Communication Networks Chapter 10: Connection-Oriented PDNs o X.25 o Frame Relay o Asynchronous Transfer Mode (ATM) Packet Switching Circuit switching was designed for voice Packet switching was designed for data Information transmitted in small packets Packets contains user data and control info user data may be part of a larger message control info includes routing (addressing) info Packets are received, stored briefly (buffered) and passed on to the next node Communication Networks - 10: Connection-Oriented PDNs 408 Prof. Jochen Seitz 1
Packet Switching Communication Networks - 10: Connection-Oriented PDNs 409 Packet-Switched Networks In packet-switched networks, data is packetized prior to transmission Each packet is a group of bits organized in a predetermined structure Each packet contains data bits as well as additional overhead information to ensure error-free transmission to intended recipients Packets may be called blocks, cells, datagrams, data units, or frames Packet assembler/disassemblers (PADs) are responsible for assembling outgoing data into packets for transmission over the packet-switching network as well as for unpacking incoming packets so that data can be delivered to intended recipients Communication Networks - 10: Connection-Oriented PDNs 410 Prof. Jochen Seitz 2
Packet-Switching Advantages and Disadvantages Advantages include: Single link shared by multiple senders and receivers No occupied links Different data rates at sender and receiver possible Establishment of packetpriority systems Charging by the volume of data (number of packets) transmitted rather than connection time Disadvantages include: Variable, unforeseeable transmission delays caused by packet processing and packet queues at packet switches Variable packet sizes leading to longer packet processing times at packet switches Overhead data in packets leading to lower data transmission efficiency and throughput than in circuitswitched networks Communication Networks - 10: Connection-Oriented PDNs 411 Virtual Circuit Diagram According to Stallings Communication Networks - 10: Connection-Oriented PDNs 412 Prof. Jochen Seitz 3
Virtual Circuit vs. Datagram Virtual circuits network can provide sequencing and error control packets are forwarded more quickly less reliable in case of link or switch failure Datagrams no call setup phase more flexible more fault-tolerant, but no QoS Communication Networks - 10: Connection-Oriented PDNs 413 Packet Size According to Stallings Communication Networks - 10: Connection-Oriented PDNs 414 Prof. Jochen Seitz 4
Circuit vs. Packet Switching Performance depends on various delays propagation delay transmission time node delay Range of other characteristics, including: transparency amount of overhead Communication Networks - 10: Connection-Oriented PDNs 415 Event Timing According to Stallings Communication Networks - 10: Connection-Oriented PDNs 416 Prof. Jochen Seitz 5
X.25 ITU-T standard for interface between host and packet switched network: Interface between Data Terminal Equipment and Data Circuit Terminating Equipment for Terminals Operating in the Packet Mode on Public Data Network almost universal on packet switched networks and packet switching in ISDN defines three layers Physical Link Packet Communication Networks - 10: Connection-Oriented PDNs 417 Characterstics of X.25 Virtual connections Permanent virtual connections Switched virtual connections Several virtual connections at the same time (up to 4096) End-to-end flow control According to the ISO/OSI Basic Reference Model: Physical layer: X.21 Transmission of data frames: HDLC Switching: X.25 Communication Networks - 10: Connection-Oriented PDNs 418 Prof. Jochen Seitz 6
X.25 & the OSI Reference Model Communication Networks - 10: Connection-Oriented PDNs 419 ITU-T X.25 Recommendation Layers 4-7 ISO/OSI layers on top of X.25 End-to-End Connections Layer 3 X.25-3 Layer 2 HDLC (LAPB) Packet Level Procedures Frame Level Procedures Packet Interface (Virtual Connections) Data Link Interface Layer 1 X.21 Physical Level Procedures Physical Interface DTE DTE/DCE Interface DCE/Network Communication Networks - 10: Connection-Oriented PDNs 420 Prof. Jochen Seitz 7
