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[11] Nu P 07 1

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  • 1. Protocolls of the OSI-layer 2 Link Layer MAC (Medium Access Control) Kapitel 7.1 Netze und Protokolle Dr.-Ing. J. Steuer Institut für Kommunikationstechnik www.ikt.uni-hannover.de Literatur: [Boss99] M.Bossert, M.Breitenbach, Digitale Netze, 1999, B.G. Teubner, Stuttgart, Leipzig, ISBN 3-519-06191-0 [Come98] D. Comer, Computernetzwerke und Internets, Prentice Hall, 1998, ISBN3-8272- 9552-1, 2001: ISBN 3-8273-7023 [Coul02] Couloris et.al.; „Verteilte Systeme“, Addison- Wesley, 2002, ISBN 3-8273-7022-1 [Hals96] F.Halshall, „Data Communications, Computer Networks and Open Systems“, 4th edition, Edison-Wesley, 1996, ISBN 0-201-42293-X [Kann] Kanbach, Körber, ISDN - Die Technik, Hüthig-Verlag [Reim95] Reimers, et.al.; Digitale Fernsehtechnik, Springer Verlag, 1995, ISBN 3-540- 58993-7 [Sieg99] Gerd Siegmund,“Technik der Netze“, 4.Auflage, Hüthig Verlag, Heidelberg, 1999, ISBN 3-7785-2637-5 [Spra91] J.D.Spragins,et.all, Telecommunications Protocols and Design, Addison Wesley Publishing Company, 1991, ISBN 0-201-09290-5 [Stall90] William Stallings, Local and Metropolitan Area Networks, 1990; MacMillen Publishing Company, ISBN 0-02-415465-2 [WAL02(1)] Walke, B., „Mobilkommunikation, Band1“, Teubner Verlag, 2002 © UNI Hannover, Institut für Allgemeine Nachrichtentechnik
  • 2. Goals understand the need for MAC (media access control) understand the most important MAC strategies for ISDN, data networks and mobile networks classify different MAC-strategies evaluate the performance of different MAC-strategies (2) The medium access control (MAC) is mandatory in case of shared usage of a media. A media could be a twisted pair, an optical fibre, the spectrum in the air (air interface), or in general any media which is able to transport communication content. Only in case one user is using one medium in one direction only, the MAC is not necessary. It could be implemented, but it need not. The control of the usage of the channel (media) could be left to the subscriber. Already in case of bidirectional usage a scheduler has to control the access from both ends of the communication. This scheduler can be simple, because one station could be declared as master. The master is responsible for the scheduling. This is a type of central control. Imagine the Master has not only to control one station but several. The master is a little bit more busy, but with sufficient memory and computing power he will be able to handle the situation. There is no principle difficulty in such centralized control task. It gets much more complicated in case there is no knowledge at the master on the states of the slaves. A protocol handling such scenario needs to operate on guesses on the behavior of the other stations competing for the usage of the medium. There will be situations of conflict, which means more than one station is accessing the medium at the same time. The result will be a corruption of messages. This situation is called decentralized and uncoordinated. The protocol must be able to detect such corruptions and to react on that, in order to allow for a proper communication. With all these issues the MAC has to deal. In this chapter we will study the principal MAC properties for different media. © UNI Hannover, Institut für Allgemeine Nachrichtentechnik
  • 3. need for MAC? Competition for the usage of a transmission medium limited to the S0-bus ISDN Sobus: signalling channel NT 4 wire Why is in the case of ISDN only the access to the signalling channel Local Area Network in competition? 802.3, coaxial cable, (LAN) with shared max 10Mbit/s (5Mbit/s) LAN media, e.g. Ethernet (3) There are networks for which many users share a common channel using a multi access scheme. We find the examples on the signaling channel for the ISDN S0 bus (in this case not for the communication channel, which is controlled by by the switch and the switch knows the status of the users and channels) on Local Area Networks (LAN) (note: latest developments in LAN´s go back to dedicated channels, using switching technologies) mobile networks packet radio networks Adhoc-networks satellite networks © UNI Hannover, Institut für Allgemeine Nachrichtentechnik
