Time Slot Synchronization

The WirelessHART (EC 62591) is a reliable, secure, cost-effective, multivendor, interoperable international wireless standard based on the Highway Addressable Remote Transducer (HART) protocol [35]. This standard supports operation in the 2.4 GHz ISM band using EEE 802.15.4 standard radios with data rate up to 250 kbps and is characterized by a time-synchronized, self-organizing, and self-healing mesh architecture. In WirelessHART, TDMA is used with 10-ms duration for each time slot to ensure contention-free transmission and a channel-hopping scheme for added system data bandwidth and robustness is employed. There are up to 15 radio frequency channels (16 with ISAlOO.lla in some world areas). OQPSK is used. Although the use of the WirelessHART standard is supposed to be easy, the implementation is more complex than the wired counterpart. 

Figure 15.6(b) shows an example of the SONET STS-1 fi ame that corresponds to a frame in Fig. 15.6(a). In this example, each VT1.5 path carried on the payload of this STS-1 frame represents a channel (time slot) in Fig. 15.6(a). The STS-1 frame is divided into two portions transport overhead and information payload, where the transport overhead is further divided into line and section overheads. The STS frame is transmitted every 125 ixs. The first three columns of the STS-1 frame contain transport overhead and the remaining 87 columns and 9 rows (a total of 783 bytes) carry the STS-1 synchronous payload envelope (SPE). The SPE contains a 9-byte path overhead that is used for end-to-end service performance monitoring. Optical carrier level-1 (OC-1) is the lowest level optical signal used at equipment and network interfaces. An OC-1 is obtained fi om a STS-1 after scrambling and electrical-to-optical conversion. 

By one time slot method, the s3mchronization can be derived from the cyclical log. The s5mchronization based on sending a synchronization signal which is cyclically received and evaluated by aU network participants. To ensure a best possible synchronization, it is required that the signal in a fixed time interval with minimal timing variances will be sent and received. 

On the SCHs, time is partitioned to frames consisting of a constant number of fixed duration time slots. All the SCHs are slot synchronized with the CCH, and on each SCH, each second contains an integer number of frames. The number of time slots per frame on channel c is denoted by = 0, Nsch, and a time slot on channel Cm is identified by the index (from 0 to - 1) of this time slot within a frame on channel c . Note that, the same time slot can have different indices on channels c,-and Cj, i j, since k is not necessarily equal to Ij. 

FoUowing an approach similar to [4] the authors of RI-EDF [8] synchronize slots based on the reception time instants of the transmission of other nodes in a distributed fashion. However, the protocol was not developed to cope with uncontrolled noncomplying traffic in the channel. 

This mechanism is particularly better than the originally proposed one when in the presence of an AP with the power management feature active and a fairly long DTIM period. In such case the transmissions instants in the former approach would be altered and the team would not synchronize. With the new mechanism, the reference node transmission instants may still be altered (implying a potential large error in % i) but this will impact the start of the round, only. The remaining transmissions in the team are stili separated in time according to the respective slots in the round. 

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