X.25 Physical Layer Interface between station node link Two ends are distinct Data Terminal Equipment DTE (user equipment) Data Circuit-terminating Equipment DCE (node) Physical layer specification is X.21 Can substitute alternative such as EIA-232 Communication Networks - 10: Connection-Oriented PDNs 421 X.21 Interface X.21 covers the first three layers of the ISO/OSI BRM: Electrical and mechanical interface Error detection and correction Circuit switching DTE PIN CCITT-IDENTIFICATION DCE G/P Signal ground or common return Ga DTE common return Gb DCE common return T Transmit R Receive C Control I Indication S Signal element timing B Byte timing (frame timing) Communication Networks - 10: Connection-Oriented PDNs 422 Prof. Jochen Seitz 8
X.25 - Link Link Access Protocol Balanced (LAPB) Subset of High-Level Data Link Control Reliable transfer of data over one link Information sent as a sequence of frames Communication Networks - 10: Connection-Oriented PDNs 423 High-Level Data Link Control Bit-oriented, code-transparent Data Link Protocol Half and full duplex mode Point-to-point and point-to-multipoint configuration Symmetrical and asymmetrical configuration Code transparency achieved by bit stuffing Piggybacking Flow control based on Sliding Window Variants of HDLC: IBM s SDLC (Synchronous Data Link Control) LAPB (Link Access Procedure, Balanced) LAPD (ISDN, Link Access Procedure for the D-Channel) LLC (IEEE 802.2, Logical Link Control) PPP (Point-to-Point Protocol) Communication Networks - 10: Connection-Oriented PDNs 424 Prof. Jochen Seitz 9
HDLC: Configurations Two types of stations: Primary station (sending commands); Secondary station (sending responses). Combined (hybrid) stations are possible, too, Information flow alternatives: Unbalanced control: Selecting Primary station requests secondary station to receive data. Unbalanced control: Polling Primary station requests secondary station to send data. Balanced control Two hybrid station may send commands or responses. Communication Networks - 10: Connection-Oriented PDNs 425 HDLC: Point-to-Point Topology HDLC Hybrid Station HDLC Hybrid Station Balanced link configuration Communication Networks - 10: Connection-Oriented PDNs 426 Prof. Jochen Seitz 10
HDLC: Point-to-Multipoint Topology HDLC Primary Station HDLC Secondary Station HDLC Secondary Station HDLC Secondary Station Unbalanced link configuration Communication Networks - 10: Connection-Oriented PDNs 427 HDLC: Operating Modes Normal Response Mode (NRM) Unbalanced configuration in which only the primary terminal may initiate data transfer. The secondary terminal transmits data only in response to commands from the primary terminal. The primary terminal polls the secondary terminal(s) to determine whether they have data to transmit, and then selects one to transmit. Asynchronous Response Mode (ARM) Unbalanced configuration in which secondary terminals may transmit without permission from the primary terminal. However, the primary terminal still retains responsibility for line initialization, error recovery, and logical disconnect. Asynchronous Balanced Mode (ABM) Balanced configuration in which either station may initiate the transmission. Communication Networks - 10: Connection-Oriented PDNs 428 Prof. Jochen Seitz 11
HDLC: Frame Format HDLC frame fields include: Flag: used to delimit the beginning and end of a packet Address: specifies the address of the intended packet recipient or sender Control: transports packet type, sequence numbers and retransmission requests Frame check: used for error checking. CRC- 16 or a 16-bit checksum may be used with HDLC frames Communication Networks - 10: Connection-Oriented PDNs 429 HDLC: Control Field 01111110 8 bit 8 bit n bit 16 bit 01111110 Frame Delimiter Address Control Information Frame Check Sequence Frame Delimiter 0 N(S) P/F N(R) I Frame [I=Information] 1 0 S S P/F N(R) S Frame [S=Supervisory] 1 1 M M P/F M M M U Frame [U=Unnumbered] Communication Networks - 10: Connection-Oriented PDNs 430 Prof. Jochen Seitz 12