  • 4. need for MAC? Competition for the usage of a transmission medium during registration phases of mobile stations (MS) mobile technologies: DECT, GSM, IS95, HSCSD, GPRS,UMTS (4) In case the MS (mobile station) is switched of, there is no knowledge in the Basestation or the Basestation controller on this subscriber. Thus there can be no dedicated channel for this subscriber. If the MS is switched on, it needs first to access a common channel and apply for a dedicated channel. The common channel is used in competition to other subscribers in the same state. Consequently we need a media access control. Abbreviations: DECT: digital enhanced cordless telephony (ETSI and ECMA standard) (see [WAL02(2)]) GSM: global system mobile (ETSI standard) (see [WAL02(1)]) IS95: Interim Standard 1995 of the Telecommunications Industry Association (TIA) USA for the first CDMA mobile system (see [WAL02(1)]) , compare ZVEI in Germany (see introduction NUP, standards) HSCSD: High Speed Circuit Switched Data (channel trunking in GSM to increase the channel capacity in SM for Data) (see [WAL02(1)]) GPRS: General Packet Radio System (packet switched enhancement of GSM) (see [WAL02(1)]) UMTS: Universal Mobile Telecommunication System (expected successor of GSM) (see [WAL02(1)]) © UNI Hannover, Institut für Allgemeine Nachrichtentechnik
  • 5. need for MAC? Station 1 Station 3 Station 5 Station 2 Station 4 The common media is the air interface, all members Basic Service Set (IEEE 802.11) of the AdHoc network are W-LAN, AdHoc-Network Competing at least during the registration process (5) In many cases, a network is installed using an infrastructure. This infrastructure takes over some central tasks and serves as an access to the wired network. In HIPERLAN/2, the Access Points (AP) take over this task. They have a wired as well as a wireless interfaces. By the way, there are a number of different technologies and protocols which could serve as wireless LAN: HIPERLAN (High PErformance Radio Local Area Network) of ETSI [WAL02(2)]) WLAN (Wireless LAN) of IEEE (802.11x) [WAL02(2)]) Bluetooth: short range data link or network and even others which partly show functionalities of LAN´s (high speed data transport in the access area): UMTS (Universal Mobile Telephone System) [WAL02(1)]) DAB (Digital Audio Broadcast) DVB (Digital Video Broadcasting) [Reim95] During this lecture we are going to concentrate on HIPERLAN due to its enhanced functionality compared to wired LAN´s. This stands also for the WLAN, but the WLAN is an American standard with some drawbacks in its functionality which is compensated by its better position in the market. The coverage area of an AP and its associated terminals is called radio cell in general. In the case of IEEE 802.11 (see below), it is called a basic service set, in Bluetooth a scatternet (to scatter=umherstreuen; the terminals are scattered in the area of coverage served by an access point). © UNI Hannover, Institut für Allgemeine Nachrichtentechnik
  • 6. need for MAC? Access Point A Access Point B Station A1 Station B1 Station A1 Station B3 Station A2 Station B2 The common media is the air Radio Cell (HIPERLAN) interface, in addition we have Basic Service Set (IEEE 802.11) Influences between the cells! (6) Wireless LAN´s can be grouped to cells or basic service sets, which are comparable to the cells of cellular telephony networks. In the HIPERLAN-System the MAC is of the scheduled type. The Access Point serves as scheduler. This scheme allows a better performance for high traffic loads. The W-LAN system in contrast operates in a random mode like the Ethernet, which is sufficient for low traffic loads. In principle it can be expected that data terminals will at least from time to time carry high traffic load. This forced the Europeans (ETSI, HIPERLAN) to deviate from the american (IEEE802.11, W-LAN) approach. The traffic channels in the HIPERLAN-scheme are dedicated to connections and thus terminals. They do not need MAC. Bat the traffic channels are assigned after request. During the request phase the data terminals compete for transmission capacity, therefore this phase is handled by a MAC protocol. © UNI Hannover, Institut für Allgemeine Nachrichtentechnik
