As for 2.4GHz, there are 14 channels. Each has 22MHz bandwidth, which is shown in Figure 1-3. Evidently, only channel 1,6,11 are the channels which are not intertwined each other.

Figure 1-3 wireless channel

1. Wi-Fi architecture
   

   
   

   Figure 1-4 Infra networks
   

   Wi-Fi has two main modes: Infra and Adhoc networks
   

   1. Infra networks
      
      1. Minimum element: BSS (Basic Service Set), shown in Figure 1-4.
         

• One or more radio hosts;
  
• AP: Access Point, i.e., BS (Base Station)
  
• Radio hosts communicate with the outside by AP
  
• Inter-connection among APs by the inter-connection with Hub/Switch /Router
  

1. SSID (Service Set Identification)
   

• Manager assigns a SSID less than 32 bytes for an AP;
  
• A BSS can exist lonely. It can also be connected to another BSS by the connection with DS (Distribution System) for the constitution of ESS.
  

1. ESS (Extension Service Set)
   

ESS is composed of several APs and DS. All of the APs share an ESSID, and an ESS can include several BSS, as shown in Figure 1-5.

Figure 1-5 ESS

1. DS (Distribution System)
   

• Distribution System, which is used to the logical connection BSS, and provide the roam distribution system for the wireless APs.
  
• When DS is used, then all of the BSSes seem like a BSS in a BSS.
  
• DS is usually denoted by Ethernet, point-to-point link or other wireless networks.
  

1. Adhoc networks
   

• No AP center control
  
• Generally no connection to the outside networks
  

Figure 1-6 Adhoc networks