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Advanced topics in Computer Networks

Advanced topics in Computer Networks. Lecture 2: ATM: an overview. University of Tehran Dept. of EE and Computer Engineering By: Dr. Nasser Yazdani. Outline. Why ATM ATM overview ATM standard. Why ATM?. Problems with IP? Best effort, lack of reliability TCP on top of IP

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Advanced topics in Computer Networks

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  1. Advanced topicsinComputer Networks Lecture 2: ATM: an overview University of Tehran Dept. of EE and Computer Engineering By: Dr. Nasser Yazdani Adv. topics in Computer Network

  2. Outline • Why ATM • ATM overview • ATM standard Adv. topics in Computer Network

  3. Why ATM? • Problems with IP? • Best effort, lack of reliability • TCP on top of IP • Being stateless • Hop to hop or point to point. • Lack of QoS. • How important is QoS? It is essential in some applications including • VoIP • Video on demand. • Teleconferencing. Adv. topics in Computer Network

  4. Supporting QoS in IP • QoS parameter • Bounded delay and jitter. • Min rate • IP should identify flow and keep state in nodes. • Contradictory to original design goal, flexibility and scalability. • Can we have both? We try. • This is the main challenge of Internet. Adv. topics in Computer Network

  5. ATM overview ATM = Asynchronous Transfer Mode. ATM = Data Network + Phone Network • Combination of Internet method of communication (packet switching) and phone companies’ method (circuit switching) • Asynchronous- The way bandwidth is allocated. The time slot is not allocated by temporal position like Synchronous transfer mode, STM, (circuit switch). • Transfer mode= Multiplexing and switching. Adv. topics in Computer Network

  6. ATM overview (cont) • Connection oriented- Need signaling like circuit switch. • Establish connection. • Tear down connection. • Create a virtual connection or circuit, then order is preserved in carrying data. • Problems? • Overhead for connection and tearing down. • Loosing connectivity in case of failure in nodes. Adv. topics in Computer Network

  7. ATM vs. STM • STM or TDM • Peak rate allocation • Fast and simple logic. • Not good for bursty traffic like data. • ATM- uses statistical multiplexing. • Giving bandwidth for label. • Has to process cells . • Congestion is possible. • Variable cell delay (Queuing). • But, network controls routing, errors, flow, copying and priorities. • Overcome to congestion and delay is fundamental to ATM. Adv. topics in Computer Network

  8. ATM vs. Phone Network • Current phone networks are synchronous (periodic). STM ATM = Asynchronous Transfer Mode • Phone networks use circuit switching. ATM networks use “Packet” Switching • In phone networks, all rates are multiple of 8 kbps. With ATM service, you can get any rate. You can vary your rate with time. • With current phone networks, all high speed circuits are manually setup. ATM allows dialing any speed. Adv. topics in Computer Network

  9. Interfaces • User to Network Interface (UNI): Public UNI, Private UNI • Network to Node Interface (NNI): • Private NNI (P-NNI) • Public NNI = Inter-Switching System Interface (ISSI) • Intra-LATA ISSI (Regional Bell Operating Co) • Inter-LATA ISSI (Inter-exchange Carriers) Broadband Inter-Carrier Interface (B-ICI) • Data Exchange Interface (DXI) Between packet routers and ATM Digital Service Units (DSU) Adv. topics in Computer Network

  10. ATM standard • Standard is different from concept. It deals with implementation. • ATM Forum is a standard body of developer, service provider and other interested bodies. • What standard does is to decide: • Size of cell • Header format • Signaling • Interfaces • Traffic control • Network management • Testing • Internetworking Adv. topics in Computer Network

  11. ATM Cell format • GFC=Generic Flow Control • (Was used in UNI but not in NNI) • The field is used for VPI in NNI • VPI/VCI=0/0 ÞIdle cell; 0/n ÞSignaling • HEC: 1 + x + x 2 + x 8 0 4 7 • U.S ANSI proposed 64 bytes data with 5 • byte header • Euro. ETSI proposed 32 bytes with 4 • bytes header • Finally, 48 bytes with 5 bytes header. Adv. topics in Computer Network

  12. Connection Identifiers • VPI/VCI are local and swapped in each switch. • VCI is allocated in connection setup time. • VPI is usually used by management. • By VPI, a set of VCI are treated as the same. • VPI/VCI=0/0 ÞIdle cell; 0/n ÞSignaling • Each cell contains a 24/28-bit connection identifier • First 8/12 bits: Virtual Path, Last 16 bits: Virtual Channel • VP service allows new VC's w/o orders to carriers Adv. topics in Computer Network

  13. Header Error Check (HEC) • Protects header only • Optional Correction mode: Correct one bit errors if no earlier errors • Discard cells with bad HEC • Used for cell delineation in SONET • Recalculated on each hop • CRC generator is 1 + x + x 2 + x 8 • Single bit error (correction) • Multibit error (discard) Detection mode Correction mode Correction mode No Error Error No error detected Adv. topics in Computer Network

  14. ATM protocol stack AAL = ATM Adaptation Layer Peer to peer connection AAL AAL ATM ATM ATM PHY PHY PHY Adv. topics in Computer Network

  15. Protocol layers • The ATM Adaptation Layer • How to break application messages to cells • The ATM Layer • Transmission/Switching/Reception • Congestion Control/Buffer management • Cell header generation/removal at source/destination • Reset connection identifiers for the next hop (at switch) • Cell address translation • Sequential delivery Adv. topics in Computer Network

  16. ATM network • ATM layer: Transporting cells • Cell encapsulation and decapsulation. • Insertion and extraction of idle cells • Cell header processing • Cell queuing • Cell coping • GFC management in UNI Adv. topics in Computer Network

  17. ATM network (cont) • Connection Admission Ctrl • Connection admission or rejection • VPI/VCI assignment and removal • Exchanging and processing signaling cells • Traffic Ctrl: Maintain agreed QoS • Usage parameter ctrl • Selective cell discarding • Implicit service priorities • Explicitly forward Adv. topics in Computer Network

  18. ATM network (cont) • ATM layer management • Configuring the network and switch fabric • Monitoring the network operation • Detecting troubles • ATM switch • Doing actual cell switching. Adv. topics in Computer Network

  19. History of ATM • 1980: Narrowband ISDN adopted • Early 80's: Research on Fast Packets • Mid 80's: B-ISDN Study Group formed • 1986 ATM approach chosen for B-ISDN • June 1989: 48+5 chosen (64+5 vs 32+4). • October 1991: ATM Forum founded • July 1992: UNI V2 released by ATM Forum • 1993: UNI V3 and DXI V1 • 1994: B-ICI V1 Adv. topics in Computer Network

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