HDLC: I Frame and S Frame I Frame Information Frame (both command or response) I (Information) 0 N(S) P/F N(R) S Frame Supervisory Frame (both command or response) containing sequence number RR (Receive Ready) RNR (Receive not ready) REJ (Reject) SREJ (Selective Reject) 1 0 S S P/F N(R) 00... RR 01... RNR 10... REJ 11... SREJ Communication Networks - 10: Connection-Oriented PDNs 431 HDLC: U Frame Unnumbered Commands SNRM... Set NRM SARM... Set ARM SABM... Set ABM DISC... Disconnect SNRME... Set extended NRM SARME... Set extended ARM SABME... Set extended ABM SIM... Set Initialization Mode UP... Unnumbered Poll UI... Unnumbered Information XID... Exchange Identification RSET... Reset TEST... Test Unnumbered Responses UA... Unnumbered Acknowledgment FRMR... Frame Reject DM... Disconnect Mode RIM... Request Initialization Mode RD... Request Disconnect UI... Unnumbered Information XID... Exchange Identification TEST... Test Communication Networks - 10: Connection-Oriented PDNs 432 Prof. Jochen Seitz 13
HDLC: MSC Notation: <Command/Response, [N(S),] P/F, N(R)> half duplex mode, Modulo 8, Window Size 7 Primary Station Set Normal Response Mode Transfer of 3 I Frames Secondary Station Acknowledgment Transfer of 2 I Frames incl. Acknowledgment Transfer of 7 Additional I Frames Then Awaiting Acknowledgement Data Transfer Ack without Data Communication Networks - 10: Connection-Oriented PDNs 433 X.25 Packet Switching Provides logical connections (virtual circuit or virtual connection) between subscribers All data in this connection form a single stream between the end stations Established permanently or on demand Communication Networks - 10: Connection-Oriented PDNs 434 Prof. Jochen Seitz 14
Virtual Circuits Call setup packets are used to establish virtual circuits; these are used to identify the (currently) best path to the destination across the network. Virtual circuit details are stored in virtual circuit tables at packet switches. The paths followed by packets in virtual circuits are called logical channels; each packet includes a logical channel number when created by the PAD. There are two major types of virtual circuits: Switched virtual circuits (SVCs): which are similar to temporary circuit-switched connections Permanent virtual circuits (PVCs): which is similar to a leased, circuit-switched connection Once a PVC is allocated, no call setup or call clearing is needed; the logical circuit is permanently stored in virtual circuit tables Communication Networks - 10: Connection-Oriented PDNs 435 X.25 Use of Virtual Circuits Communication Networks - 10: Connection-Oriented PDNs 436 Prof. Jochen Seitz 15
Time X.25: Packet Link Control Header Paketkopf (Packet Header) Start of Frame (Flag) Address Control GFI Logical Channel Packet Control Transmission Direction According to HDLC-LAPB According to X.25 User Information User Information According to higher layers and application Link Control Trailer GFI General Format Identifier Logical Channel Packet Control Frame Check Sequence End of Frame (Flag) Identifies packet format According to HDLC-LAPB Identifies the logical channel the packet belongs to Type of packet and additional control information Communication Networks - 10: Connection-Oriented PDNs 437 X.25: Packet Header General Format Identifier, GFI: Bit 5 and 6: Sequence numbering scheme: 01 = Modulo 8; 10 = Modulo 128 Bit 7 = Delivery Confirmation Bit Bit 8 = Data Qualifier Bit (for important control information) Bits 7 and 8 are only relevant for special packet types Logical Channel Identifier: Divided into group and number (which is not important ) 2 12 = 4096 logical channels might be simultaneously active on one subscriber line Packet Type Identification: Currently 22 different packet types Packet for data exchange: 1 st Bit in Octet 3 = 0 All other packets: 1 st Bit in Octet 3 = 1 Bit 8 7 6 5 4 3 2 1 Octet 1 Octet 2 GFI Q D 0 1 Logical Channel Group (LCG) Logische Channel Number (LCN) Octet 3 Packet Type Identification Communication Networks - 10: Connection-Oriented PDNs 438 Prof. Jochen Seitz 16
X.25: Encapsulation Data Layer 3 Packet Header Data Layer 2 Flag Address Control Information FCS Flag Layer 1 Bit stream according to X.21 Direction of Transmission Communication Networks - 10: Connection-Oriented PDNs 439 PDN Error Correction Processes PDNs employ node-to-node (aka hop-to-hop or point-to-point) error detection and correction Each packet is checked for errors at each packet switch before being forwarded to the next hop on its path If no errors are detected, an ACK is sent to the previous hop If errors are detected, a NAK is sent to the previous hop which triggers retransmission of the packet This process means that PDNs are store-and-forward networks; packets are stored at switching nodes until positive acknowledgements are received Communication Networks - 10: Connection-Oriented PDNs 440 Prof. Jochen Seitz 17
