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Handbook Of Fiber Optic Data Communication, Thi...

Fiber optics as a transmission technology came about in the 1980s when it became economically feasible to mass produce fiber optic cable. Advances in laser and semiconductor technologies compounded the growth and within decades fiber optic based transmission systems were the bedrock of long haul high capacity data transmission.

Handbook of Fiber Optic Data Communication, Thi...


In the nineties and 2000s TDM (time division multiplexing), FDM (frequency division multiplexing) and WDM (wavelength division multiplexing) were the three technologies proposed to resolve the chronic capacity crisis. Demand for bandwidth was growing exponentially and the existing transmission technologies were failing to keep up let alone inspire confidence that they could scale to meet future requirements. The problem was that although SDH, a TDM based technology, was widely deployed and was capable of aggregating client data input to meet 10 GBPS or more of transport, there were inherent restrictions. With TDM each client is assigned a timeslot thus making the transmission shared by all the users. In effect SDH/Sonet takes asynchronous or synchronous electrical signals and multiplexes them onto a single optical bit rate. This however requires optical-to- electrical or optical-to-electrical-to-optical (OEO) signal conversion. Furthermore, the optical technology for transporting the data provided a theoretical available bandwidth that could exceed several Terahertz (10 to the power of 12). The problem was though that TDM could not be made to work to take advantage of this vast bandwidth because the electrical signaling it required simply cannot work at those frequencies. The same was true for FDM systems it just wasn't possible to use frequency multiplexing, at the electrical level, at these high end frequencies. WDM on the other hand takes multiple optical signals and multiplexes them onto a single fiber and there is no signal conversion. Therefore being purely optical DWM was able to scale to use the full optical bandwidth potential.

In a Cochrane review, Elsner et al (2013a) examined the effects for improving aphasia in patients after stroke. These investigators searched the Cochrane Stroke Group Trials Register (April 2013), the Cochrane Central Register of Controlled Trials (CENTRAL) (The Cochrane Library, March 2012), MEDLINE (1948 to March 2012), EMBASE (1980 to March 2012), CINAHL (1982 to March 2012), AMED (1985 to April 2012), Science Citation Index (1899 to April 2012) and 7 additional databases. They also searched trials registers and reference lists, hand-searched conference proceedings and contacted authors and equipment manufacturers. These researchers included only RCTs and randomized controlled cross-over trials (from which they only analyzed the first period as a parallel group design) comparing tDCS versus control in adults with aphasia due to stroke. Two review authors independently assessed trial quality and extracted the data. If necessary, they contacted study authors for additional information. These investigators collected information on drop-outs and adverse events from the trials. They included 5 trials involving 54 participants. None of the included studies used any formal outcome measure for measuring functional communication, which is measuring aphasia in a real-life communicative setting. All 5 trials measured correct picture naming as a surrogate for aphasia. There was no evidence that tDCS enhanced speech and language therapy outcomes. No adverse events were reported and the proportion of drop-outs was comparable between groups. The authors concluded that currently there is no evidence of the effectiveness of tDCS (anodal tDCS, cathodal tDCS) versus control (sham tDCS). Moreover, they stated that it appears that cathodal tDCS over the non-lesioned hemisphere might be the most promising approach.

Frey et al (2012) established a novel approach for fiber tracking based on navigated TMS mapping of the primary motor cortex and proposed a new algorithm for determination of an individualized fractional anisotropy value for reliable and objective fiber tracking. A total of 50 patients (22 females, 28 males; median age of 58 years, range of 20 to 80) with brain tumors compromising the primary motor cortex and the cortico-spinal tract underwent pre-operative MRI and navigated TMS mapping. Stimulation spots evoking muscle potentials (MEP) closest to the tumor were imported into the fiber tracking software and set as seed points for tractography. Next the individual FA threshold, namely, the highest FA value leading to visualization of tracts at a pre-defined minimum fiber length of 110 mm, was determined. Fiber tracking was then performed at a fractional anisotropy value of 75 % and 50 % of the individual FA threshold. In addition, fiber tracking according to the conventional knowledge-based approach was performed. Results of tractography of either method were presented to the surgeon for pre-operative planning and integrated into the navigation system and its impact was rated using a questionnaire. Mapping of the motor cortex was successful in all patients. A fractional anisotropy threshold for cortico-spinal tract reconstruction could be obtained in every case. TMS-based results changed or modified surgical strategy in 23 of 50 patients (46 %), whereas knowledge-based results would have changed surgical strategy in 11 of 50 patients (22 %). Tractography results facilitated intra-operative orientation and electrical stimulation in 28 of 50 (56 %) patients. Tracking at 75 % of the individual FA thresholds was considered most beneficial by the respective surgeons. The authors concluded that fiber tracking based on navigated TMS by the proposed standardized algorithm represents an objective visualization method based on functional data and provides a valuable instrument for pre-operative planning and intra-operative orientation and monitoring. This was a small study and it did not validate navigated TMS findings with improved health outcomes. 041b061a72

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