Ethernet Protocols
- (Interlaken, Switzerland - Alvin Wei-Cheng Wong)
The Evolution of Ethernet
- The History of Ethernet
Ethernet was created in the 1970s by DEC, Intel, and Xerox. In fact, it was called DIX Ethernet. It was later called thick Ethernet because of the use of coaxial cable. In the mid 1980s, it was upgraded to support more capabilities and speeds. This was called Ethernet 2, and around the same time, IEEE was creating standards for Ethernet-like networks; these were called 802.3. Throughout the years, we have seen now Ethernet has evolved into 10 Mb/s, 100 Mb/s, and later 1 Gb/s, and today’s 10 Gb/s in the form of IEEE standard 802.3AE.
- Ethernet Standards
Ethernet standards define both the Layer 2 protocols and the Layer 1 technologies. For the Layer 2 protocols, as with all 802 IEEE standards, Ethernet relies on the two separate sublayers of the data link layer to operate, the Logical Link Control (LLC) and the MAC sublayers.
The protocol is split into two sublayers. The media access control (MAC) sublayer will deal with access to the media and define the MAC addresses as forms of identification for all devices in an Ethernet network. The logical link control (LLC) sublayer will deal with interaction with upper layers. And so, as Ethernet components process each packet, they will make references to, for example, IP at upper layers by using the fields in the frame header.
In Ethernet technologies, all machines can sense the channel and determine whether there are signals from other transmitters. And this is the carrier sens part of the protocol. This allows machines to sense the channel and detect collisions, which makes it a collision detection part of the protocol.
Ethernet provides service up to the data link layer. At the data link layer, Ethernet divides the data stream received from the upper layers and encapsulates it into frames, before passing them on to the physical layer.
The OSI Model
The Open Systems Interconnection model (OSI model) is a conceptual model that characterises and standardises the communication functions of a telecommunication or computing system without regard to its underlying internal structure and technology. Its goal is the interoperability of diverse communication systems with standard communication protocols. The model partitions a communication system into severn abstraction layers. A layer serves the layer above it and is served by the layer below it.
The OSI model is represented as a stack because data that is sent across the network has to move through each layer at both the sending and receiving ends. The sending computer generally initiates the process at the application layer and the data is sent down the stack to the physical layer and across the network to the receiving computer. On the receiving end, the data is received at the physical layer and the data packet is sent up the stack to the application layer. For example, the application layer starts the process. Small pieces of information relative to the transmission of information are added to the data at each layer; this is called encapsulation. The process is then reversed on the receiving side to get back to just the data.
Layer 1: The Physical Layer
The first layer of the OSI model is the physical layer. This layer specifies the electrical and mechanical requirements for transmitting data bits across the transmission medium. It involves sending and receiving the data stream on the carrier - whether that carrier uses electrical (cable), light (fiber optic) or radio, infrared or laser (wireless) signals. The physical layer specifications include the following: Voltage changes, Timing of voltage changes, Data rates, Maximum transmission distances, Physical connectors to the transmission medium, Physical topology or layout of the network.
The physical layer addresses digital versus analog signaling, baseband versus broadband signaling, synchronously or asynchronously transmissions, and how signals are divided into channels (multiplexing).
Layer 2: The Data Link Layer
- The Data Link Layer
The data link layer is responsible for maintaining the data link between two hosts or nodes. Its characteristics and functions are as follows:
- Defines and manages the ordering of bits to and from data segments called packets
- Management of frames, which contains data arranged in an organized manner, which provides for an orderly and consistent method of sending data bits across the medium
- Responsible for flow control, which is the process of managing the timing of sending and receiving data so that it doesn't exceed the capacity of the physical connection
- Responsible for error notification, including receiving and managing error messaging related to physical delivery of packets
- Network devices that operate at this layer include Layer 2 switches (switching hubs) and bridges.
The data link layer is divided into two sublayers:
- Logical Link Control (LLC) sublayer provides the logic for the data link. Thus, it controls the synchronization, flow control, and error checking functions of the data link layer.
- Media Access Control (MAC) sublayer provides control for accessing the transmission medium. It is responsible for moving data packets from one network interface card (NIC) to another, across a shared transmission medium. Physical addressing is handled at the MAC sublayer. MAC is also handled at this layer. This refers to the method used to allocate network access to computers and prevent them from transmitting at the same time, causing data collisions. Common MAC methods include Carrier Sense Multiple Access/Collision Detection (CSMA/CD), used by Ethernet networks, Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA), used by AppleTalk networks, and token passing, used by Token Ring and Fiber Distributed Data Interface (FDDI) networks.
- The Logical Link Control (LLC) Sublayer
The Ethernet LLC sublayer handles the communication between the upper layers and the lower layers. This is typically between the networking software and the device hardware. The LLC sublayer takes the network protocol data, which is typically an IPv4 packet, and adds control information to help deliver the packet to the destination node. The LLC is used to communicate with the upper layers of the application, and transition the packet to the lower layers for delivery.
LLC is implemented in software, and its implementation is independent of the hardware. In a computer, the LLC can be considered the driver software for the NIC. The NIC driver is a program that interacts directly with the hardware on the NIC to pass the data between the MAC sublayer and the physical media.
- (Ethernet Protocol - Learncisco)
- The MAC Sublayer
MAC constitutes the lower sublayer of the data link layer. MAC is implemented by hardware, typically in the computer NIC. The specifics are specified in the IEEE 802.3 standards.
- CSMA/CD
Ethernet is a layer 2 protocol, which provides among other things addressing to a MAC address but also an access method. The access method is actually called CSMA/CD or Carrier Sense Multiple Access / Collision Detection, a set of rules determining how network devices respond when two devices attempt to use a data channel simultaneously (called a collision). Standard Ethernet networks use CSMA/CD to physically monitor the traffic on the line at participating stations.
CSMA/CD is a type of contention protocol. If no transmission is taking place at the time, the particular station can transmit. If two stations attempt to transmit simultaneously, this causes a collision, which is detected by all participating stations. After a random time interval, the stations that collided attempt to transmit again. If another collision occurs, the time intervals from which the random waiting time is selected are increased step by step. This is known as exponential back off.
[More to come ...]