  • 7. when is the MAC function not needed? (7) © UNI Hannover, Institut für Allgemeine Nachrichtentechnik
  • 8. influences on the MAC distance of stations to each other (power and delay) visibility of stations (SNR, Signal to Noise Ratio)) throughput of the network, e.g. in Kbit/s fairness of the usage of the transmission media by competing terminals transmission speed of channels burstiness of traffic packet length (8) The distance between stations influences the ability to detect collisions, e.g. the IEEE802.3 is limited to 2500m (collision domain) Systems with radio connections need to „see“ each other in order to be able to detect collisions The required throughput of the network influences the complexity of the MAC. If the throughput is high compared to the capacity we need to establish MAC protocols with high efficiency; if the throughput is low in contrast to the capacity we can allow for a less efficient MAC. This consideration led to the application of the slotted ALOHA for the access to the RACH (Random Access Channel) of the GSM system. This channel is used only for the request from the mobile Station to the network in order to apply for dedicated channels. The volume of the signaling is low. Thus we can allow for a low efficiency. Similar thoughts let us find that a bulk data transfer on a Local Area Network (LAN) needs a MAC which minimizes the collisions. If we stick to the random MAC than CSMA/CD is the choice. Fairness is often an opposite requirement to throughput. A fair MAC allows for each traffic source on average the same transmission capacity. Even if we implement no priorities fairness is difficult to establish. The MAC for the Dual Queueing Distributed Bus - a Metropolitan Area Network (IEEE 802.6) - tried to establish fairness. The DQDB is a very good subject for study purposes, but it did not gain an important position in the market. The transmission speed of the channels dictate the time available in the protocol stack. The higher the transmission speed, the more critical is the time spent in the stack. Take the ATM Protocoll with a targeted speed of 2.5Gbit/s. There is not much time left to perform complicated operations in the stack. Thus the MAC has to be extremly time efficient. A high burstiness of traffic generates a high peak load in the stack. This effects the stack in the same mannor than a high transnmission speed. Long packets put less burdon on the stack compared to short packets © UNI Hannover, Institut für Allgemeine Nachrichtentechnik
  • 9. MAC principles Scheduling vs. Random access: scheduling means stations ready to send are waiting until it is their turn to operate Under the random access scheme a station tries to access the transmission media as soon as it has to send something (immediately) Discuss differences with respect to efficiency Random Scheduler Access (9) © UNI Hannover, Institut für Allgemeine Nachrichtentechnik
  • 10. MAC principles Scheduling random access Scheduling random access without with sensing fixed demand without with sensing fixed demand sensing(ALOHA) (CS) assignment assignment sensing(ALOHA) (CS) assignment assignment central distributed central distributed before before & during pure slotted control control before before & during pure slotted control control ALOHA ALOHA transmission transmission ALOHA ALOHA transmission transmission (CSMA) (CSMA/CD) (CSMA) (CSMA/CD) How does the channel selection and the channel assignment of a PDH-, SDH-, ISDN- and Ethernet-LAN-system fit into the above scheme? (10) © UNI Hannover, Institut für Allgemeine Nachrichtentechnik
