Broadband ISDN and ATM

1. Broadband ISDN and ATM 발표자 : 박종민 Jmpark@cs.chonbuk.ac.kr

2. Contents • Introduction to B-ISDN and ATM • The B-ISDN ATM Reference Model • Physical Layer in ATM • Virtual Circuits versus Circuit Switching • Transmission in ATM Networks • ATM Switches ( Knockout, Batcher-Banyan ) • Data Link Layer in ATM

3. Network Layer in ATM • Cell Formats • Connection Setup • Routing and Switching • Service Categories • Quality of Service • Congestion Control • ATM LANs • ATM AAL Layer Protocols • Structure of the ATM Adaptation Layer • AAL 1, AAL 2, AAL 3 / 4, AAL 5 • Comparison of AAL Protocols • Service Specific Connection-Oriented Protocol(SSCOP)

4. 1. Introduction to B-ISDN & ATM • B-ISDN(Broadband Integrated Services Digital Network) • A single physical network integrates variety of services • Problems • QoS Requirements for services widely different • Voice : Real time(low delay jitter), tolerates occasional losses • Data : Usually no real-time requirements, error-free, guaranteed delivery • Video : High bandwidth, low delay and jitter • Traffic characteristics also widely different • Certain applications require synchronization among multiple traffic streams

5. ATM(Asynchronous Transfer Mode) • Underlying technology that makes B-ISDN possible • Transmit all information in small, fixed size packet (ATM cell) • Cell switching technology • Flexible and handle both constant(audio, video) and variable(data) rate traffic • Easier than traditional multiplexing techniques using fiber optics • Broadcasting(television distribution) • Connection-oriented : Cell order is guaranteed • Intended speed : 155 Mbps, 622 Mbps, gigabit speeds later • 155 Mbps(exact choice) : made for compatibility with AT&T’s SONET

6. B-ISDN ATM Reference Model

7. Physical Layer(deals with physical medium) • Voltage, bit timing, various other issues. • Designed to be independent of the transmission medium • ATM Layer(deals with cells and cell transport) • Defines the layout of a cell • Establishment and release of virtual circuits • Congestion control • AAL Layer(ATM Adaptation Layer) • Allows users to send packets larger than a cell • Segments packets, transmits the cells individually, and reassembles then at the other end • User plane • Deals with data transport, flow control, error correction, and other user functions • Control plane is concerned with connection management

9. 2. Physical Layer in ATM • B-ISDN service : Compromise between circuit and packet switching • Virtual Circuits • Connection oriented • Implemented internally with packet switching • Connection Types • Permanent virtual circuits (No setup time) • Switched virtual circuits (Like telephone call) • Circuit establishment • The route is chosen from source to destination • All switch along the way make table entries so they can route any packet on that virtual circuit

11. Transmission in ATM Network • Synchronous transmission mode • Asynchronous transmission mode • Not required that the stream of cells coming out of a computer be continuous(Gaps between the data cells are filled by idle cells) • Does not standardize the format for transmitting cells • All ATM links are point-to-point(unidirectional) • Multicasting : Enter a switch on one line and exit it on multiple lines

12. ATM Switches • General Model for an ATM cell switch • ATM switches are generally synchronous • Cell arrive on the input lines asynchronously • Master clock marks the beginning of a cycle • Common goal • Switch all cells with as low a discard rate as possible • Never reorder the cells on a virtual circuit

13. Input queueing : head-of-line blocking • Output queueing

14. The Knockout Switch • Output queueing

15. Batcher-Banyan Switch

19. 3. Data Link Layer in ATM • TC(Transmission Convergence) sublayer • Cell Transmission • takes a sequence of cells • add HEC to each one ( HEC : Header Error Control ) • convert the result to bit stream • match the bit stream to the speed of the underlying physical transmission system by inserting OAM cells as filler • OAM : Operation And Maintenance • Cell Reception • takes an incoming bit stream • locates cell boundaries • verifies the header • processes the OAM cells • passes the data cell up to the ATM layer

20. Locating the cell boundaries • HUNT : Shifting bits into the shift registers one at a time looking for a valid HEC • PRESYNCH : shifts in the next 424 bits(5-bytes) without examining them

21. 4. Network Layer in ATM Network • Connection-oriented layer • Basic element : virtual circuit(virtual channel) • Unusual for a connection-oriented protocol • Does not provide any acknowledgements • Used for real-time traffic(audio, video) • Guarantee : cells sent will never arrive out of order • Supports a two-level connection hierarchy

22. Cell Formats • UNI(User-Network Interface) • Define boundary between a host and an ATM network(between the customer and the carrier) • NNI(Network-Network Interface) • Define boundary between two ATM switches GFC : General Flow Control PTI : Payload Type VPI : Virtual Path Identifier CLP : Cell Loss Priority VCI : Virtual Channel Identification HEC : Header Error Check 40 bits 40 bits

