Token Bus. Message Exchange in Token Bus. Example. Problems with Token Bus

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Hilda Jennings
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1 Token us Token -procedure: Only someone who possesses a certain ken (= bit sequence), may send. One example for a ken network: IEEE 80. Token us All s should be treated equally, i.e. they have pass on the ken cyclically or this: logical ordering of all s a ring In a bus pology, ordering is made regarding addresses: 9 Usage e.g. as a field bus (eldbus in German) in industrial environments with a high degree of noise. Purpose: e.g. roboter control; a few masters, many slaves (they only listen). Data rate is not that important, but guarantees in response times are necessary (not possible with Ethernet) Page Message Exchange in Token us Two types of messages are used: Token messages T ID, nextid ID Data messages M ID ID Data Token messages are used for passing on the sending permission ID nextid Data messages contain the data be sent Having the ken, a is allowed send a message. After this (or if nothing is be sent) the ken is passed on. Traffic on the bus e.g.: T 5,7 M 7 T 7, M T, T, T,6 M 6 T 6,9 T 9,5 M 5 T 5,7 High overhead for ken exchange: 5 it times for each ken message (a full small size Ethernet frame)! In contrast that, in Token Ring networks only one bit is be switched 0, i.e. only one bit time for a ken message. Thus, the number of participating s should be low, or the number of masters should be low, you can lerate very many slaves nextid Page Problems with Token us irst problem: a leaves the logical ring Easy solution: the leaving sends a message its predecessor indicating the new successor Second problem: a comes in the logical ring To allow new s join, in periodically intervals, a window is opened between neighbors, e.g. between 5 and 7. New s with IDs 6 6 now can apply. Problem: conflict risk! Several s could apply join in this window Conflict resolution: survival of the fittest. Consider last bits of the IDs: send a request join with a duration specified by the last bits of your ID: 00 send short 0 send longer 0 send even longer send longest If you hear anybody else sending longer than you, give up. If you survive without conflict, join. If there is no resolution (two or more s are sending for the same time, no one is sending longer), repeat with the second-last bit pair, etc. Page Example Assume a global window. Competirs: Winner! In case of some configuration error, two identical IDs are present: after checking all pairs of the IDs, continue by adding random bit pairs. Page

2 ut Industrial Ethernet Token Ring The Token-us approach is more and more displaced by Ethernet variants, e.g.: EtherCAT (since 00, ast Ethernet based on a bus, star or tree pology (very flexible) Uses TP or optical fiber as medium Synchronization necessary between all s A master polls the other s with a single Ethernet frame each has its one time slot read out /write in data Ethernet Powerlink ( Introduction of time slots and a cyclic timing schedule Whole time axis is divided in isochronous and asynchronous phase Isochronous: for time-critical data transfer Asynchronous: for non-time-critical data transfer A managing node assigns time slots (in both phases!): in the isochronous phase all s, in the asynchronous phase one particular Page 5 Token -procedure: Only someone who possesses a certain ken (= bit sequence), may send. based on standard IEEE 80.5 Token Ring the s share a ring of point--point connections the ken is cyclically passed on particularly suitable for rings Token Ring (/6/00 Mbit/s) Characteristics: guaranteed access, no collisions very good utilization of the network capacity, high efficiency fair, guaranteed response times possible: multiple kens however: complex and expensive Passing on the ken Page 6 Token Ring Sending and Receiving Characteristics Medium: twisted pair, coaxial cable or optical fiber Capacity of resp. 6 Mit/s (00Mb/s with optical fiber) Differential Manchester Code on layer The s are actively attached, i.e. received signals are regenerated (same principle as for repeaters, therefore no restriction of the ring s expansion) Station Point--Point Connection Active connecr Initial state Data are received the ring serially Data addressed a connecr s are copied Data are serially passed on along the ring the ring the ring Transmission state The ring is divided Own data are sent serially Data coming in the ring are evaluated by the the ring Page 7 Page 8

3 Access within a Token Ring Access within a Multiple Token Ring Example: Station sends. Station waits for free ken (transmission authorization, -yte- Token).. Station changes free ken in an occupied one (occupied ken = frame header). Afterwards, sends the frame. (Station may send further frames, if the ken holding timer (default 0 ms) is not exceeded). Station terminates the frame and waits until the frame passed the whole ring and arrives again.. Station copies the frame. Station removes it the ring and produces a new, free ken. remove entfernen kopieren copy Same example: Station sends. Station waits for free ken (transmission authorization).. Station changes free ken in an occupied one (occupied ken = frame header). Afterwards, sends the frame. (Station may send further frames, if the ken holding timer (default 0 ms) is not exceeded). Station terminates the frame and produces a new, free ken immediately.. Station copies the frame. Station removes it the ring. remove entfernen kopieren copy Page 9 Page 0 rame ormat for Token Ring rame ormat for Token Ring If the ring is inactive, only the -yte-token (SD, AC, ED) circulates. If a wants send, it sets a certain bit in this ken 0. ree ken, if a certain bit within AC is set. /6 /6 any DA SA Data CS rame End Delimiter Control (ED) (C) Access Control (AC) Start Delimiter (SD) End Delimiter (ED) rame Status SD and ED serve for marking the frame. They contain invalid sequences of the Differential Manchester code. Access control contains the ken bit, further a monir bit, priority bits and reservation bits. rame control marks the kind of the frame: Data, control, yte rame status contains confirmation bits A and C. If a frame arrives at the with the destination address, bit A is set. If the processes the frame, also bit C is set. When the sending gets the frame back, it can see whether the receiving is not working (A = 0, C = 0), if the frame was not accepted (A =, C = 0), or whether the frame was received correctly (A =, C = ). To protect against bit errors, both bits are doubly present. The addresses and the checksum are identical Ethernet. its of access control: Themonir bit serves for recognition of a second frame circulation Thepriority bits make possible several priorities. They indicate the priority of the ken. If a wants send with priority n, it must wait for a ken of priority n or higher. Thereservation bits permit a reserve the next frame for itself. If a wants do this, it registers its priority in the reservation bits. This is only possible, if not already a higher priority is registered. During the next ken generation, the priority is copied in the priority bits. Page Page

