Chapter 9 ETHERNET
Ethernet is the leading LAN technology in use today. Ethernet can be related but it is actually implemented in the lower half of the Data Link layer, which is known as the Media Access Control (MAC) sub layer, and the Physical layer only. The success of Ethernet is due to the following factors:
· Simplicity and ease of maintenance
· Ability to incorporate new technologies
· Reliability
· Low cost of installation and upgrade
Ethernet shared media and collision detection techniques were adapted from ALOHANET radio network. Ethernet was designed to accommodate multiple computers that were interconnected on a shared bus topology. All Ethernet nodes share the media to receive the data sent to it, each node needs a unique address. IP addresses use to communicate between networks while MAC address used within networks across the local media. In Ethernet, different MAC addresses are used for Layer 2 unicast, multicast, and broadcast communications. A unicast MAC address is the unique address used when a frame is sent from a single transmitting device to single destination device. Unicast IP and MAC destination address are used by the source to forward a packet. With a broadcast, the packet contains a destination IP address that has all ones (1s) in the host portion. Broadcast IP and MAC destination addresses are used by the source to forward a packet to all hosts on the network. Recall that multicast addresses allow a source device to send a packet to a group of devices. Devices that belong to a multicast group are assigned a multicast group IP address. Multicast IP and MAC destination addresses deliver packet/frame to specific group of member hosts.
In the last few years, switches have quickly become a fundamental part of most networks. Switches allow the segmentation of the LAN into separate collision domains. Switch acting as a bridge between two shared media hubs. Two collision domains, one for each media LAN. Switch at the center of the LAN, each computer has its own collision domain.
Logical Link Control (LLC)
Handles the communication between the upper layers and the lower layers, typically hardware. Implemented in software and is independent of the physical equipment; can be considered as the NIC driver software in a PC. Provides sub addressing to identify the network protocol that uses the link layer service. Destination Service Access Point (DSAP) and the Source Service Access Point (SSAP): identifies a protocol, or set of protocols, in the next higher OSI layer, the Network layer.
The two Datalink sublayers are as follows:
Logical Link Control (LLC)
Handles the communication between the upper layers and the lower layers, typically hardware. Implemented in software and is independent of the physical equipment; can be considered as the NIC driver software in a PC. Provides sub addressing to identify the network protocol that uses the link layer service. Destination Service Access Point (DSAP) and the Source Service Access Point (SSAP): identifies a protocol, or set of protocols, in the next higher OSI layer, the Network layer.
Media Access Control (MAC)
Ethernet MAC sublayer has the following responsibilities: Data Encapsulation: Fame assembly before transmission, and frame parsing/error detection during and after reception. Media Access Control: Controls frames on and off the media, including initiation of frame transmission and recovery from transmission failure. Addressing: Provides physical address (MAC address) that enables frames to be delivered to destination hosts. Media Access Control is implemented by hardware, typically in the host NIC or equivalent.
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