CST 311 - Week 7

 The topic of this week's studies was the link layer and LANs (local area networks).  Some of the topics covered were the link layer, error detection, error correction, multiple access links, and switched local area networks.

The purpose of the link layer is to detect and correct errors as well as how to allow multiple nodes to share a broadcast channel.  These links can be directly connected through a physical medium or wirelessly.  The link layer also regulates the access to a channel that multiple nodes are trying to send on and supports reliable delivery of frames, flow control, and error detection/correction.

The techniques used to detect and correct errors are parity checking and cyclic redundancy checks (CRC).  Parity checking sets the parity bit so that there are an even number of 1's.  At the receiver, it checks for an even number of 1's, and if there is not then an error is detected.  Two-D parity checking is more trusted because it eliminated the problem that single parity bit checking can have if there are an even number of bit errors which will not flag the detection of an error.  CRC is another error detection technique that can detect all burst errors which makes it the go-to for WiFi and Ethernet.

Multiple access protocols determine how nodes share the broadcast channel.  There are three classes of these protocols.  Taking Turns, which acts exactly how it sounds.  Random Access which will randomly assign nodes a set time for sending.  And channel partitioning which divides the channel into smaller time slots that nodes routinely use.  This can be done using either time division multiple access (TDMA) or frequency division multiple access (FDMA).

LANs use addresses that are called MAC addresses.  These addresses are hard codded into devices which makes these addresses portable, unlike IP that changes from one LAN to the next.  MAC addresses are used locally to get frames from one interface to another on a physically connect interface or on the same network.  To relate the IP and MAC addresses, an address resolution protocol (ARP) is used.  Each node on the LAN has an ARP table that contains the mappings of the IP address to the MAC address of each device on the LAN.