  • 11. MAC principles (Examples) Scheduling random access Scheduling random access without with sensing fixed demand without with sensing fixed demand sensing(ALOHA) (CS) assignments assignment sensing(ALOHA) (CS) assignments assignment central distributed central distributed before before & during pure slotted control control before before & during pure slotted control control ALOHA ALOHA transmission transmission ALOHA ALOHA transmission transmission (CSMA) (CSMA/CD) (CSMA) (CSMA/CD) • TDMA/reservation • slotted ALOHA: • Polling GSM, Random Access Channel (RACH) • Token Passing • CSMA/CD: Ethernet (11) © UNI Hannover, Institut für Allgemeine Nachrichtentechnik
  • 12. TDMA (Time Division Multiple Access) frame i frame i+1 frame i+2 time 1 guard time station 1 station 2 station m-2 station m-1 station m control data data data data data time 2 no packet ready? control information? yes guard time? wait for advantages and disadvantages? assigned slot transmit packet (12) © UNI Hannover, Institut für Allgemeine Nachrichtentechnik
  • 13. Reservation in TDMA frame i frame i+1 frame i+2 time 1 guard time n<m slot 1 slot 2 slot n-2 slot n-1 slot n control data data data data data time 2 no packet ready? advantages and disadvantages? yes • Number of stations is not limited sharply reservation and waiting • traffic behaviour as pure TDMA forassigned slot • reservation has to be performed by signalling transmit (not shown here) packet Application: HIPERLAN (13) © UNI Hannover, Institut für Allgemeine Nachrichtentechnik
  • 14. Polling scheduling with central control station 11 station k station R station station k station R message closed message closed by go ahead from by go ahead from station k station 1 poll poll message closed poll station m station k by go ahead from station 1 central controller station m central controller • efficient with high load from all stations • inefficient if stations have low or zero load • half duplex links need resynch, full duplex links can stay synchronized (14) © UNI Hannover, Institut für Allgemeine Nachrichtentechnik
  • 15. Token passing (scheduling with distributed control) station 11 station station rr station 22 station station station kk station • transmission medium of ring (IEEE 802.5) or bus (IEEE 802.4) type [logically the bus behaves like a ring • a token (packet with permission to send) is handed from station to station • all stations read all packets • all packets are circulating on the ring and have to be removed after one turn • the packet control requires receiving and sending of all packets in all stations, which requires highly reliable stations and adds delay in each station • the token is a distinctive bit pattern, how can the transmission be data transparent? • if no station has to transmit, a free token is circulating. What happens if a bit error occurs? • priority handling is possible (15) © UNI Hannover, Institut für Allgemeine Nachrichtentechnik
  • 16. Random Access with no sensing (ALOHA and slotted ALOHA) shared transmission medium: station 11 station station rr station 22 station station or station kk station yes wait two way no yes delay to start of propagation time transmit acknoledgement? packet to transmit? next slot quantized to slot times no delay k packets compute random transmission time backoff integer k Discuss high load scenario! Do you see the advantage of the slotted ALOHA? (16) The slotted ALOHA- mechanismn is implemented in the registratition phase of the GSM system. © UNI Hannover, Institut für Allgemeine Nachrichtentechnik
  • 17. Random Access with sensing Carrier Sense with Multiple Access (CSMA) shared transmission medium: station 11 station station rr station 22 station station station kk station yes Medium wait two way no yes carrier is free delay to start of propagation time transmit acknowledgement? packet to transmit? sense quantized to next slot strategy slot times no Medium is occupied delay k packets compute random transmission time backoff integer k (17) © UNI Hannover, Institut für Allgemeine Nachrichtentechnik
  • 18. Random Access with sensing before and during transmission /Collision Detection (CSMA/CD) shared transmission medium: station 11 station station rr station 22 station station Implemented in IEEE 802.3 station kk station no yes no Collision detected/ carrier transmit packet to transmit? jamming received? sense strategy yes Medium is occupied delay k packets transmit yamming compute random abort transmission transmission time Signal* backoff integer k * Only in case of collision detected (18) © UNI Hannover, Institut für Allgemeine Nachrichtentechnik