X.25: Network Access Protocol Packet multiplexing on layer 3 Packets are delivered in statistical time multiplex in demand 4096 different layer 3 virtual connections use one physical link and one HDLC connection Virtual connections are differentiated by the Logical Channel Identifier Packet multiplexing at the interface between DCE and DTE: N DTE A 1 1 4 3 2 1 3 Bidirectional Packet Flow Packet belonging to logical channel N Logical Channel 1 2 3 4 Virtual Connection between DTE A and... DTE B DTE C DTE D DTE E DCE Communication Networks - 10: Connection-Oriented PDNs 441 X.25: Logical Channels and Identifiers Logical Channel Identifiers LCIs group all packets that belong to a virtual X.25 connection on a network link Both directions use the same identifier LCIs are valid on a single link only The subscriber only knows the LCIs on the subscriber line LCIs are used for packet multiplexing On all links of a virtual connection, different LCIs are used All LCIs are independent of each other Communication Networks - 10: Connection-Oriented PDNs 442 Prof. Jochen Seitz 18
X25: Assignment of LCI Logical Channel Identifiers are assigned For each link independently During connection establishment by the initiating entity Call collision may occur Both entities connected via one link try to establish a new X.25 connection using the same LCI Call collision can be avoided using a special LCI allocation method Logical Channel Identifiers are grouped for Permanent Virtual Circuits (PVC) for incoming only Switched Virtual Circuits (SVC) for outgoing only Switched Virtual Circuits (SVC) for incoming and outgoing Switched Virtual Circuits (SVC) Communication Networks - 10: Connection-Oriented PDNs 443 X.25: Allocation of LCIs LCI - Logical Channel Identifiers Usage 0 not available for virtual connections 1... for Permanent Virtual Circuits (PVC)... - LIC Low Number Incoming Calls... HIC High Number Incoming Calls for incoming Switched Virtual Circuits (SVC)... - LTC Low Number Two Way Calls... HTC High Number Two Way Calls for both incoming and outgoing Switched Virtual Connections (SVC)... - LOC Low Number Outgoing Calls... HOC High Number Outgoing Calls... 4095 for outgoing Switched Virtual Circuits (SVC) - Currently not used LCIs Communication Networks - 10: Connection-Oriented PDNs 444 Prof. Jochen Seitz 19
X.25: Local Character of LCIs With virtual connections, complete addresses of sender and receiver have to be transmitted only during connection setup. Only current virtual connections get an LCI, which is valid only on one link. There is no relation between the network addresses of the DTEs and the LCIs to be used. Whenever a new virtual connection is setup, a currently unused (temporary) LCI is chosen. Thus, there may be several virtual connections between the same two DTEs. LCI 1 2... 4096 2100 logical 2900 channels... 4095 DTE A DTE/DCE Network 0 1 452 3732 4095 DTE/DCE DTE C 2 LCI 3... 2100 4096 2900 logical... 4095 channels DTE B Communication Networks - 10: Connection-Oriented PDNs 445 X.25: Packet Call Request Bit Octet 1 2 3 4 8 7 6 5 4 3 2 1 GFI 0 0 0 1 LCN LCG Packet Type 0 0 0 0 1 0 1 1 Address Length of Calling DTE Address Length of Called DTE Addresses of DTEs (calling, called) 0 0 0 0 0 0 Length of Performance Features Performance Features Short information of calling user Communication Networks - 10: Connection-Oriented PDNs 446 Prof. Jochen Seitz 20
X.25: Connection Setup (1) Connection setup at the calling DTE: Selection of a currently unused LCI according to allocation rules Creation of a Call Request packet: Entry of LCI Entry of (complete) DTE addresses Destination Address and Source Address Length of Address might go up to15 digits Entry of requested performance features, e.g.: Reverse Charging Acceptance Througout Class Transmission Rate Short User Data Communication Networks - 10: Connection-Oriented PDNs 447 X.25: Connection Setup (2) X.25 Call Request packet is encapsulated into an HDLC Information Frame. Packet is sent to the DCE, to which the initiating DTE is connected. DCE is forwarding the packet into the network, which is responsible for delivering it to the receiving DCE. The receiving DCE is transmitting an X.25 Incoming Call Packet with an accordingly selected LCI to the called DTE. Communication Networks - 10: Connection-Oriented PDNs 448 Prof. Jochen Seitz 21
