ATM(Asynchronous Transfer Mode)
Presented By- Surbhi & Vasundhara
Content: What is ATM Network? ATM Protocol Architecture ATM Devices and the Network Environment ATM network interfaces ATM Cell Format ATM switching operation ATM Service Categories ATM Adaption Layer (AAL 1 to AAL 5) Advantages of ATM Network
What is ATM?• ATM Net = Data Net + Phone Net• Combination of Internet method of communication (packet
switching) and phone companies’ method (circuit switching)• International Telecommunication Union-Telecommunications
Standards Section (ITU-T) standard for cell relay • ATM networks are connection-oriented.• ATM virtual circuit: which is an end-to-end connection that has
defined end points and routes but does not have bandwidth dedicated to it.
• Bandwidth is allocated on demand by the network as users have traffic to transmit
ATM Protocol architecture
ATM Protocol architecture(cont.) ATM cell: fixed size packets for information flow Reduces overhead of processing and space (support high rate) Planes in ATM network: User plane: Provides for user information transfer, along with
associated flow and error control Control plane: Performs call control and connection control
functions Management plane: plane mgmt. & resources management as
per parameter of protocol entities.
ATM Protocol architecture(cont.) Physical layer :
Cells are converted into a bit stream The transmission and receipt of bits on the physical medium are
controlled. ATM cell boundaries are tracked. Cells are packaged into the appropriate types of frames for the
physical medium. Min data rate: 155.52 mbps : Max data rate: 622.08 Mbps
ATM Layer: independent of service demanded (common to all) Deals with AT cell transmission & use of logical connections
ATM Devices and the Network Environment An ATM network is made up of an ATM switch and ATM
endpoints. An ATM switch is responsible for cell transit reads and updates the cell header information and quickly
switches the cell to an output interface towards its destination. Examples of ATM endpoints are workstations, routers, digital
service units (DSUs), LAN switches, and video coder-decoders (Codec’s).
ATM Network Interfaces
ATM network interfaces
UNI(connects ATM end systems (such as hosts and routers) to
an ATM switch )
Public UNI Private UNI
NNI(Connects two ATM switches)
Public UNI Private UNI
ATM Cell Format
Each cell consists of 53 octets Small, fixed-length cells are well suited to transfer voice
and video traffic.
Header (5 Bytes) Payload (48 bytes)
CELL format in ATM Header(UNI)
GFC (4 bits) VPI (4 bits)
VPI (4 bits) VCI (4 bits)
VCI (8 bits)
VCI (4 bits) PT (3 bits) CLP (1 bit)
HEC (8 bits)
ATM cell header
Payload (48 bytes)
Generic Flow Control (GFC)—Provides local functions, such as identifying multiple stations that share a single ATM interface. This field is typically not used and is set to its default value of 0 (binary 0000).
CELL format in ATM Header(UNI)
GFC (4 bits) VPI (4 bits)
VPI (4 bits) VCI (4 bits)
VCI (8 bits)
VCI (4 bits) PT (3 bits) CLP (1 bit)
HEC (8 bits)
ATM cell header
Payload (48 bytes)
Virtual Path Identifier (VPI)—In conjunction with the VCI, identifies the next destination of a cell as it passes through a series of ATM switches on the way to its destination
CELL format in ATM Header(UNI)
GFC (4 bits) VPI (4 bits)
VPI (4 bits) VCI (4 bits)
VCI (8 bits)
VCI (4 bits) PT (3 bits) CLP (1 bit)
HEC (8 bits)
ATM cell header
Payload (48 bytes)
Virtual Channel Identifier (VCI)—In conjunction with the VPI, identifies the next destination of a cell as it passes through a series of ATM switches on the way to its destination.
