EE 122 Lecture 19 Asynchronous Transfer Mode ATM Ion Stoica Nov 13 2001 based on some on line slides of J Kurose K Rose Goals Provide better services than best effort Able to carry both data and voice telephony traffic Can be implemented at very high speeds istoica cs berkeley edu 2 ATM Protocol Stack Physical layer ATM layer performs routing similar in function to IP data link layer ATM Adaptation Layer AAL performs segmentation and reassembly multiplexing similar in function to the transport layer Source Destination AAL AAL ATM ATM ATM Phys Phys Phys istoica cs berkeley edu 3 Design Decisions Fixed size packets cells Based on virtual circuit istoica cs berkeley edu 4 Why Fixed Size Easier to implement high speed switches Easier to do processing when the cell length is known in advance Easier to implement parallel and pipeline solutions when the processing of all cells take the same time A cell is in general much smaller than the maximum packet size A high priority cell needs to wait less before being transmitted a low priority cell will take less time to be transmitted than a packet of maximum size When the is empty the first data bits are in general transmitted faster Cell size 48 byte payload 5 byte header A compromise between US 64 byte payload and Europe 32 byte payload istoica cs berkeley edu 5 ATM Cell Header VCI virtual channel ID will change from link to link through network PT Payload type e g control cell versus data cell CLP Cell Loss Priority bit CLP 1 implies low priority cell can be discarded if congestion HEC Header Error Checksum cyclic redundancy check istoica cs berkeley edu 6 ATM Cell Header First 4 bits in VCI GFC Generic Flow Control Local significance at the end host Arbitrate the access to the link if a shared medium is used to connect to ATM GFC bits can be overwritten by the newtork istoica cs berkeley edu 7 ATM VCs Advantages of ATM VC approach QoS performance guarantee for connection mapped to VC bandwidth delay delay jitter Drawbacks of ATM VC approach Inefficient support of datagram traffic One VC between each source dest pair does not scale N 2 connections needed VC introduces call setup latency processing overhead for short lived connections istoica cs berkeley edu 8 Virtual Circuit VC Forwarding Each router maintains a routing table A routing entry input port input VCI output port output VCI VCI Virtual Circuit Identifier Upon a cell arrival at interface i Input port uses i and the packet s VCI v to find the routing entry i v i v Replaces v with v in the packet header Forwards packet to output port I istoica cs berkeley edu 9 VC Forwarding Example in in VCI out out VCI in in VCI out out VCI source 5 3 5 4 11 1 2 3 4 1 2 3 4 1 7 4 1 destination 1 2 3 4 1 2 3 4 11 1 2 3 4 1 2 3 4 1 7 in in VCI out out VCI 2 11 3 7 istoica cs berkeley edu 10 Virtual Path Identifier VPI ATM splits the VCI in two 16 bits Virtual Path Identifier VPI 8 12 bits VCI Use to implement hierarchical routing All VCI between two subnets share the same VPI Backbone switches switch based on VPI Switches in edge networks switch based on the entire VCI Backbone Public Network Virtual path Network A istoica cs berkeley edu Network B 11 Virtual Path Identifier VPI ATM splits the VCI in two 16 bits Virtual Path Identifier VPI 8 12 bits VCI Use to implement hierarchical routing All VCI between two subnets share the same VPI Backbone switches switch based on VPI Switches in edge networks switch based on the entire VPI Backbone Public Network Virtual path Network A istoica cs berkeley edu Network B 12 ATM Adaptation Layer Goal allow existing protocols and applications to run on top of ATM AAL is implemented only at endpoints AAL has two sub layers Convergence Sub layer CS Segmentation and Reassembly SAR sub layer Usually CS adds Common Part Convergence Sub layer CPCS header and trailer Checksum istoica cs berkeley edu 13 AAL Structure Protocol Data Unit PDU CPCS header PDU CPCS trailer ATM cell AAL ATM cell Payload data 48 header header trailer Example of AAL header information Type e g first cell last cell in PDU Sequence of the cell within PDU istoica cs berkeley edu 14 ATM Quality of Service QoS Constant bit rate CBR Variable bit rate real time VBR rt Variable bit rate non real time VBE nrt Available bit rate ABR Unspecified bit rate UBR istoica cs berkeley edu 15 ATM QoS cont d VBR rt similar to Guaranteed Service in Intserv Traffic is specified by a token bucket and the end toend delay is specified CBR very similar to VBR rt but the source is expected to send at a specified rate e g telephony traffic Specified by a token bucket with very small bucket e g 1 cell VBR nrt similar to the Controlled load Service in Intserv Traffic specified by token bucket but no hard delay guarantees istoica cs berkeley edu 16 ATM QoS cont d UBR similar to best effort service but There can still be an admission control However UBR allows the source to specify a maximum rate which can be used in the admission control ABR implements congestion control use explicit notification Source sends periodic Resource Management RM cells Each switch put in the RM cell the available bit rate of the output link after the RM cell traverses all switches it will have the minimum available rate amongst all routers along the path Routers can use fair queueing to compute the available bit rate Receiver sends back the RM cell to the sender Sender adjust its rate accordingly istoica cs berkeley edu 17 AAL Types AAL 1 support CBR AAL 2 support VBR AAL 3 4 support variable length packets AAL 5 more efficient support for data packets used to implement to support data packets and UBR service istoica cs berkeley edu 18 IP Over ATM Classic IP only 3 networks e g LAN segments MAC 802 3 and IP addresses IP over ATM Replace network e g LAN segment with ATM network ATM addresses IP addresses ATM network Ethernet LANs Ethernet LANs istoica cs berkeley edu 19 IP Over ATM Issues IP datagrams into ATM AAL5 PDUs From IP addresses to ATM addresses ATM network Ethernet LANs istoica cs berkeley edu 20 Datagram Transmission in IP overATM Network At source IP layer finds mapping between IP ATM dest address using ARP passes datagram to AAL5 AAL5 encapsulates data segments to cells passes to ATM layer ATM network moves cell along VC to destination At destination AAL5 reassembles cells into original datagram if CRC OK datgram is passed to IP istoica cs berkeley edu 21 Summary ATM was an effort to build a new network to support data and voice
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