X.25: Phases Three Phases: Connection Establishment Phase: finished with the arrival of a Call Connected Packet at the calling DTE. Data Phase: Full duplex, bidirectional data transmission. Disconnection Phase: finsihed with the arrival of a Clear Confirmation Packets at the DTE that wanted to disconnect. Communication Networks - 10: Connection-Oriented PDNs 449 X.25: Message Sequence Chart Interface of calling DTE/DCE Interface of called DTE/DCE Call Request Packet Incoming Call Packet Call Establishment Phase Call Accepted Packet Establishment Phase Connected Packet Data Phase Disconnection Phase Data Packet Data Packet Data Packet Clear Indication Packet Clear Confirmation Packet Data Packet Data Packet Data Packet Clear Request Packet Clear Confirmation Packet Data Phase Disconnection Phase The arrival of a Clear Confirmation packet at the called entity might not necessarily be the result of a Clear Confirmation packet of the calling entity Communication Networks - 10: Connection-Oriented PDNs 450 Prof. Jochen Seitz 22
X.25: Data Packet Sequence numbers might be either Modulo 8 or Modulo 128 Virtual connections are always end-to-end: Sequence numbers are used for sequence guarantees and acknowledgements just like in HDLC, but here, these numbers are used end-to-end and not only on one link Control information of the data packet: P(S): Packet Send Sequence Number P(R): Packet Receive Sequence Number M-Bit (More Data Bit): Several packets belong to the same context E.g. a segmented transport layer PDU Communication Networks - 10: Connection-Oriented PDNs 451 X.25: Data Packet Sequence numbers are Modulo 8: Bit 8 7 6 5 4 3 2 1 Octet 1 2 3 4... GFI Q D 0 1 LCG LCN P(R) M P(S) 0 User Information (e.g. transport layer PDU) Communication Networks - 10: Connection-Oriented PDNs 452 Prof. Jochen Seitz 23
Issues with X.25 key features include: call control packets, in band signaling multiplexing of virtual circuits at layer 3 layers 2 and 3 include flow and error control hence have considerable overhead not appropriate for modern digital systems with high reliability Communication Networks - 10: Connection-Oriented PDNs 453 Important X.25 PDN Standards X.25: defines interface between DTE and DCE in public data networks X.21: specifies the interface between user terminal equipment and PDN packet-switching nodes X.3: specifies packet assembly/disassembly processes X.28: governs asynchronous dial-up access to PDNs X.29: governs synchronous dial-up access to PDNs X.75: defines the interface between different public packet-switching networks, both domestic and international X.121: defines a global addressing scheme for PDNs Communication Networks - 10: Connection-Oriented PDNs 454 Prof. Jochen Seitz 24
Advancement: Frame Relay Influenced by: Requests for higher throughput / bandwidth Lower error rates on the physical medium supersedes extensive error detection and correction mechanisms as in X.25 Standardization: Initial proposals for the standardization of Frame Relay were presented to the Consultative Committee on International Telephone and Telegraph (CCITT) in 1984 ITU-T Standard I.122: Packet Mode Bearer Services for ISDN In 1990, Cisco, Digital Equipment Corporation (DEC), Northern Telecom, and StrataCom formed a consortium to focus on Frame Relay technology development: Frame Relay Specification with Extensions Establishment of Frame Relay Forum, which is now the Broadband Forum (http://www.broadband-forum.org/) Communication Networks - 10: Connection-Oriented PDNs 455 Frame Relay: Frame New HDLC Variant: LAPF (Link Access Procedure, Frame Mode) Frame Header Flag Information FCS Flag DLCI EA C/R DLCI EA DE BECN FECN 8 7 6 5 4 3 2 1 8 7 6 5 4 3 2 1 Bit DLCI = Data Link Connection Identifier C/R = Command/Response Field FECN = Forward Explicit Congestion Notification BECN = Backward Explicit Congestion Notification DE = Discard Eligibility Indicator EA = Address Field Extension Bit Communication Networks - 10: Connection-Oriented PDNs 456 Prof. Jochen Seitz 25