  • 19. MAC of the ISDN S0-Bus Transmission in frames on S0 Activation procedure in the physical layer Frame synchronization in the MAC layer Frame recognition by violation of code rules Distributed MAC for the d-channel only MAC for the user channel by framing S0-bus ISDN Sobus: NT 4 wire (19) © UNI Hannover, Institut für Allgemeine Nachrichtentechnik
  • 20. S0-Frame (2) Leitungscodierung S0 AMI-Code 01011100 + + - - inv. AMI-Code 01011100 + + - - (20) AMI - Alternate Mark Inversion: Eine Markierung (1) wird abwechselnd mit positivem bzw. negativem Impuls dargestellt. Pseudoternärer Code: zwei logische Zustände (Null, Eins) werden auf 3 physikalische Zustände abgebildet (pos. Impuls +, kein Impuls 0, neg. Impuls -) Am ISDN-Basisanschluß eingesetzt wird ein invertierter AMI-Code: nicht die (1), sondern die (0) wird abwechselnd mit positivem bzw. negativem Impuls dargestellt. © UNI Hannover, Institut für Allgemeine Nachrichtentechnik
  • 21. S0-Frame (3) 2 bit offset between frame from NT TE and NT TE in order to allow the terminals to read the e-bit before they write the next d-bit 48 bits in 250 microseconds NT TE D L. F L. B1B1B1B1B1B1B1B1 E D A FA N B2B2B2B2B2B2B2B2 E D M B1B1B1B1B1B1B1B1 E D S B2B2B2B2B2B2B2B2 E D L. F L. 0 1 0 2 bits offs et TE NT D L. F L. B1B1B1B1B1 B1B1B1 L. D L. FA L. B2B2 B2 B2 B2 B2B2B2 L. D L. B1 B1B1 B1B1B1 B1 B1 L. D L. B2 B2 B2 B2 B2 B2 B2 B2 L. D L. F L. t A Aktivierungsbit FA Hilfsrahmenbit . an diesen Stellen ist der B1 Bit des 1. B-Kanals L DC-Ausgleichbit Code gleichanteilsfrei How is a collision B2 Bit des 2. B-Kanals M Multiframingbit = 0 D Bit des D-Kanals N = FA detected? E Bit des Echokanals S Spare = 0 F Rahmenbit (21) Diagramm zeigt die möglichen physikalischen Zustände, die ein Bit im Rahmen annehmen kann (Null als positiver oder negativer Impuls, Eins als kein Impuls). 2 Bit Versatz zwischen den Rahmen (E-Bit muß angekommen sein, bevor D-Kanal-Bits wieder gesendet werden dürfen => Zugriffssteuerung) Zusammenfassung von 2 Abtastperioden (je 125µs) in einem S0-Rahmen (250 µs) 48bit/250µsec = 192 kbit/s Datenrate je Richtung auf S0 Echokanal zur D-Kanal-Zugriffsteuerung: NT spiegelt das zuletzt vom im D-Kanal empfangenen Bits im nächsten Echo-Kanal zurück. Ausgleichsbits zur Herstellung der Gleichanteilsfreiheit nach jeden logischen Kanal in Richtung NT Coderegelverletzung am Beginn eines jeden Rahmen wie folgt festgelegt: 1.Coderegelverletzung durch F-Bit (+,Verletzung mit letztem D- bzw. L-Bit); nächste Coderegelverletzung zwischen L-Bit(-) und (spätestens) FA-Bit (-). A-Bit (Bus aktiviert) Literatur: [Kann] © UNI Hannover, Institut für Allgemeine Nachrichtentechnik
  • 22. S0-Frame (4) Why is there no e-bit for the other direction in the Frame from TE NT? 48 bits in 250 microseconds NT TE D L. F L. B1B1B1B1B1B1B1B1 E D A FA N B2B2B2B2B2B2B2B2 E D M B1B1B1B1B1B1B1B1 E D S B2B2B2B2B2B2B2B2 E D L. F L. 0 1 0 2 bits offs et TE NT D L. F L. B1B1B1B1B1 B1B1B1 L. D L. FA L. B2B2 B2 B2 B2 B2B2B2 L. D L. B1 B1B1 B1B1B1 B1 B1 L. D L. B2 B2 B2 B2 B2 B2 B2 B2 L. D L. F L. t A Aktivierungsbit FA Hilfsrahmenbit . an diesen Stellen ist der B1 Bit des 1. B-Kanals L DC-Ausgleichbit Code gleichanteilsfrei B2 Bit des 2. B-Kanals M Multiframingbit = 0 D Bit des D-Kanals N = FA E Bit des Echokanals S Spare = 0 F Rahmenbit (22) Diagramm zeigt die möglichen physikalischen Zustände, die ein Bit im Rahmen annehmen kann (Null als positiver oder negativer Impuls, Eins als kein Impuls). 2 Bit Versatz zwischen den Rahmen (E-Bit muß angekommen sein, bevor D-Kanal-Bits wieder gesendet werden dürfen => Zugriffssteuerung) Zusammenfassung von 2 Abtastperioden (je 125µs) in einem S0-Rahmen (250 µs) 48bit/250µsec = 192 kbit/s Datenrate je Richtung auf S0 Echokanal zur D-Kanal-Zugriffsteuerung: NT spiegelt das zuletzt vom im D-Kanal empfangenen Bits im nächsten Echo-Kanal zurück. Ausgleichsbits zur Herstellung der Gleichanteilsfreiheit nach jeden logischen Kanal in Richtung NT Coderegelverletzung am Beginn eines jeden Rahmen wie folgt festgelegt: 1.Coderegelverletzung durch F-Bit (+,Verletzung mit letztem D- bzw. L-Bit); nächste Coderegelverletzung zwischen L-Bit(-) und (spätestens) FA-Bit (-). A-Bit (Bus aktiviert) Literatur: [Kann] © UNI Hannover, Institut für Allgemeine Nachrichtentechnik
