Asynchronous Transfer Modes By: Megan Cwiklinski Adam Nasset Brad Samples Will Vanlue

Outline Introduction Introduction Why Cells and Cells in Practice Why Cells and Cells in Practice Why Virtual Circuits Why Virtual Circuits Types of Virtual Circuits and Paths Types of Virtual Circuits and Paths Cell Structure Cell Structure Quality and Classes of Service Quality and Classes of Service Benefits Benefits

Introduction ATM is a data link layer protocol ATM is a data link layer protocol Uses fixed sized cells Uses fixed sized cells Utilizes Synchronous Optical Network Utilizes Synchronous Optical Network Widely used by multiplex services and DSL Widely used by multiplex services and DSL

Why Cells? Use small data cells to reduce jitter Use small data cells to reduce jitter Important when carrying voice traffic Important when carrying voice traffic Good job includes Good job includes –Evenly spaced stream of data items –Measure in time variables

Why Cells? Designed to run at 155 Mbit/s Designed to run at 155 Mbit/s –Typical 1500-byte data packet sent in microseconds Disadvantage Disadvantage –Queuing delays can be larger than transmittal time –Packets expensive to implement at high speeds

Why Cells? Cells developed to carry large datagrams and provide short queuing delays Cells developed to carry large datagrams and provide short queuing delays Breaks up packets into 48-byte chunk with 5-byte header to be reassembled later Breaks up packets into 48-byte chunk with 5-byte header to be reassembled later

Cells in Practice Reduced need for small packet cells Reduced need for small packet cells However, Asymmetric Digital Subscribers Lines use ATM However, Asymmetric Digital Subscribers Lines use ATM On slow links, makes sense to use ATM On slow links, makes sense to use ATM Provides compelling business advantage for small companies Provides compelling business advantage for small companies

Why virtual circuits? Data is passed from source to destination over multiple real circuits Data is passed from source to destination over multiple real circuits ATM cell header contains: ATM cell header contains: – 8- or 12-bit Virtual Path Identifier (VPI) – 16-bit Virtual Channel Identifier (VCI)

Why virtual circuits? VPI & VCI make up a unique path for a virtual circuit VPI & VCI make up a unique path for a virtual circuit Switching is done by altering VPI & VCI Switching is done by altering VPI & VCI – VPI & VCI may not stay constant throughout a circuit – The circuit is consistent for the entire message Can be used as multiplexing layer Can be used as multiplexing layer Hierarchically identified first by VPI, then by VCI Hierarchically identified first by VPI, then by VCI – Simpler switching by only looking at one identifier

Types of virtual circuits and paths Static Static Dynamic Dynamic Switched Switched

Static Permanent Virtual Circuits (PVCs) and Permanent Virtual Paths (PVPs) Permanent Virtual Circuits (PVCs) and Permanent Virtual Paths (PVPs) – Circuits are made of explicitly defined segments between endpoints – Simple to understand; not very scalable – No dynamic rerouting in case of failure

Dynamic Soft Permanent Virtual Circuits (SPVCs) and Soft Permanent Virtual Paths (SPVPs) Soft Permanent Virtual Circuits (SPVCs) and Soft Permanent Virtual Paths (SPVPs) – Dynamically built from specified service contract and endpoints – Types of service contracts ABR (Available Bit Rate) ABR (Available Bit Rate) CBR (Constant Bit Rate) CBR (Constant Bit Rate) UBR (Unspecified Bit Rate) UBR (Unspecified Bit Rate) VBR (Variable Bit Rate) VBR (Variable Bit Rate)

Switched Switched Virtual Circuits (SVCs) Switched Virtual Circuits (SVCs) – Created on demand; one time use – Defined by service contract and endpoints – Example: Individual telephone calls

ATM Cell Structures 7430 VPI VPIVCI VCI VCIPTCLP HEC Payload (48 bytes) 7430GFCVPI VPIVCI VCI VCIPTCLP HEC UNI ATM CellNNI ATM Cell

Generic Flow Control Permits transmissions of several terminals to be multiplexed on the same user interface Permits transmissions of several terminals to be multiplexed on the same user interface Contains 4 bits Contains 4 bits

Virtual Path/Circuit Identifier Virtual paths are semi-permanent connections between endpoints Virtual paths are semi-permanent connections between endpoints Connections are identified by the virtual circuit Connections are identified by the virtual circuit

Payload Type The first bit indicates user data or control data The first bit indicates user data or control data If the cell contains user data If the cell contains user data –Second bit indicates congestion –Third bit indicates whether the cell is the last in a series of cells that represent a single AAL5 frame

Cell Loss Priority Used for buffer management Used for buffer management Only 1 bit Only 1 bit –0 means no congestion problems –1 means congestion and should be discarded

Header Error Correction Used to detect and correct errors in the header Used to detect and correct errors in the header Correction attempted on single bit errors Correction attempted on single bit errors Calculated using the polynomial x 8 +x 2 +x+1 Calculated using the polynomial x 8 +x 2 +x+1

Classes of Service Constant Bit Rate (CBR) Constant Bit Rate (CBR) Variable Bit Rate-Non Real Time (VBR- NRT) Variable Bit Rate-Non Real Time (VBR- NRT) Variable Bit Rate-Real Time (VBR-RT) Variable Bit Rate-Real Time (VBR-RT) Available Bit Rate (ABR) Available Bit Rate (ABR) Unspecified Bit Rate (UBR) Unspecified Bit Rate (UBR)

Technical Parameters Cell Loss Ratio (CLR) Cell Loss Ratio (CLR) Cell Transfer Delay (CTD) Cell Transfer Delay (CTD) Cell Delay Variation (CDV) Cell Delay Variation (CDV) Peak Cell Rate (PCR) Peak Cell Rate (PCR) Sustained Cell Rate (SCR) Sustained Cell Rate (SCR) Burst Tolerance (BT) Burst Tolerance (BT)

Class of Service CBR VBR – NRT VBR – RT ABRUBR CLRyesyesyesyesno CTDyesnoyesnono CDVyesyesyesnono PCRyesyesyesnoyes SCRnoyesyesnono PCR noyesyesnono Flow Control nononoyesno

Benefits Scalability Scalability –Hardware –Speed –Size Versatility Versatility –Voice and Video data –LAN and WAN

Benefits contd. Consistency Consistency –Efficient switching –Predictable traffic flow –Guaranteed delivery –International standards

Questions?