ATM Logical connection
Two levels of ATM connections:• virtual path connections• virtual channel connections
Indicated by two fields in the cell header:• virtual path identifier VPI(identifies a particular VP link)
• virtual channel identifierVCI (numerical tag
to identifies a particular VC)
Physical Link
Virtual Paths
Virtual Channels
ATM switching operations
CELL format in ATM Header(UNI)
GFC (4 bits) VPI (4 bits)
VPI (4 bits) VCI (4 bits)
VCI (8 bits)
VCI (4 bits) PT (3 bits) CLP (1 bit)
HEC (8 bits)
ATM cell header
Payload (48 bytes)
• Payload Type (PT)—Indicates in the first bit whether the cell contains user data or control data. If the cell contains user data, the bit is set to 0. If it contains control data, it is set to 1.
• The second bit indicates congestion (0 = no congestion, 1 = congestion).
• The third bit indicates whether the cell is the last or not.
CELL format in ATM Header(UNI)
GFC (4 bits) VPI (4 bits)
VPI (4 bits) VCI (4 bits)
VCI (8 bits)
VCI (4 bits) PT (3 bits) CLP (1 bit)
HEC (8 bits)
ATM cell header
Payload (48 bytes)
Cell Loss Priority (CLP)—Indicates whether the cell should be discarded if congestion as it moves through the network.
If the CLP bit equals 1, the cell should be discarded in preference.
CELL format in ATM Header(UNI)
GFC (4 bits) VPI (4 bits)
VPI (4 bits) VCI (4 bits)
VCI (8 bits)
VCI (4 bits) PT (3 bits) CLP (1 bit)
HEC (8 bits)
ATM cell header
Payload (48 bytes)
Header Error Control (HEC)—Calculates checksum only on the first 4 bytes of the header. HEC can correct a single bit error in these bytes, thereby preserving the cell rather than discarding it.
ATM Service CategoriesATM
Service Categories
Real-Time Service
Constant bit rate(CBR)
Real-Time Variable bit rate(rt-VBR)
NON-REAL-TIME service
Non real-time variable bit rate(nrt-
VBR)
Available Bit rate(ABR)
Unspecified BIT
Rate(UBR)
Guaranteed Frame
Rate(GFR)
Tolerance of JITTER matters
here(tight constraints
of delay)
Busty traffic characteristics
(un-tight constraints of
delay)
Fixed rate throughout lifetime of
connection(telephony, video conf)
Time sensitive application
(video compression effect rate)
Data rate varies
Relative low delay and minimal loss cell
(End system have specify peak cell
rate)
FIFO based transmission acc. To
left resources (variable delay and
data rate)
Fair distribution of resources among
ABR sources(if left given to UBR)
Used for IP and Ethernet based sub
networks(sp. For frame based
network)
ATM Adaption layer
Higher Layers
ATM Adaptation Layer (AAL)Convergence Sub layer (CS)
Segmentation and Reassembly Sub layer (SAR)
ATM Layer (ATM) Virtual Channel (VC) Virtual Path (VP)
Physical Layer (PL) Transmission Convergence Sub layer (TC)
Physical Medium Sub layer (PM)
ATM Adaptation Layer Defines the rules for breaking up a higher level protocol data unit
(PDU) into ATM cells for transmission on the network, and for reassembling the PDU at the other end.
Segmentation and reassembly (SAR): breaks data into cells at the sender and reassembles cells into larger data units at the receiver.