4 Ring Maintenance To check the correct function of the ring, a monir is introduced. If this crashes, another is raised as monir : if a recognizes that the monir is inactive, this sends a certain ken (CLAIM_TOKEN). This can be done by several s simultaneously. If such a message arrives with a smaller ID then suppress it. If a message arrives with larger ID then pass it on. If a CLAIM_TOKEN message arrives with own ID: this is the new monir. iber Distributed Data Interface (DDI) DDI is a high performance ken ring LAN based on optical fibers ANSI standard XT9.5 Data rates of 00 Mit/s Range of up 00 km (MAN?) Support of up 000 s, with distances of maximally km Often used as ackbone for small LANs Host ridge 80. LAN Tasks of the monir : New generation of the ken after a ken loss Reaction ring collapse Removal of frame fragments Deletion of old, circulating frames or each problem an own ken is defined. Additionally if necessary also timers are used. Page 80.5 LAN ridge DDI ring Successor: DDI-II, supports besides normal data also synchronous circuit switched PCM data (speech) and ISDN traffic Variant: CDDI (Copper Distributed Data Interface), with 00 Mit/s over Twisted Pair Page Structure of DDI DDI Configurations Wiring within DDI: optical fiber rings with opposite transmission direction During normal operation, only the primary ring is used, the secondary ring remains in readiness Normaler asic double Doppel-Ring ring Double ring Doppelter with connected Ring mit Einzelring-Erweiterung single Einfacher Simple DDI DDI-Ring ring NAC If the ring breaks, the other one (also called protection ring) can be used. If both rings break or if a precipitates, the rings can be combined in only one, which has double length: DAC SAC SAC = Dual Attachment Station = Single Attachment Station Two classes of s exist: (Dual Attachment Station) can be attached both rings, the cheaper (Single Attachment Station) are only attached one ring. DAC = Dual Attachment Concentrar NAC =Null Attachment Concentrar SAC = Single Attachment Concentrar y means of the concentrars several rings can be linked. Page 5 Page 6

5 Transmission within DDI Synchronous Transmission of Data Coding /5 code, thus coding of bits of data in 5 bits which are transferred Synchronization Transmission of a long preamble in order synchronize the receiver the sender clock pulse. The clocks of all s must run stable on at least 0.005%. With such a stability, frames with up 500 byte of data can be transferred without the receiver losing the clock pulse. Procols The fundamental procols of DDI are similar IEEE 80.5 (ken ring): in order transmit data, a must acquire the ken. Then it transfers its frame and takes it the ring when it returns it. Due the expansion of DDI, a single ken is unpractical. Therefore, DDI transfers in the multiple ken mode. Original transmission principle within DDI: Use of asynchronous frames, i.e. sending can be started any time. Additionally, with DDI-II also the use of synchronous frames for circuit switched PCM or ISDN data (telephony) is possible: every 5 µs a master produces synchronous frames for reaching the 8000 samples / second necessary for PCM. every frame consists of 6 byte for non-circuit-switched data and up 96 byte for circuit switched data (up 96 PCM channels per frame). if a once uses fixed slots in a frame, these are considered for it as reserved until the releases them expressly (implicit reservation). unused synchronous slots of the frame are assigned on request any. Ring and management also are similar IEEE 80.5, additionally a function for deviating traffic the protection ring is included. Page 7 Page 8 Data frames with DDI The data frames are similar those used in the ken ring format: Conclusion: Local Area Networks 8 /6 /6 Up 80 Preamble DA SA Data CS rame Control (C) Start Delimiter (SD) End Delimiter (ED) rame Status Thepreamble is used for the synchronization as well as for the preparation of the s a following transmission Start and end delimiter are being used for marking the frame rame control specifies the type of the frame: data, control, synchronously/asynchronously, Here, also several kens are differentiated for the possible traffic types. rame status contains confirmation bits as in IEEE 80.5 Addresses and the CS are as in IEEE 80.5 yte Token Preamble S D C E D Page 9 Three main approaches: Ethernet for networks sporadic, bursty traffic Token us for networks with a few sending s having timing guarantees Token Ring for networks with many s having timing guarantees. Also suited for MANs Today s trend: Ring networks (DDI) still in use in older MANs, no longer used in the LAN area ield busses still in use as field busses, but Ethernet variants take over the market Everything tends Ethernet Page 0

Content. Deterministic Access Polling(1) Master-Slave principles: Introduction Layer 2: Media Access Control

Content. Deterministic Access Polling(1) Master-Slave principles: Introduction Layer 2: Media Access Control Content Introduction Layer 2: Frames Error Handling Media Access Control General approaches and terms Network Topologies Media Access Principles (Random) Aloha Principles CSMA, CSMA/CD, CSMA / CA Media