Data Link Layer and Ethernet

COM211 Communications and NetworksCDA CollegeTheodoros ChristophidesEmail: theo_christopher@hotmail.comwww.cdacollege.ac.cy/site/info-com-technology-ll/

Data Link Layer

provides a means for exchanging data over a common local media.

allows the upper layers to access the media using techniques such as framing

controls how data is placed on the media and is received from the media using techniques such as media access control and error detection

Frame - The Data Link layer PDU exists as a connecting layer between the

software processes of the layers above it and the Physical layer below it.

Data Link Frame

Data - The packet from the Network layer Header - Contains control information,

such addressing, and is located at the beginning of the PDU

Trailer - Contains control information added to the end of the PDU

Typical Fields

Start and stop indicator fields - beginning and end limits of the frame

Naming or addressing fields Type field - The type of PDU contained in the

frame Quality - control fields A data field -The frame payload (Network layer

packet)

Data Link Sublayers

upper sublayer defines the software processes that provide services to the Network layer protocols.

lower sublayer defines the media access processes performed by the hardware.

Data Link Sublayers

Logical Link Control (LLC) places information in the frame that identifies which Network layer protocol is being used for the frame. This information allows multiple Layer 3 protocols to utilize the same network interface and media.

Media Access Control (MAC) provides Data Link layer addressing and delimiting of data according to the physical signaling requirements of the medium and the type of Data Link layer protocol in use. MAC address

Communication

Half duplex communication means that the devices can both transmit and receive on the media but cannot do that simultaneously.

Full duplex, both devices can transmit and receive on the media at the same time.

Ethernet

Operates across two layers of the OSI model. The model provides a reference to which 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) sublayer, and the Physical layer only.

Ethernet CSMA/CD

Data that is transmitted in the form of packets. To sense the channel prior to packet

transmission. Transmitting packets only if the channel is idle,

else defer the transmission until channel becomes idle.

After packet transmission is started, the node monitors its own transmission to see if the packet had a collision.

If the packet is observed to be undergoing a collision, the transmission is aborted and the packet is retransmitted after a random interval of time

MAC Address MAC Address or Hardware Address which is a

unique 6 Byte address. MAC Address is represented in Hex Decimal

format e.x. 00:05:6A:FE:20:AA The first 3 bytes identify a vendor (prefix) and

the last 3 bytes are unique for every host or device

MAC Sublayer

Data EncapsulationFrame delimiting AddressingError detection

Media Access Control Logical Topology

Ethernet Frame Fields

Preamble and Start Frame Delimiter Fields Destination MAC Address Field Source MAC Address Field Length/Type Field Data and Pad Fields