Frame Relay: Topology LAN 4 Router B Frame Relay-Network C Router LAN 3 A Router LAN 1 DLCI 3 Router LAN 2 Communication Networks - 10: Connection-Oriented PDNs 457 Frame Relay: Explicit Congestion Notification Congestion avoidance policy: Forward Explicit Congestion Notification The FECN bit can be set to 1 to indicate that congestion was experienced in the direction of the frame transmission, so it informs the destination that congestion has occurred Backwards Explicit Congestion Notification The BECN bit can be set to 1 to indicate that congestion was experienced in the network in the direction opposite of the frame transmission, so it informs the sender that congestion has occurred. Communication Networks - 10: Connection-Oriented PDNs 458 Prof. Jochen Seitz 26
Comparison Frame Relay X.25 Througput Latency Jitter Error Detection & Correction Functionality X.25 slow (200 kbit/s 2 Mbit/s) high No countermeasure Special mechanisms (retransmission on layer 2 and 3) complex Frame Relay fast (2Mbit/s typically, 45 Mbit/s max) low (due to less overhead) No countermeasure Responsibility of higher layers simple Frame Relay discards frames (in congestion or with wrong FCS) higher layers have to correct this problem. Isochronous transmission is not possible with both protocols. Communication Networks - 10: Connection-Oriented PDNs 459 Comparison Frame Relay X.25 Taken from The Basic Guide to Frame Relay Networking (Frame Relay Forum, http://www.frforum.com) Communication Networks - 10: Connection-Oriented PDNs 460 Prof. Jochen Seitz 27
Broadband Applications According to ITU-T Interactive Services Conversational Services e.g. video conferencing, interactive work on shared documents Messaging Services e.g. transfer of multimedia messages (videos, high resolution pictures) Retrieval Services e.g. video on demand, access to high resolution pictures Distribution Services... without User Individual Presentation Control (Broadcast Services) e.g. television broadcast... with User Individual Presentation Control e.g. Broadband videotext Communication Networks - 10: Connection-Oriented PDNs 461 Broadband Traffic STM Synchronous Transfer Mode Isochronous data traffic (constant data rate and constant latency) Example: Narrowband-ISDN Circuit-switched communication channels ATM Asynchronous Transfer Mode Transport of small data packets (ATM cells) Data rate and latency are variable, but guarantees are possible Packet-switched communication (connection-oriented: virtual channels) Communication Networks - 10: Connection-Oriented PDNs 462 Prof. Jochen Seitz 28
ATM: Principle Technology for backbone networks Statistical (asynchronous, demand-driven) time multiplex (ATDM) Cell header includes identifier of virtual connection Mixing of different cell rates possible Different throughput for different virtual connections Support of variable data rates: Guaranteed base rate Bursty traffic possible, if there are enough resources ATM cell: Cell Header User Data 5 Octets 48 Octets Communication Networks - 10: Connection-Oriented PDNs 463 ATM: Multiplexing Advantages of small size of ATM cells: Easy Multiplexing of different streams Support for different bit rates Support of different services Easy processing of ATM cells: High data rates Complex end systems, network equipment can be kept simple ATM ist a fundamental concept, on which different networks can be based. Constant Bit Rate Variable Bit Rate 155 Mbit/s Communication Networks - 10: Connection-Oriented PDNs 464 Prof. Jochen Seitz 29
ATM: Cell Header (UNI) GFC (Generic Flow Control): local control functions such as identification of several terminals connected to the same ATM interface VPI, VCI (see next slide) PT (Payload Type): type of data in the information field (user data or control data) CLP (Cell Loss Priority): determines whether a cell can be preferentially deleted or not in case of a transmission bottleneck. Cells with CLP-0 have higher priority than cells with CLP-1. HEC (Header Error Control) At NNI, the VPI field is 4 bits longer, as the GFC is not needed there Header User Information 5 Bytes 48 Bytes GFC VPI VPI VCI VCI VCI PT CLP HEC 1 2 3 4 5 [Byte] Communication Networks - 10: Connection-Oriented PDNs 465 ATM: Virtual Channel and Virtual Connection VC VC... VC VP VP Transmission Path Definition of two different connection types: Virtual Channel (VC): unidirectional virtual connection: Virtual Path (VP): Bundle of virtual channels with the same end points Accordingly identified in the cell header: Virtual Channel Identifier (VCI) Virtual Path Identifier (VPI) Communication Networks - 10: Connection-Oriented PDNs 466 Prof. Jochen Seitz 30
