new ccna material

new ccna material.1. Sending Frames802.1H1 uses only CTStoself to send frames within the allocated time, instead of using RTS/CTS as discussed earlier in this chapter.Another feature that makes 802.1In more efficient is its use of blockacknowledgment, which contrasts with the way other 802.11 protocols acknowledge each unicast packet. Blocking acknowledgment works by sending a number of data frames and then acknowledging the sent frames. This is similar to the way TCP operates.Another point of knowledge for sending frames is the requirement to understand how data frames are sent in a normal 802.11a/b/g environment, which is covered in detail in Chapter 7. Here is just a quick overview. Each sending station must wait before sending the next frame until the previous frame has been sent. This is called DistributedInterframeSpace (DIFS) oThis DIFS can cause more overhead than is actually needed. 802.11n improves this DIFS mechanism by utilizing a smaller interframe interval called Reduced Interframe Space (RIFS). This approach reduces latency and total overhead.2. Antenna considerations The number of antennas at the transmitter and receiver can be different. How they work is described below. If a sender can send on 3 antennas, then it has 3 data streams. If it can transmit on 3 antennasreceive, then it has 3 receive chains. In the literature, this is referred to as 3 power. Two receive chains and two data streams are referred to as 2x2.This is important because the Cisco 1250AP is a 2-power device. If you have a 2x2 portable computer, then you can look into the meaning of this. When using a specific multiplexing technique, the number of data streams is the same as the minimum number of antennas. In this case, you will have two data streams.Finally, even if you don't have an 802.11H client, you can expect a 30% improvement on top of these features.6.3 Exam Preparation Tasks