  • 23. S0-Frame - frame detection Case 1: d-bit from the NT at the end of the frame will be „0“ and negative, the L-bit will be „0“ and positive in order to compensate. The F-bit will be „0“ and positive in in 250 microseconds in conflict with the coding rule. 48 bits order to be NT TE D L. F L. B1B1B1B1B1B1B1B1 E D A FA N B2B2B2B2B2B2B2B2 E D M B1B1B1B1B1B1B1B1 E D S B2B2B2B2B2B2B2B2 E D L. F L. 0 1 0 2 bits offs et TE NT D L. F L. B1B1B1B1 B1 B1 B1 B1 L. D L. FA L. B2B2 B2 B2 B2 B2B2 B2 L. D L. B1 B1 B1 B1 B1B1B1 B1 L. D L. B2 B2 B2 B2 B2 B2B2 B2 L. D L. F L. t A Aktivierungsbit FA Hilfsrahmenbit . an diesen Stellen ist der B1 Bit des 1. B-Kanals L DC-Ausgleichbit Code gleichanteilsfrei B2 Bit des 2. B-Kanals M Multiframingbit = 0 D Bit des D-Kanals N = FA E Bit des Echokanals S Spare = 0 F Rahmenbit (23) Diagramm zeigt die möglichen physikalischen Zustände, die ein Bit im Rahmen annehmen kann (Null als positiver oder negativer Impuls, Eins als kein Impuls). 2 Bit Versatz zwischen den Rahmen (E-Bit muß angekommen sein, bevor D-Kanal-Bits wieder gesendet werden dürfen => Zugriffssteuerung) Zusammenfassung von 2 Abtastperioden (je 125µs) in einem S0-Rahmen (250 µs) 48bit/250µsec = 192 kbit/s Datenrate je Richtung auf S0 Echokanal zur D-Kanal-Zugriffsteuerung: NT spiegelt das zuletzt vom im D-Kanal empfangenen Bits im nächsten Echo-Kanal zurück. Ausgleichsbits zur Herstellung der Gleichanteilsfreiheit nach jeden logischen Kanal in Richtung NT Coderegelverletzung am Beginn eines jeden Rahmen wie folgt festgelegt: 1.Coderegelverletzung durch F-Bit (+,Verletzung mit letztem D- bzw. L-Bit); nächste Coderegelverletzung zwischen L-Bit(-) und (spätestens) FA-Bit (-). A-Bit (Bus aktiviert) Literatur: [Kann] © UNI Hannover, Institut für Allgemeine Nachrichtentechnik
  • 24. S0-Frame - frame detection Case 2: d-bit from the NT at the end of the frame will be „0“ and positive, the L-bit will be „1“in order to compensate. The F-bit will be „0“ and positive in in 250 microseconds in conflict with the coding rule. 48 bits order to be NT TE D L. F L. B1B1B1B1B1B1B1B1 E D A FA N B2B2B2B2B2B2B2B2 E D M B1B1B1B1B1B1B1B1 E D S B2B2B2B2B2B2B2B2 E D L. F L. 0 1 0 2 bits offs et TE NT D L. F L. B1B1B1B1B1 B1B1B1 L. D L. FA L. B2B2 B2 B2 B2 B2B2B2 L. D L. B1 B1B1 B1B1B1 B1 B1 L. D L. B2 B2 B2 B2 B2 B2 B2 B2 L. D L. F L. t A Aktivierungsbit FA Hilfsrahmenbit . an diesen Stellen ist der B1 Bit des 1. B-Kanals L DC-Ausgleichbit Code gleichanteilsfrei B2 Bit des 2. B-Kanals M Multiframingbit = 0 D Bit des D-Kanals N = FA E Bit des Echokanals S Spare = 0 F Rahmenbit (24) Diagramm zeigt die möglichen physikalischen Zustände, die ein Bit im Rahmen annehmen kann (Null als positiver oder negativer Impuls, Eins als kein Impuls). 2 Bit Versatz zwischen den Rahmen (E-Bit muß angekommen sein, bevor D-Kanal-Bits wieder gesendet werden dürfen => Zugriffssteuerung) Zusammenfassung von 2 Abtastperioden (je 125µs) in einem S0-Rahmen (250 µs) 48bit/250µsec = 192 kbit/s Datenrate je Richtung auf S0 Echokanal zur D-Kanal-Zugriffsteuerung: NT spiegelt das zuletzt vom im D-Kanal empfangenen Bits im nächsten Echo-Kanal zurück. Ausgleichsbits zur Herstellung der Gleichanteilsfreiheit nach jeden logischen Kanal in Richtung NT Coderegelverletzung am Beginn eines jeden Rahmen wie folgt festgelegt: 1.Coderegelverletzung durch F-Bit (+,Verletzung mit letztem D- bzw. L-Bit); nächste Coderegelverletzung zwischen L-Bit(-) und (spätestens) FA-Bit (-). A-Bit (Bus aktiviert) Literatur: [Kann] © UNI Hannover, Institut für Allgemeine Nachrichtentechnik
