit(virtual channel)
– Unusual for a connection-oriented protocol
• Does not provide any acknowledgements
• Used for real-time traffic(audio, video)
– Guarantee : cells sent will never arrive out of order
• Supports a two-level connection hierarchy
Cell Formats
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UNI(User-Network Interface)
– Define boundary between a host and an ATM network
(between the customer and the carrier)
40 bits
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NNI(Network-Network Interface)
– Define boundary between two ATM switches
40 bits
GFC : General Flow Control
VPI : Virtual Path Identifier
VCI : Virtual Channel Identification
PTI : Payload Type
CLP : Cell Loss Priority
HEC : Header Error Check
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GFC : flow control or priority
VPI : select a particular virtual path
VCI : select a particular virtual circuit
PTI : define the type of payload
Payload type
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Meaning
000
User data cell, no congestion, cell type 0
001
User data cell, no congestion, cell type 1
010
User data cell, congestion experienced, cell type 0
011
User data cell, congestion experienced, cell type 1
100
Maintenance information between adjacent switches
101
Maintenance information between source and destination switch
110
Resource Management cell(used for ABR congestion control)
111
Reserved for future function
CLP : set by host to differentiate between high-priority traffic and low-priority traffic
HEC : checksum over the header
Connection Setup
• Messages used for connection establishment and release
Message
Meaning when sent by host
Meaning when sent by network
SETUP
Please establish a circuit
Incoming call
CALL PROCEEDING
I saw the incoming call
Your call request will be attempted
CONNECT
I accept the incoming call
Your call request was accepted
CONNECT ACK
Thanks for accepting
Thanks for making the call
RELEASE
Please terminate the call
The other side has had enough
RELEASE COMPLETE
Ack for release
Ack for release
• ATM address
– 20 bytes(based on OSI addresses)
• Byte 1 : indicate which of three formats the address is in
• Bytes 2-3 : country(alternatively, international organization)
• Bytes 4 : format of the rest of the address
3-byte(authority), 2-byte(domain), 2-byte(area), 6-byte(address)
– 15 digit decimal ISDN telephone number
Connection setup in an ATM network
Connection release
Routing and Switching
• Route on VPI field, but not VCI field
– Exception : route on VCI at the final hop in each direction
• Advantage
– No new routing decisions have to be made
– Routing of individual cells is easier when all VCs for a given path are
always in the same bundles
– Easier to switch a whole group of VCs
– Easier for carriers to offer closed user groups(private network) to
corporate customers
• Examples : Rerouting a VP reroutes all of its VCs
Service Categories
• The ATM service categories
Class
CBR
Description
Example
Constant bit rate
T1 circuit
Variable bit rate : real time
Real-time videoconference
Variable bit rate : non-real time
Multimedia email
ABR
Available bit rate
Browsing the Web
UBR
Unspecified bit rate
Background file transfer
RT-VBR
NRT-VBR
• Characteristics of the ATM service categories
Service characteristic
CBR
RT-VBR
NRT-VBR
ABR
UBR
Bandwidth guarantee
Yes
Yes
Yes
Optional
No
Suitable for real-time traffic
Yes
Yes
No
No
No
Suitable for bursty traffic
No
No
Yes
Yes
Yes
Feedback about congestion
No
No
No
Yes
No
Quality of Service
• Some of the quality of service parameters
Parameter
Acronym
Meaning
Peak cell rate
PCR
Maximum rate at which cells will be sent
Sustained cell rate
SCR
The long-term average cell rate
Minimum cell rate
MCR
The minimum acceptable cell rate
Cell delay variation tolerance
CDVT
The maximum acceptable cell jitter
Cell loss ratio
CLR
Fraction of cells lost or delivered too late
Cell transfer delay
CTD
How long delivery takes(mean and maximum)
Cell delay variation
CDV
The variance in cell delivery times
Cell error rate
CER
Fraction of cells delivered without error
Severely-errored cell block ratio
Cell misinsertion rate
SECBR
CMR
Fraction of blocks garbled
Fraction of cells delivered to wrong destination
• The probability density function for cell arrival times
Congestion Control
• Admission Control.
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Must describe the traffic to be offered and the service expected.
Check to see if it is possible to handle this connection.
If no route can be located, the call is rejected.
Denying admission should be done fairly.
• Resource Reservation.
