Introduction
Let’s first identify the following elements in a wireless network
- Wireless hosts. As in the case of wired networks, hosts are the end-system devices that run applications. A wireless host might be a smartphone, tablet, laptop, or IoT devices that being connected to the Internet
- Wireless links. A host connects to a base station or to another wireless host through a wireless communication link. Different wireless link technologies have different transmission rates with different ranges.
- Base station. This is responsible for sending and receiving data to and from a wireless host that is associated with that base station. It will often also be responsible for coordinating the transmission of multiple wireless hosts with which it is associated. Examples are Cell towers in cellular networks and access point in 802.11 wireless LANs
- Network Infrastructure. This is the larger network with with a wireless host may wish to communicate
Wireless Links and Network Characteristics
Wireless links differ from their wired counterpart in a number important ways
- Decreasing signal strength. EM radiation attenuates as it passes through matter. There’s path loss as the distance between sender and receiver increase
- Interference from other sources. Radio sources transmitting in the same frequency band will interfere with each other.
- Multipath propagation. Multipath propagation occurs when portions of the electromagnetic wave reflect off objects and the ground, taking paths of different lengths between a sender and receiver
Thus, bit errors would be more common in wireless links than in wired links, and it have a stronger link-level RDT protocols that retransmit corrupted frames
For the receiver side, the signal-to-noise(SNR) is a relative measure of the strength of the received signal and noise, typically measured in units of dB. Another concept relating to this is the BER, bit errors rate
- For a given modulation scheme, the higher the SNR, the lower the BER
- For a given SNR, a modulation technique with a higher bit transmission rate will have a higher BER
- Dynamic selection of the physical-layer modulation technique can be used to adapt the modulation technique to channel conditions
CDMA
Recall that when hosts communicate over a shared medium, a protocol is needed so that the signals sent by multiple senders do not interfere at the receivers. CDMA is used in wireless communication.
In a CDMA protocol, each bit being sent is encoded by multiplying the bit by a signal (the code) that changes at a much faster rate(the chipping rate) than the original sequence.
WiFi: 802.11 Wireless LANs