libro cisco 200-301

libro cisco 200-301.In FIGS. 1-3, when a portable computer sends data over a wireless network, that data must be encoded and processed using a sequence of code pieces and subsequently modulated by radio waves. In this figure, a fragment code with a bit value of 1 is expanded as the 00110011011 code slice sequence and a fragment code with a bit value of 0 is expanded as 11001100100. thus, after the data bits are sent, 1001 generates the following code slice sequence.00110011011 11001100100 11001100100 001100110111 00 1The code slice sequence can be decoded back to the value 1001 on the AP located at the receiving end. Remember that due to interference, some bits in the code slice sequence may be lost or inverted, i.e., 1 may become 0 and 0 may become I. This is not a big deal, since more than 5 bits need to be inverted for the value to change between 1 and 0. Thus, the use of code slice sequences can make 802.11 networks more resilient to interference.Likewise, since more bits are used for code slice (carrier) transmission than actual data, the code slice rate is higher than the data count rate.(2) BarkerCodeIn order to obtain IMbit/s and 2Mbit/s rates, 802.11 uses Barker codes. BarkerCode uses 11 code pieces when encoding data. BarkerCode is the ideal code for modulating radio waves, and the mathematics associated with it is beyond the scope of this book. Finally, for the exam, it is important to understand that each data bit sent out is encoded into an 11-bit Barker code, which is then modulated using DSSS.(3) Complementary Keying When using DSSS, Bacode works fine for lower data rates such as IMbit/s, 2Mbit/s> 5.5Mbit/s and 1 IMbit/s. For higher data rates, DSSS uses a different coding method that allows the 802.11 standard to reachrates of 5.5 Mbit/s and 11 Mbit/s. CCK (Complementary Code Keying) uses a series of codes called complementarysequences. This series of codes has 64 unique code words. A single code word can represent up to12 Chapter 1 Introduction to Wireless Networking Concepts