Ethernet at Layers 1 and 2

Contents

The OSI Model

Picture 1

The OSI model as shown in Picture 1 above comprises of 7 (seven) Levels. Each interface interlinks with the next level in the cycle.

The OSI Model is a model that illustrates how data communications should take place. It segregates the process into seven groups, called layers. Into these layers are integrated the protocol standards developed by the ISO. (Reference 1)

The low- levels, the physical and Data layer aid in converting data into binary pulses also known as bits to send through the transporting medium. The reverse also happens in which data is received in these electrical pulses and then converted to Bits at the physical layer and then to Frames at the Data Link Layer.

Part A - Layer 1

Operation in an Ethernet LAN

The Physical Layer, also known as Layer 1 in the OSI model comprises of the electrical signalling and cables. This is the interface where information enters and exits the OSI model. This is the only layer in the OSI model; were data is physically transferred over the network interface.

The main functions of the Physical Layer are: (Reference 2)

To Define Hardware Specifications: This is the details of operations; which include cables, connectors, wireless transceivers, network interface cards and other ...

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Part A - Layer 1

Operation in an Ethernet LAN

The main functions of the Physical Layer are: (Reference 2)

To Define Hardware Specifications: This is the details of operations; which include cables, connectors, wireless transceivers, network interface cards and other hardware devices that function on the Physical Layer, examples include RJJ45, NRZ, Coaxial, Twisted Pair.
Encoding and Signalling: The Physical Layer is responsible for various signalling and encoding that transform bits that reside in the computer to signals that can be sent over a network.
Data Transmission and Reception: The Physical Layer Transmits and receives Data. It acts as the physical gateway for information.
Topology and Physical Network Design: considered the design of many hardware- related topology Networks like the Local Area Network and Wide Area Network topologies.

The layer also informs the driver software for the Media Attachment Unit (MAU) what needs to be sent across the media, although this does not account for the operation of Network interface cards.

CSMA/CD

Carrier Sense Multiple Access\ Collusion Detection is the protocol used in Ethernet networks to ensure that only one network node is transmitting on the network wire at any one time.

Carrier Sense: Every Ethernet device listens to the wire before attempting to send information. If there is a transmission already occurring, the devices will standby its transmission.

Multiple Access: more than one device could be on standby awaiting transmission.

Collusion Detention: when multiple Ethernet devices accidentally transmit at one time, they are able to detect this error. (Reference 3)

This is the method used by Ethernet to physically maintain the transmission line between network devices on the network. If a collusion occurs, it is noted if a transmission occurs and another collusion occurs, a random waiting time is activated which is called the Exponential Back Off

CSMA/CD is a contention protocol, standardised By the IEEE 802.3 and ISO 8802.

Part B- Layer 2

Operation of the Data Link Layer

The Data Link layer handles Data Frames between the Network and the Physical layer. The layer provides access to the networking media and enables data to locate its intended destination on a network. This transit is controlled by the Media Access Control addresses (MAC Address). The Layer uses the MAC addressing system to define a hardware or data link address in order for multiple stations to share the same medium and still uniquely address each other. The layers main responsibility is to provide error free communication link, especially with Hop to Hop delivery.

The following are the key tasks performed at the data link layer: (Reference 4)

Logical Link Control (LLC): refers to the functions required for the establishment and control of Logical links on a network. The LLC provides services to the network layer above it and hides the rest of the details of the data link layer to allow different technologies to work seamlessly with the higher layers.

Media Access Control (MAC): refers to the procedures used by devices to control access to the network medium. Since many networks use a shared it is necessary to have rules for managing the medium to avoid conflicts. Ethernet uses the CSMA/CD method of media access control.

Data Framing: The data link layer is responsible for the final encapsulation of higher-level messages into frames that are sent over the network at the physical layer

Addressing: The data link layer is the lowest layer in the OSI model that is concerned with addressing: labelling information with a particular destination location. Each device on a network has a unique number, usually called a hardware address or MAC address that is used by the data link layer protocol to ensure that data intended for a specific machine gets to it properly.

Error Detection and Handling: The data link layer handles errors that occur at the lower levels of the network stack

Collusion Domain

This is a group of Ethernet devices in CSMA/CD that compete for access of the network. Only one device in the collision may transmit at any one time, and the other devices in the domain listen to the network in order to avoid data .