Introduction to Ethernet at Data Link Layer

Introduction to Ethernet

Ethernet is the prominent LAN technology; Ether net is a collection of technologies that provides data-link and physical layer specifications to control the access to a shared network media. Ethernet was developed by Xerox in 1972 which has speed around 2.5 Mbps. Xerox, Intel and DEC standardized to Ethernet 1. Later in mid-1980’s IEEE (Institute of Electrical and Electronic Engineers) standardized as IEEE 802.3.
Ethernet technology uses Carrier Sense Multiple access with collision detection (CSMA/CD) to effectively control access of devices to a shared network segment. We know how the CSMA/cd works if the collision occurred when devices simultaneously transmitted csma/cd send jam signal and give random back off timers to collided devices thus effectively maintain collision. Mainly collision occurs in half-duplex which use only one pair of wire. Full-duplex uses two wires which can be used as point-to-point connection between sender and receiving device, this gives us an collision free communication and has much more speed compared to half-duplex.

Advantages of Ethernet over other LAN technologies:

Inexpensive
Highly flexible and scalable
Easy to setup
Vendor friendly

Ethernet Network Media

We already know different types of network media such as coaxial cable, twisted pair, and fiber optic cable. In older days Ethernet mainly depended on coaxial cable and used two type of coaxial cable 10Base2 (Thinnet) and 10Base5 (Thicknet). Thinnet used thinner coaxial cable to interconnect devices it have 10 Mbps speed and have distance coverage of 185 m and BNC connectors is used. Thicknet which has more diameters compared to thinnnet but it can give a distance coverage about 500 m Vampire Tap is used as connector.
Twisted pair cables are twisted in order to reduce the EMI. Twisted pair cable is most commonly used over Ethernet. There are different categories of twisted pair cables depending on twists and coverage.RJ-45 is used as connector.
Fiber Optic Cables use light to transmit data over the network thus it is highly immune to EMI. Have two main categories of fiber single mode and multimode. Single mode fiber has lesser diameter and allow light to enter in one angle only where as in multimode which is greater in diameter and allow light to enter in different angle but has less coverage area compared to single mode.

Ethernet Network Topologies

Topologies define the network as physically and logically. Bus topology and star topologies are fundamental Ethernet topologies.

Ethernet at the Data Link Layer

Ethernet has specification of data link layer and physical layers. Data link layer is responsible for hardware addressing and framing the packets received from the Network layer.

Ethernet MAC Addressing

We already know that MAC (Media Access Control) address is burned in to the Ethernet NIC of each device in the network and this address is globally unique. That means end devices can only have one physical address were as interconnecting device such as router can have multiple data link addresses.MAC address are 48 bits in length and represented by 12 hexadecimal numbers. This mac address should have to be unique so it is divided into two parts.

In this first six hexadecimal is administered by IEEE and assigned to manufacture and last six hexadecimal is administered by the vendor this make mac address unique. Since MAC, address is burned in to the RAM, it is also known by Burned in Addresses (BIAs).
Ethernet using mac address in this higher order bit 47 and 46 has special purpose.

Bit in MAC address	Value	Description
Individual Group (47^th bit)	0	The address is the mac address of the device
Individual Group (47^th bit)	1	Address represent broadcast or multicast addresses
Universal Local (U/l) Global/Local (G/l) 46^th bit	0	Globally administered address by the IEEE
Universal Local (U/l) Global/Local (G/l) 46^th bit	1	The lower 24 address is the locally governed address that is code assigned by the vendor.

Ethernet Framing and Frame Format

We know that data link layer is responsible for framing packets from network; it encapsulates the packet with L2 header and trailers and passed down to the physical layer. Data link layer can also detect the error occurred during transmission but don’t able to rectify the errors.

The above figure shows the Ethernet frame format as per 802.3 standards.
Preamble: It’s alternating zeroes and ones of 56 bits which is used to synchronize the receiver with the signal.
Start Field Delimiter: It is one byte length (10101011) that indicates a valid frame is about to start. Note that preamble and SFD is not actual frame.
Destination Address: The destination address is the mac address of the device to which the frame is transmitting.
Source Address: This 6 byte field is used to identify the sender of the frame. We cannot use broadcast and multicast address in this field.
Type or Length: This 16 bit field is used to identify which network layer protocol is used. If the value is 0800 it means that IPv4 is used.08DD corresponds to IPV6.The 802.3 standard use this field as Length; minimum frame size for Ethernet is 512 bits (64 bytes) including headers.
A frame that is smaller than 64 bytes will be discarded as a runt. The required fields in an Ethernet header add up to 18 bytes; so get only 46 bytes for payload if the payload reaches this it is padded to reach 46 bytes.

Maximum frame size is 1518 bytes (1500 payload + 18 header fields) .Frame that is larger than the maximum will be discarded as a giant. This runts and giants discarded by the receiver will be retransmitted using TCP at higher layers.

Data: Payload is the actual data transmitted through the Ethernet frame it size can be in 46 to 1500 bytes.

Frame Check Sequence: It is used to store CRC to detect the errors.

About Jas Jashim

Jas Jashim Author and Founder of ccnaguru.com.He is electronics and communication engineer by education, working in networking field.Started this blog as an hobby to help you in ccna studies.