– Reserving resources in advance, usually at call setup time.
– Bandwidth can be reserved by having the SETUP message along each
line it traverses.
• Rate-Based Congestion Control.
– CBR and VBR : No dynamic congestion control are possible( real-time ).
– UBR : Nobody cares.
– ABR : How congestion should be detected, signaled, and controlled ?
• Rate-based solution
– After every k data cells, transmits a special RM cell
– RM(Resource Management) cell
• Travels along the same path as the data cells
• Contains ER(Explicit Rate).
• When it gets the destination, it is examined, updated, and sent back
– Congestion control mechanism
• Overloaded switches generate RM cells and ship them back to the sender
• Overloaded switches set the middle PTI bit
ATM LANs
• How to provide connectionless LAN service over the ATM network
– Introduce a connectionless server into the network
– Every host has a (potential) ATM virtual circuit to every other host
LES : LAN Emulation Server
BUS : Broadcast/Unknown Server
5. ATM ALL Layer Protocol
• AAL : ATM Adaptation Layer
• Goal
– Provide useful services to application programs
– Shield application programs from the mechanical processing
• Original service classes supported by AAL(now obsolete)
• Defined four protocols to handle services(AAL 1 ~ AAL4)
Structure of the ATM Adaptation Layer
• ATM adaptation layer
Convergence sublayer (service specific part)
Convergence sublayer (common part)
Segmentation reassembly sublayer
• Convergence sublayer
– Provide the interface to the application
– Service specific part
– Common part
• SAR(Segmentation And Reassembly) sublayer
– Sender : add header and trailers to the data
– Receiver : reassemble the cells into message
AAL 1
• Used for transmitting class A traffic
– Simple, connection-oriented, real-time constant bit rate
– Ex. Uncompressed audio and video
• CS sublayer
– Break up the input message or stream into 46- or 47- byte unit
– Does not have any protocol header
• Cell format
– SN(Sequence Number), SNP(Sequence Number Protection: checksum)
– P cell : used when message boundaries must be preserved
AAL 2
• Used for transmitting class B traffic
– Rate can vary strongly in time
– Message boundary preserved
– Ex. Compressed audio or video
• As in AAL1, the CS sublayer does not have a protocol header
• Cell format
– 1-byte header, 2-byte trailer, 45-byte data bytes
– IT(Information Type) : indicate that cell is the start, middle, or end
– LI(Length Indicator) : tells how big the payload is
AAL 3/4
• Protocols for classes C and D
• Stream or message mode
– Message mode : Boundaries are preserved
– Stream mode : Boundaries are not preserved
• Multiplexing of several sessions onto one virtual circuit
• AAL 3 / 4 CS sublayer message format
– CPI(Common Part Indicator) : Message type
– Btag, Etag : Used to frame message
– BA size, Length : Payload length
• Cell format : ST(Segment Type), MID(Multiplexing ID)
AAL 5
• SEAL(Simple Efficient Adaptation Layer)
• Service
– Reliable service, Unreliable service
– Unicast and Multicast(not guaranteed) are supported
– Supports message mode and stream mode
• CS sublayer
– UU(User to User) : available for a higher layer for its own purpose
• Sequencing, multiplexing
– Length : Payload length(not counting padding)
• SAR sublayer : does not add headers or trailers
Comparison of AAL Protocols
• Some difference between the various AAL protocols
Item
AAL 1
AAL 2
AAL 3/4
AAL 5
Service class
A
B
C/D
C/D
Multiplexing
No
No
Yes
No
Message delimiting
None
None
Btag/Etag
Bit in PTI
Advance buffer allocation
No
No
Yes
No
User bytes available
0
0
0
1
CS padding
0
0
32-Bit word
0-47 bytes
CS protocol overhead(bytes)
0
0
8
8
CS checksum
None
None
None
32 Bits
SAR payload bytes
46-47
45
44
48
SAR protocol overhead(bytes)
1-2
3
4
0
SAR checksum
None
None
10 Bits
None
SSCOP
• AAL protocols does not provide for simple end-to-end reliable
transport connections
• SSCOP
– Service Specific Connection-Oriented Protocol
– Used for control, not for data transmission
– Fundamentally a dynamic sliding window protocol
• No piggybacking
• Periodically, sender asks to send back the bit map giving the window status