23. GFC : flow control or priority • VPI : select a particular virtual path • VCI : select a particular virtual circuit • PTI : define the type of payload • CLP : set by host to differentiate between high-priority traffic and low-priority traffic • HEC : checksum over the header

24. Connection Setup • Messages used for connection establishment and release • ATM address • 20 bytes(based on OSI addresses) • Byte 1 : indicate which of three formats the address is in • Bytes 2-3 : country(alternatively, international organization) • Bytes 4 : format of the rest of the address 3-byte(authority), 2-byte(domain), 2-byte(area), 6-byte(address) • 15 digit decimal ISDN telephone number

25. Connection setup in an ATM network Connection release

26. Routing and Switching • Route on VPI field, but not VCI field • Exception : route on VCI at the final hop in each direction • Advantage • No new routing decisions have to be made • Routing of individual cells is easier when all VCs for a given path are always in the same bundles • Easier to switch a whole group of VCs • Easier for carriers to offer closed user groups(private network) to corporate customers

27. Examples : Rerouting a VP reroutes all of its VCs

29. Service Categories • The ATM service categories • Characteristics of the ATM service categories

30. Quality of Service • Some of the quality of service parameters

31. The probability density function for cell arrival times

32. Congestion Control • Admission Control. • Must describe the traffic to be offered and the service expected. • Check to see if it is possible to handle this connection. • If no route can be located, the call is rejected. • Denying admission should be done fairly. • Resource Reservation. • Reserving resources in advance, usually at call setup time. • Bandwidth can be reserved by having the SETUP message along each line it traverses. • Rate-Based Congestion Control. • CBR and VBR : No dynamic congestion control are possible( real-time ). • UBR : Nobody cares. • ABR : How congestion should be detected, signaled, and controlled ?

33. Rate-based solution • After every k data cells, transmits a special RM cell • RM(Resource Management) cell • Travels along the same path as the data cells • Contains ER(Explicit Rate). • When it gets the destination, it is examined, updated, and sent back • Congestion control mechanism • Overloaded switches generate RM cells and ship them back to the sender • Overloaded switches set the middle PTI bit

34. ATM LANs • How to provide connectionless LAN service over the ATM network • Introduce a connectionless server into the network • Every host has a (potential) ATM virtual circuit to every other host LES : LAN Emulation Server BUS : Broadcast/Unknown Server

35. 5. ATM ALL Layer Protocol • AAL : ATM Adaptation Layer • Goal • Provide useful services to application programs • Shield application programs from the mechanical processing • Original service classes supported by AAL(now obsolete) • Defined four protocols to handle services(AAL 1 ~ AAL4)

36. Structure of the ATM Adaptation Layer • ATM adaptation layer • Convergence sublayer • Provide the interface to the application • Service specific part • Common part • SAR(Segmentation And Reassembly) sublayer • Sender : add header and trailers to the data • Receiver : reassemble the cells into message

38. AAL 1 • Used for transmitting class A traffic • Simple, connection-oriented, real-time constant bit rate • Ex. Uncompressed audio and video • CS sublayer • Break up the input message or stream into 46- or 47- byte unit • Does not have any protocol header • Cell format • SN(Sequence Number), SNP(Sequence Number Protection: checksum) • P cell : used when message boundaries must be preserved

39. AAL 2 • Used for transmitting class B traffic • Rate can vary strongly in time • Message boundary preserved • Ex. Compressed audio or video • As in AAL1, the CS sublayer does not have a protocol header • Cell format • 1-byte header, 2-byte trailer, 45-byte data bytes • IT(Information Type) : indicate that cell is the start, middle, or end • LI(Length Indicator) : tells how big the payload is

40. AAL 3/4 • Protocols for classes C and D • Stream or message mode • Message mode : Boundaries are preserved • Stream mode : Boundaries are not preserved • Multiplexing of several sessions onto one virtual circuit

41. AAL 3 / 4 CS sublayer message format • CPI(Common Part Indicator) : Message type • Btag, Etag : Used to frame message • BA size, Length : Payload length • Cell format : ST(Segment Type), MID(Multiplexing ID)

42. AAL 5 • SEAL(Simple Efficient Adaptation Layer) • Service • Reliable service, Unreliable service • Unicast and Multicast(not guaranteed) are supported • Supports message mode and stream mode • CS sublayer • UU(User to User) : available for a higher layer for its own purpose • Sequencing, multiplexing • Length : Payload length(not counting padding) • SAR sublayer : does not add headers or trailers

43. Comparison of AAL Protocols • Some difference between the various AAL protocols

44. SSCOP • AAL protocols does not provide for simple end-to-end reliable transport connections • SSCOP • Service Specific Connection-Oriented Protocol • Used for control, not for data transmission • Fundamentally a dynamic sliding window protocol • No piggybacking • Periodically, sender asks to send back the bit map giving the window status