  • 25. S0-Frame - frame detection Case 3: d-bit from the NT at the end of the frame will be „1“, the L-bit will be „0“ and positive if the last “0” before the L-bit was negative in order to compensate or the L-bit will be “1” if the last “0” before the L-bit was “0” and positive(compensation not necessary). The F-bit will be 48 bits and microsecondsbe in conflict with the coding rule. „0“ in 250 pos. to NT TE D L. F L. B1B1B1B1B1B1B1B1 E D A FA N B2B2B2B2B2B2B2B2 E D M B1B1B1B1B1B1B1B1 E D S B2B2B2B2B2B2B2B2 E D L. F L. 0 1 0 2 bits offs et TE NT D L. F L. B1B1B1B1B1 B1B1B1 L. D L. FA L. B2B2 B2 B2 B2 B2B2B2 L. D L. B1 B1B1 B1B1B1 B1 B1 L. D L. B2 B2 B2 B2 B2 B2 B2 B2 L. D L. F L. t A Aktivierungsbit FA Hilfsrahmenbit . an diesen Stellen ist der B1 Bit des 1. B-Kanals L DC-Ausgleichbit Code gleichanteilsfrei B2 Bit des 2. B-Kanals M Multiframingbit = 0 D Bit des D-Kanals N = FA E Bit des Echokanals S Spare = 0 F Rahmenbit (25) Diagramm zeigt die möglichen physikalischen Zustände, die ein Bit im Rahmen annehmen kann (Null als positiver oder negativer Impuls, Eins als kein Impuls). 2 Bit Versatz zwischen den Rahmen (E-Bit muß angekommen sein, bevor D-Kanal-Bits wieder gesendet werden dürfen => Zugriffssteuerung) Zusammenfassung von 2 Abtastperioden (je 125µs) in einem S0-Rahmen (250 µs) 48bit/250µsec = 192 kbit/s Datenrate je Richtung auf S0 Echokanal zur D-Kanal-Zugriffsteuerung: NT spiegelt das zuletzt vom im D-Kanal empfangenen Bits im nächsten Echo-Kanal zurück. Ausgleichsbits zur Herstellung der Gleichanteilsfreiheit nach jeden logischen Kanal in Richtung NT Coderegelverletzung am Beginn eines jeden Rahmen wie folgt festgelegt: 1.Coderegelverletzung durch F-Bit (+,Verletzung mit letztem D- bzw. L-Bit); nächste Coderegelverletzung zwischen L-Bit(-) und (spätestens) FA-Bit (-). A-Bit (Bus aktiviert) Literatur: [Kann] © UNI Hannover, Institut für Allgemeine Nachrichtentechnik
  • 26. The end (26) © UNI Hannover, Institut für Allgemeine Nachrichtentechnik
  • 27. need for MAC? Competition for the usage of a transmission medium limited to the S0-bus ISDN Sobus: signalling channel NT 4 wire Why is in the case The user channel, the B-channel with 64Kbit/s, is needed of ISDN only the permanently during a communication session, otherwise the access to the Shannon sample rate of 125µs can not be guaranteed. signalling channel Therefore the B-channel will be a dedicated channel which is in competition ? assigned by the switch during call set up phase and released at the end of the connection. The traffic on the signalling channel is highly bursty, it can not be foreseen when it is needed. Therefore the D-channel is a packet channel, for which the users have to compete. The usage will be assigned for individual packets or sequences of packets. (27) There are networks for which many users share a common channel using a multi access scheme. We find the examples on the signaling channel for the ISDN S0 bus (in this case not for the communication channel, this is dedicated by the switch) on Local Area Networks (LAN) (note: latest developments in LAN´s go back to dedicated channels, using switching technologies) mobile networks packet radio networks Adhoc-networks satellite networks © UNI Hannover, Institut für Allgemeine Nachrichtentechnik
  • 28. when is the MAC function not needed? E.g. to communicate between two points, a communication media need to be used. If this medium is used unidirectional there is no need for MAC (media access control)! no MAC necessary, but might be implemented (28) © UNI Hannover, Institut für Allgemeine Nachrichtentechnik
  • 29. MAC principles Scheduling vs. Random access: scheduling means stations ready to send are waiting until it is their turn (efficient channel control under high load! But,high overhead under low load conditions) Under the random access scheme a station tries to access the transmission media as soon as it has to send something (immediately) (collisions under high load! But, low overhead under low load conditions) Discuss differences with respect Random to efficiency Scheduler Access (29) © UNI Hannover, Institut für Allgemeine Nachrichtentechnik
