CST311 - Week 7

 

CST311 – Intro to Computer Networks 

This week, we are learning about the Link layer as an intermediate layer between the Network and physical layer. In layer 2, offers nodes are communicated without any layer 3 devices with a direct physical connection. The network datagram is received and inserted into the frame structure before transmission. The frame structure format depends on the link layer protocol, which encapsulates the datagram. One of the most important services a layer link layer offers is error detection and correction without retransmission. Unlike the checksum in the transport layer and network layer, the robust bit error detection and correction mechanisms are implemented in hardware.

This layer is implemented in both hardware and software, acting as a bridge that delivers frames across nodes using both physical and many software protocols. For example, Link layer services, such as framing, link access, and error corrections, are implemented in the network interface controller (NIC) or the network adapter.

The network multiple access problem occurs when multiple devices share the same transmission link and compete over the communication channel. While there are two links designed for link access, point-to-point link, and broadcast link, the latter is most common in Link layer LAN architecture.  The point-to-point link allows one sender and one receiver, while the broadcast link has several nodes. Since shared link medium like shared wireless or shared wired permits nodes to transmit frames at the same time, transmission collisions will occur at the receiver, resulting in lost frames. Hence, channel partitioning protocols, random access protocols, and taking-turns protocols were designed to manage transmissions and ensure efficient data transmission.

TDM and FDM are two divisions techniques of multiplexing used in channel partitioning to eliminate collision and divide bandwidth among nodes. ALOHA is a fully decentralized protocol type of random access designed to retransmit frames fully first by allowing collision, then retransmit collided frames again with probability p. Carrier sense multiple access (CSMA) and CSMA with collision detection (CSMA/CD) protocols are designed to listen for idle channel before transmitting.

The module introduces the MAC address or link-layer addresses, which are often fixed addresses of network adapters (excluding L2 devices). Another important protocol is the Address Resolution Protocol (ARP), designed to translate IP addresses into MAC addresses. We also reviewed link-layer switches and compared their functions to those of layer 3 routers.