CST 311 W 7

This week, we learned about the link layer, error detection and correction, MAC protocols, and LANs and switches. In chapter 8, we learned about network security, principles of cryptography, message integrity and digital signatures, and end point authentication.

The link layer transfers datagrams from one node to an adjacent node over a link. Error detection and correction is not 100% reliable but is done through single- and two-dimensional bit parity. In 2d bit parity, single bit errors are found and corrected using the checksum.

MAC protocols are distributed algorithms that find how nodes share a channel. The communication uses the channel. MAC protocols have 3 classes: channel partitioning, random access, and taking turns. Channel partitioning divides the channel into smaller pieces like time slots, frequency or code then allocates each piece to a node for use. The random-access protocol doesn’t divide the channel and can allow collusions which are bad. It can recover from collisions, however. In taking turns, the nodes take turns vu the nodes with more to send take longer.

In the ARP: address resolution protocol, it determines the interface’s MAC address from its IP address. It contains an ARP table which has each IP node on a LAN where the IP/MAC address for some LAN nodes are known. It also has the TTL: Time to Live time where the address mapping expires after about 20 minutes. If the nodes are on the same LAN, and the destinations MAC address is not on the ARP table, the source will broadcast the ARP query with the destinations IP address. The destinations MAC address will be FF-FF-FF-FF-FF-FF. The destination will get this ARP packet and reply to the source with their MAC address. The source saves the UP to MAC address in the ARP table until the info times out.  

Ethernet is the most popular Lan wired technology and widespread with rates of 10Mbps-10Gbps. In the past, the bus architecture was used but it often has collisions. Now, the star architecture is used where a switch is in the center and the nodes don’t have collisions. The Ethernet switch is a link-layer device that stores and forwards ethernet frames. It looks at the MAC address and forwards the frames to other links where the frame needs to be forwarded.  Hosts have a dedicated connection to a switch where packets are buffered.