  • 30. MAC principles Scheduling random access Scheduling random access without with sensing fixed PDH-, SDH demand without with sensing fixed PDH-, SDH demand sensing(ALOHA) (CS) assignment assignment sensing(ALOHA) (CS) assignment assignment central distributed central distributed before before & during pure slotted control control before before & during pure slotted control control ALOHA ALOHA transmission transmission ALOHA ALOHA transmission transmission ISDN ISDN (CSMA) (CSMA/CD) (CSMA) (CSMA/CD) Ethernet-LAN How does the channel selection and the channel assignment of a PDH-, SDH-, ISDN- and Ethernet-LAN-system fit into the above scheme? (30) © UNI Hannover, Institut für Allgemeine Nachrichtentechnik
  • 31. TDMA (Time Division Multiple Access) View on a shared media: frame i frame i+1 frame i+2 time 1 guard time station 1 station 2 station m-2 station m-1 station m control data data data data data time 2 View on a single terminal: control information? no packet Frame length and Frame start (Frame delimiter) ready? guard time? yes Data packets from stations suffer from wait for assigned slot different latency times which creates the danger of overlapping (collision) transmit advantages and disadvantages? packet Efficient for high traffic load, inappropriate for low traffic load (31) © UNI Hannover, Institut für Allgemeine Nachrichtentechnik
  • 32. S0-Frame (3) 48 bits in 250 microseconds NT TE D L. F L. B1B1B1B1B1B1B1B1 E D A FA N B2B2B2B2B2B2B2B2 E D M B1B1B1B1B1B1B1B1 E D S B2B2B2B2B2B2B2B2 E D L. F L. 0 1 0 2 bits offs et TE NT D L. F L. B1B1B1B1B1 B1B1B1 L. D L. FA L. B2B2 B2 B2 B2 B2B2B2 L. D L. B1 B1B1 B1B1B1 B1 B1 L. D L. B2 B2 B2 B2 B2 B2 B2 B2 L. D L. F L. t Let the d-bit be a logical „0“, the e-bit will be a “0” in any case, because the logical “0” is dominant! In this case the “blue” terminal will not detect a collision and will continue!!! The “green” terminal which sent a “1” will get back a “0” and will stop transmitting! Are there other cases? (32) © UNI Hannover, Institut für Allgemeine Nachrichtentechnik
  • 33. S0-Frame (3a) 48 bits in 250 microseconds NT TE D L. F L. B1B1B1B1B1B1B1B1 E D A FA N B2B2B2B2B2B2B2B2 E D M B1B1B1B1B1B1B1B1 E D S B2B2B2B2B2B2B2B2 E D L. F L. 0 1 0 2 bits offs et TE NT D L. F L. B1B1B1B1B1 B1B1B1 L. D L. FA L. B2B2 B2 B2 B2 B2B2B2 L. D L. B1 B1B1 B1B1B1 B1 B1 L. D L. B2 B2 B2 B2 B2 B2 B2 B2 L. D L. F L. t Let the d-bit be a logical „0“, and let another terminal send a “0” as well. In this case a collision is not detected!!! A collision is detected only, if one TE is transmitting a logical „1“ and the other a „0“. The „0“ will continue and the „1“ will stop! (33) © UNI Hannover, Institut für Allgemeine Nachrichtentechnik
  • 34. S0-Frame (4) Why is there no e-bit for the other direction in the Frame from TE NT? Because the NT is not in competition with other terminals! 48 bits in 250 microseconds NT TE D L. F L. B1B1B1B1B1B1B1B1 E D A FA N B2B2B2B2B2B2B2B2 E D M B1B1B1B1B1B1B1B1 E D S B2B2B2B2B2B2B2B2 E D L. F L. 0 1 0 2 bits offs et TE NT D L. F L. B1B1B1B1B1 B1B1B1 L. D L. FA L. B2B2 B2 B2 B2 B2B2B2 L. D L. B1 B1B1 B1B1B1 B1 B1 L. D L. B2 B2 B2 B2 B2 B2 B2 B2 L. D L. F L. t A Aktivierungsbit FA Hilfsrahmenbit . an diesen Stellen ist der B1 Bit des 1. B-Kanals L DC-Ausgleichbit Code gleichanteilsfrei B2 Bit des 2. B-Kanals M Multiframingbit = 0 D Bit des D-Kanals N = FA E Bit des Echokanals S Spare = 0 F Rahmenbit (34) Diagramm zeigt die möglichen physikalischen Zustände, die ein Bit im Rahmen annehmen kann (Null als positiver oder negativer Impuls, Eins als kein Impuls). 2 Bit Versatz zwischen den Rahmen (E-Bit muß angekommen sein, bevor D-Kanal-Bits wieder gesendet werden dürfen => Zugriffssteuerung) Zusammenfassung von 2 Abtastperioden (je 125µs) in einem S0-Rahmen (250 µs) 48bit/250µsec = 192 kbit/s Datenrate je Richtung auf S0 Echokanal zur D-Kanal-Zugriffsteuerung: NT spiegelt das zuletzt vom im D-Kanal empfangenen Bits im nächsten Echo-Kanal zurück. Ausgleichsbits zur Herstellung der Gleichanteilsfreiheit nach jeden logischen Kanal in Richtung NT Coderegelverletzung am Beginn eines jeden Rahmen wie folgt festgelegt: 1.Coderegelverletzung durch F-Bit (+,Verletzung mit letztem D- bzw. L-Bit); nächste Coderegelverletzung zwischen L-Bit(-) und (spätestens) FA-Bit (-). A-Bit (Bus aktiviert) Literatur: [Kann] © UNI Hannover, Institut für Allgemeine Nachrichtentechnik