ATM: Traffic Engineering CBR (constant bit rate): a peak cell rate (PCR) is specified, which is constant, and a maximum jitter value VBR (variable bit rate): an average cell rate is specified, which can peak at a certain level for a maximum interval before being problematic (typical e.g. for MPEG streams) ABR (available bit rate): a minimum guaranteed rate is specified; if the rate cannot be sustained, the sender is notified UBR (unspecified bit rate): no guarantees, just best effort transmission ABR VBR CBR UBR t Communication Networks - 10: Connection-Oriented PDNs 467 ATM and AAL End System A AAL Service-dependent AAL connections End System B AAL ATM physical layer Service-independent ATM connections ATM physical layer Application ATM-Network ATM Layer: service-independent transport of ATM cells, multiplexing and demultiplexing AAL Layer: ATM Adaptation Layer support of different services Communication Networks - 10: Connection-Oriented PDNs 468 Prof. Jochen Seitz 31
ATM-Networks and Broadband- ISDN ATM-Networks provide data transmission services For realtime applications For applications with variable bit rates Broadband-ISDN is realized as an ATM-network in Germany: T-Net-ATM ATM is also applied in broaband data networks with high bit rates ATM provides the infrastructure for IP networks LANE (LAN Emulation) CIP (Classical IP over ATM) MPOA (Multi-Protocol over ATM) Standardization: ITU-T, ATM-Forum, IETF Communication Networks - 10: Connection-Oriented PDNs 469 Adaptation of Data Units of Tele Service and Bearer Service Application (Layers 4-7) Tele Service Adaptation of Data Units Bearer Service (Layers 1-3) Communication Networks - 10: Connection-Oriented PDNs 470 Prof. Jochen Seitz 32
ATM: Support of Voice and Data Services Services packets bitstream packets bitstream ATM Adaptation Layer cells cells cells cells ATM-Switch ATM subnetwork Communication Networks - 10: Connection-Oriented PDNs 471 B-ISDN Reference Model (I) 3-dimensional reference model Three vertical planes User Plane Control Plane Management Plane Three hierarchical Layers Physical Layer ATM Layer ATM Adaptation Layer Signaling information is transfered separately from the user information (Out-of- Band-Signalling) Control Plane Higher Layers ATM Adaptation Layer ATM Layer Management Plane User Plane Layer Management Plane Management Physical Layer Layers Planes Communication Networks - 10: Connection-Oriented PDNs 472 Prof. Jochen Seitz 33
B-ISDN Reference Model (II) Management plane Control plane User plane Higher layers Signaling Class A Class B Class C Class D ATM adaptation layer Signaling AAL AAL1 AAL2 AAL3/4 or AAL5 ATM layer Physical layer Communication Networks - 10: Connection-Oriented PDNs 473 ATM Adaptation Layer AAL Different AAL for different service types AAL0 : empty AAL = no adaptation required AAL consist of sublayers Segmentation and Reassembly Sublayer (SAR-Sublayer) Convergence Sublayer (CS) Common Part Convergence Sublayer (CPCS) Service Specific Convergence Sublayer (SSCS) Communication Networks - 10: Connection-Oriented PDNs 474 Prof. Jochen Seitz 34
B-ISDN Reference Configuration S B T B U B V B B-TE1 B-NT2 B-NT1 B-LT B-ET TE2 oder B-TE2 R B-TA Functional groups: B-ET B-ISDN Exchange Termination B-LT B-ISDN Line Termination B-NT1,2 B-ISDN Network Termination B-TA B-ISDN Terminal Adaptor B-TE1 B-ISDN Terminal B-TE2 Broadband-Terminal or Network TE2 ISDN-Terminal or -DTE Communication Networks - 10: Connection-Oriented PDNs 475 References KASERA, S.: ATM Networks: Concepts and Protocols. New York: Irwin/Mcgraw Hill, 2006. ISBN 978-0071477321. STALLINGS, W.: Business Data Communications. 6th edition, New York: Prentice Hall, 2009, ISBN 978-0136065432. STALLINGS, W.: Data and Computer Communications. 8th edition, New York: Prentice Hall, 2008, ISBN 978-0135071397. STALLINGS, W.: ISDN and Broadband ISDN with Frame Relay and ATM. New York: Prentice Hall, 1998. ISBN 978-0139737442. STAMPER, D.A.; CASE, T.L.; DORNAN, A.: Business Data Communications. 6th edition, New York: Prentice Hall, 2002, ISBN 978-0130094285. Communication Networks - 10: Connection-Oriented PDNs 476 Prof. Jochen Seitz 35