Overall, it is cheap, easy, and quick to implement the distributed backbone network. Collapsed backbone. This type of backbone uses single, powerful router as the central connection point for multiple subnetworks. As Figure 4 shows, the central device is the highest level of the backbone.arallel backbone. Parallel backbone is a variation of the collapsed backbone, where devices are having more than one connection between them. As Figure 5 shows, there are multiple connections between the high level routers and the network segments.
The most common method to measure visual acuity is the Snellen eye chart. Like all eye charts, the Snellen chart tests a patient's ability to resolve high-contrast letters and is satisfactory if their vision is good. Unfortunately, the chart does not have enough sensitivity for patients with poor vision.
A visual acuity chart is used to test distance visual acuity. It uses capital letters of different sizes to test vision in literate adults. The test is conducted at 6 metres (20 feet in USA).The test result is given as a fraction that indicates the distance in metres at which that row of the chart can be read by a normal eye.he bottom number represents the size of the letter seen. The larger the bottom number the larger the letter on the chart (E.g. 6/48 indicates a bigger letter than 6/12). Normal visual acuity is recorded as 6/6 (20/20 in USA).