Rechnernetze

Physikalische Verbindungen

Allgemeines

Zwischen Master und Slave können verschiedene Verbindungstypen aufgesetzt werden. Zwei definierte Typen sind

Eine SCO-Verbindung ist eine Punkt-zu-Punkt-Verbindung zwischen einem Master und einem einzelnen Slave in einem Piconet. Der Master betreibt die SCO-Verbindung über reservierte Slots in regelmäßigen Intervallen. Die ACL-Verbindung ist eine Punkt-zu-Mehrpunkt-Verbindung zwischen dem Master und sämtlichen Slaves, die an dem Piconet teilhaben. In den nicht für SCO-Verbindungen reservierten Slots kann der Master eine ACL-Verbindung auf einer Pro-Slot-Basis zu jedem Slave, einschließlich jener, zu denen eine SCO-Verbindung besteht.

SCO LINK

The SCO link is a symmetric, point-to-point link between the master and a specific slave. The SCO link reserves slots and can therefore be considered as a circuit-switched connection between the master and the slave. The SCO link typically supports time-bounded information like voice. The master can support up to three SCO links to the same slave or to different slaves. A slave can support up to three SCO links from the same master, or two SCO links if the links originate from different masters. SCO packets are never retransmitted.

The master will send SCO packets at regular intervals, the so-called SCO interval TSCO (counted in slots) to the slave in the reserved master-to-slave slots. The SCO slave is always allowed to respond with an SCO packet in the following slave-to-master slot unless a different slave was addressed in the previous master-to-slave slot. If the SCO slave fails to decode the slave address in the packet header, it is still allowed to return an SCO packet in the reserved SCO slot.

The SCO link is established by the master sending an SCO setup message via the LM protocol. This message will contain timing parameters such as the SCO interval TSCO and the offset DSCO to specify the reserved slots. In order to prevent clock wrap-around problems, an initialization flag in the LMP setup message indicates whether initialization procedure 1 or 2 is being used. The slave shall apply the initialization method as indicated by the initialization flag. The master uses initialization 1 when the MSB of the current master clock

(CLK27) is 0; it uses initialization 2 when the MSB of the current master clock

(CLK27) is 1. The master-to-slave SCO slots reserved by the master and the

slave shall be initialized on the slots for which the clock satisfies the following equation:

BLUETOOTH SPECIFICATION Version 1.1 page 46 of 1084

Baseband Specification

CLK27-1 mod TSCO = DSCO for initialization 1

(CLK27,CLK26-1) mod TSCO = DSCO for initialization 2

The slave-to-master SCO slots shall directly follow the reserved master-toslave

SCO slots. After initialization, the clock value CLK(k+1) for the next master-

to-slave SCO slot is found by adding the fixed interval TSCO to the clock

value of the current master-to-slave SCO slot:

CLK(k+1) = CLK(k) + TSCO

3.3 ACL LINK

In the slots not reserved for SCO links, the master can exchange packets with

any slave on a per-slot basis. The ACL link provides a packet-switched connection

between the master and all active slaves participating in the piconet.

Both asynchronous and isochronous services are supported. Between a master

and a slave only a single ACL link can exist. For most ACL packets, packet

retransmission is applied to assure data integrity.

A slave is permitted to return an ACL packet in the slave-to-master slot if and

only if it has been addressed in the preceding master-to-slave slot. If the slave

fails to decode the slave address in the packet header, it is not allowed to

transmit.

ACL packets not addressed to a specific slave are considered as broadcast

packets and are read by every slave. If there is no data to be sent on the ACL

link and no polling is required, no transmission shall take place.