Convergence sub layer: adds sequencing and control information to aid SAR process
TCPIP
AALATM
TCPIP
AALATMATM
Network
PDU
PDUCS-header Pad CS-trailer
Cellheader
Cellpayload
SAR-PDUheader
SAR-PDUpayload
SAR-PDUtrailer
SAR-PDUheader
SAR-PDUpayload
SAR-PDUtrailer
Higherlayer
CSlayer
SARlayer
ATMlayer Cell
headerCell
payload
Adaptation Layer : A detailed view
ATM Adaptation Layer
Connectionless Date Transfer
Bit Rate
Connection Mode
Examples of Services
CircuitEmulation
Constant Bit Rate Video and Audio
Constant Variable
Connection Oriented Connectionless
Variable Bit Rate Video and Audio
Connection- oriented Data Transfer
AALTYPE AAL 1 AAL 2
AAL 3/4AAL 5
Class A Class B Class C Class D
Related Not Related
Service
Timing betweenSource andDestination
AAL3/4AAL 5
AAL Type 1 Protocol Supports Class A traffic, i.e., constant bit rate data with specific requirements for
delay, delay jitter, and timing, e.g., PCM voice. Summary of AAL 1 functions
CS Functions Handling of cell delay variation Forward error correction for high quality video and audio Report end-to-end performance status
SAR FUNCTIONS Segmentation and reassembly of user information Sequence numbering Error protection Mapping between CS PDU and SAR PDU
Consists of 1 octet header (PCI) and 47 octets of payload Sequence Number (SN): A 1-bit Convergence Sub layer Indication and 3-bit
sequence count to detect deletion or miss insertion of cells Sequence Number Protection (SNP): 3-bit CRC with even parity for detecting
and correcting SN error
The 4 bit RTS is transferred by the CSI bit in successive SAR-PDU headers with an odd SN (SN = 1, 3, 5, 7)
For P format operations, the CSI value in SAR-PDU headers with an even SN (SN = 0, 2, 4, 6) is set to 1
SN SNP SAR-PDU Payload
4 bits 4 bits 47 Octets
Cyclic Redundancy Check Even Parity
1 bit3 bit
1 bit 3 bits
Convergence Sub layer Indication (CSI)
Sequence Number (SN)
AAL Type 2 protocol Deals with variable bit-rate information Intended for analog applications ,such as video and audio(CBR
not required)
Supports variable bit rate data where there is no timing relationship between source and destination, e.g., X.25, frame relay.
Convergence sublayer divided into two parts: Common Part Convergence Sublayer (CPCS) Service Specific Convergence Sublayer (SSCS)
AAL Type 3/4 protocol
AAL Type ¾ protocol
Types of services provided by AAL ¾ • Connectionless(independent packet) &
connection-oriented(multiple SAR logical connection)• Message mode(cell mode) & stream mode(block
mode)CPCS-PDU payloadCpcs header Cpcs-pdu trailerpad
CPI Btag BAsize AL ETag Length of payload
1 octet1 octet 1 octet 2 octet 2 octet1 octet
CPCS= common part convergence sub layer
SAR= Segmentation & Reassembly
PDU= protocol data unit
CPCS-H= caps header
CPCS-T = cpcs trailer
Sar-H= Sar headerSAR- H = SAR TailerATM-H = Atm headerBOM= beginning of msgCOM = continuation of message
AAL Type 5 Protocol AAL5 is a simple and efficient AAL (SEAL) to perform a subset
of the functions of AAL3/4 The CPCS-PDU payload length can be up to 65,535 octets and
must use PAD (0 to 47 octets) to align CPCS-PDU length to a multiple of 48 octets
CPCS-PDU Payload PAD CPI Length
1 2 40 - 47
CPCS UU
1
CRC-32
PAD PaddingCPCS-UU CPCS User-to-User IndicatorCPI Common Part IndicatorLength CPCS-PDU Payload LengthCRC-32 Cyclic Redundancy Chuck
AAL Type 5 SAR-PDU
PAD CPCS-PDU Trailer
CPCS-PDU
SAR-PDU Payload
CPCS-SDU
SAR-PDU Payload
SAR-PDU Payload
SAR-PDU Payload
SAR-PDU Payload
SAR-PDU SAR-PDU SAR-PDU SAR-PDUSAR-PDU
CPCS-PDU Payload
Advantages of ATM
Dynamic bandwidth for bursty traffic :meeting application needs and delivering high utilization of networking resources
Smaller header with respect to the data to make the efficient use of bandwidth.
Can handle Mixed network traffic very efficiently fixed size cell. Cell network: All data is loaded into identical cells that can be transmitted
with complete predictability and uniformity. Class-of-service support for multimedia traffic allowing applications with
varying throughput and latency requirements to be met on a single network.
Scalability in speed and network size supporting link speeds of (622 Mbps). Common LAN/WAN ATM technology can be used either as a LAN
technology or a WAN technology.
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