Δίκτυα Δακτυλίου. Token Ring - Polling

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1 Δίκτυα Δακτυλίου Token Ring - Polling

2 Όλοι οι κόμβοι είναι τοποθετημένοι σε ένα δακτύλιο. Εκπέμπει μόνο ο κόμβος ο οποίος έχει τη σκυτάλη (token). The token consists of a number of octets in a specific format that gets passed around. A station receives a token and starts transmitting. Every system receives, looks at it and retransmits it (time delay is introduced called station delay) Ένα πλαίσιο περνά από όλους τους κόμβους του δακτυλίου.

3 Ο κόμβος ο οποίος μετάδωσε ένα πλαίσιο είναι υπεύθυνος να το αφαιρέσει από τον δακτύλιο αφού συμπληρώσει κύκλο. Επίσης ο κόμβος είναι υπεύθυνος να ελευθερώσει τη σκυτάλη είτε μόλις ολοκληρώσει την μετάδοση του πλαισίου είτε μόλις αφαιρέσει το πλαίσιο από τον δακτύλιο. Κάθε πλαίσιο που επιστρέφει στο αποστολέα περιέχει επίσης και bits επαλήθευσης (A και C).

4 Τι μπορεί να πάει λάθος; Καθυστέρηση: Υπάρχει μέγιστος επιτρεπτός χρόνος κατά τον οποίο κάθε κόμβος μπορεί να κρατεί τη σκυτάλη (Token Holding Time (THT)). Υπάρχεί μέγιστος επιτρεπτός χρόνος για να συμπληρώσει μια περιστροφή η σκυτάλη (Target Token Rotation Time) Απώλεια της σκυτάλης. Κάθε κόμβος μετρά το χρόνο από τη προηγούμενη σκυτάλη. Αν αυτός υπερβαίνει τα 2.5ms, τότε ζητά τη σκυτάλη. Αν δεν υπάρχει κόμβος με ψηλότερη προτεραιότητα, τότε ο κόμβος αυτός ξαναδημιουργεί τη σκυτάλη. «Επείγοντα» πλαίσια Η σκυτάλη υποστηρίζει επίσης Προτεραιότητες

5 Token: SD AC ED SD: start delimiter tells that something is coming (used for synchronization) ED: end delimiter. Signifies the end of the frame. ED has 8 bits associated with it. ED is written generically in the following way: ED: JK1JK1IE If IE = 00 it is a token (i.e., this is the last field, get off) If it is a frame: (a) I = 1 This particular frame received is the first or an intermediate frame of a sequence. (b) I = 0 End of transmission by this station (c) E = 0 This is set to 0 by the source station. It can be set to 1 by any station detecting an error. In that case, it comes back to the source station that knows that something went wrong.

6 AC: Access field (8 bits): 4 th bit is the token bit (T) T=0 token comes T=1 frame comes PPP = priority bits (in the AC field) (provide up to 8 levels of priority in accessing the ring) RRR = reserve bits (in the AC field) (we reserve 8 priorities) (used to request that the next token be transmitted at the priority requested) M (management) bit Station looks at the priority bits of the token and if priority is less than the priority of the station then it resets the priority bits to its priority. Highest priority will get the token next when it is free. Occasionally we drop the priority to let those with small priorities to come in play and transmit their information.

7 In the AC field there is also an M-bit which is normally set to zero by the source station. There is a management (monitor station) in our ring. When the frame reaches that station it sets the M bit to 1. When the frame comes back to the source M=1. If by mistake the frame is not taken out, the monitor station receives it, sees that M=1 and takes the frame out. It then sends a token since it recognized that something is wrong. Source: Turn bit T into an 1 and put in DA, SA, information (data), FCS, ED and transmit. Then wait until it senses something coming back. Recognizes the T=1, its own SA, strips everything except first 3 fields and makes the token bit 0. Note: We cannot have multiple tokens in the ring.

8 Summary of Operation: 1. Capture token, transmit frames: A given station carries out the following operations: It receives some information, recognized by SD, and it sees if it is a token or a frame. It looks at the priority of the waiting frames. If they are of higher priority it can capture token. If not, it will pass it on. Converts T = 1 and transmits waiting frame. 2. Intermediate Station (could be destination): Not destination: Receives and passes frame on. It can change the priority bits if its priority >> RRR so that it can reserve place for later transmission. It can also change IE bits. Destination: Recognizes address, copy frame and pass it on. (also will change some bits, i.e., AC bits) 3. Source (Originating) station: Waits for the frame to return. It will generate a new token to be passed on if timer expires (once it gets the token initially it sets a timer called the token holding time) OR if not exceeded and no more frames OR if RRR bits have greater priority than its own.

9 Άλλα πρωτόκολλα και Τεχνολογίες Ασύρματα τοπικά δίκτυα IEEE Ασύρματα δίκτυα ευρείας ζώνης (Broadband Wireless) ΙΕΕΕ Bluetooth Βλέπε A. Tanenbaum, σελίδες

10 Polling 1. Roll-call polling (central station interrogates everybody - central control) 1 2 Control 3 station 4 Propagation time is longer in a roll-call polling scheme 2. Hub Polling Passing of control from one station to the other Distributed control Token passing fits in this category