11-ATM - Data and Computer Communications Communications...

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Unformatted text preview: Data and Computer Communications Communications Chapter 11 – Asynchronous Transfer Chapter Mode Mode Eighth Edition by William Stallings Lecture slides by Lawrie Brown Asynchronous Transfer Mode Asynchronous One man had a vision of railways that would link all the One mainline railroad termini. His name was Charles Pearson and, though born the son of an upholsterer, he became Solicitor to the city of London. There had previously been a plan for gaslit subway streets through which horse-drawn traffic could pass. This was rejected on the grounds that such sinister tunnels would become lurking places for thieves. Twenty years before his system was built, Pearson envisaged a line running through "a spacious archway," well-lit and wellline ventilated. His was a scheme for trains in a drain. —King Solomon's Carpet, Barbara Vine (Ruth Rendell) ATM ATM a streamlined packet transfer interface similarities to packet switching transfers data in discrete chunks supports multiple logical connections over a supports single physical interface single ATM uses fixed sized packets called cells with minimal error and flow control data rates of 25.6Mbps to 622.08Mbps Protocol Architecture Protocol Reference Model Planes Reference 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 ATM Logical Connections ATM virtual channel connections (VCC) basic unit of switching between two end users analogous to virtual circuit in X.25 full duplex fixed size cells also for also user-network exchange (control) user-network network-network exchange (network mgmt & routing) ATM Virtual Path Connection ATM virtual path connection (VPC) bundle of VCC with same end points Advantages of Virtual Paths Advantages simplified network architecture increased network performance and increased reliability reliability reduced processing short connection setup time enhanced network services Call Establish Establish ment ment Using VPs Using Virtual Channel Connection Uses Uses between end users end to end user data vontrol signals VPC provides overall capacity • VCC organization done by users between end user and network control signaling between network entities network traffic management routing VP/VC Characteristics VP/VC quality of service switched and semi-permanent channel switched connections connections call sequence integrity traffic parameter negotiation and usage traffic monitoring monitoring VPC only virtual channel identifier restriction within VPC Control Signaling - VCC Control to establish or release VCCs & VPCs to uses a separate connection methods are: 1. 2. 3. 4. semi-permanent VCC meta-signaling channel user to network signaling virtual channel user to user signaling virtual channel Control Signaling - VPC Control methods for control signalling for VPCs: 1. 2. 3. Semi-permanent Customer controlled Network controlled ATM Cells ATM ATM Header Fields ATM generic flow control Virtual path identifier Virtual channel identifier payload type cell loss priority header error control Generic Flow Control (GFC) Generic control traffic flow at user to network interface control (UNI) to alleviate short term overload (UNI) two sets of procedures every connection subject to flow control or not if subject to flow control uncontrolled transmission controlled transmission may be one group (A) default may be two groups (A and B) flow control is from subscriber to network GFC - Single Group of Connections Connections 1. 2. 3. If TRANSMIT=1 send uncontrolled cells any If time. If TRANSMIT=0 no cells may be sent time. If HALT received, TRANSMIT=0 until If NO_HALT NO_HALT If TRANSMIT=1 & no uncontrolled cell to send: 1. 2. 4. If GO_CNTR>0, TE may send controlled cell and If decrement GO_CNTR decrement If GO_CNTR=0, TE may not send controlled cells TE sets GO_CNTR to GO_VALUE upon TE receiving SET signal receiving Use of HALT Use to limit effective data rate on ATM should be cyclic to reduce data rate by half, HALT issued to to be in effect 50% of time to done on regular pattern over lifetime of done connection connection Two Queue Model Two uses two counters each with current & uses initial values: initial GO_CNTR_A GO_VALUE_A GO_CNTR_B GO_VALUE_B Header Error Control Header Effect of Error in Cell Header Header Impact of Random Bit Errors Impact on HEC Performance on Transmission of ATM Cells Transmission I.432 specifies several data rates: 622.08Mbps 155.52Mbps 51.84Mbps 25.6Mbps two choices of transmission structure: Cell based physical layer SDH based physical layer Cell Based Physical Layer Cell no framing imposed vontinuous stream of 53 octet cells cell delineation based on header error cell control field control Cell Delineation State Diagram Diagram Impact of Random Bit Errors on Cell Delineation Performance Performance Acquisition Time v Bit Error Rate Rate SDH Based Physical Layer SDH imposes structure on ATM stream eg. for 155.52Mbps use STM-1 (STS-3) frame can carry ATM and STM payloads specific connections can be circuit specific switched using SDH channel switched SDH multiplexing techniques can combine SDH several ATM streams several STM-1 Payload for SDHSTM-1 Based ATM Cell Transmission ATM Service Categories ATM Real time - limit amount/variation of delay Constant bit rate (CBR) Real time variable bit rate (rt-VBR) Non-real time - for bursty traffic Non-real time variable bit rate (nrt-VBR) Available bit rate (ABR) Unspecified bit rate (UBR) Guaranteed frame rate (GFR) Constant Bit Rate (CBR) Constant fixed data rate continuously available tight upper bound on delay uncompressed audio and video video conferencing interactive audio A/V distribution and retrieval Real-Time Variable Bit Rate Real-Time (rt-VBR) for time sensitive applications rt-VBR applications transmit data at a rate that rt-VBR varies with time varies tightly constrained delay and delay variation eg. compressed video eg. produces varying sized image frames original (uncompressed) frame rate constant so compressed data rate varies hence can statistically multiplex connections Non-Real-Time Variable Bit Rate (nrt-VBR) (nrt-VBR) if can characterize expected bursty traffic flow ATM net allocates resources based on this eg. airline reservations, banking transactions to meet critical response-time requirements giving improve QoS in loss and delay end system specifies: peak cell rate peak sustainable or average rate sustainable measure of how bursty traffic is Unspecified Bit Rate (UBR) Unspecified may be additional capacity over and above that may used by CBR and VBR traffic used for application that can tolerate some cell loss or for variable delays variable not all resources dedicated to CBR/VBR traffic unused cells due to bursty nature of VBR eg. TCP based traffic cells forwarded on FIFO basis best effort service Available Bit Rate (ABR) Available application specifies peak cell rate (PCR) application and minimum cell rate (MCR) and resources allocated to give at least MCR spare capacity shared among all ARB spare sources sources eg. LAN interconnection ATM Bit Rate Services ATM Summary Summary Asynchronous Transfer Mode (ATM) architecture & logical connections ATM Cell format transmission of ATM cells ATM services ...
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