Introduction to data Link Layer

Data Link Layer

  • Communication at DLL is node-to-node (two end host and the routers as nodes). Links-Cable, air. Two types Broadcast and point-to-point. In broadcast link is shared and in point-to-point the link is dedicated between two ends
  • Services:
  • Located between physical and network layer – When packet is travelling in the internet, DLL of a node(host or router) is responsible for delivering a datagram to the next node in the path.
  • Framing – link addres(dst and src) IP add(src, dst) data, CRC
  • Flow Control
  • Error Control
  • Congestion control
  • Although the TCP/IP reference model is primarily supported as the standard model based on TCP/IP protocol suite, the focus of the TCP/IP reference model does not clearly separate and distinguish the functionality when referring lower layer physical transmission.
  • In light of this, the open systems interconnection, or OSI reference model is often recognized as the model for reference to IEEE 802 standards due to the clear distinction and representation of the behavior of lower layers which closely matches the LAN/MAN reference model standards that are defined as part of the documented IEEE 802-1990 standards for local and metropolitan area networks. In addition, the model that is generally in reference to the ISO protocol suite, provides an extended breakdown of upper layer processing.


As data travels from upper layers to lower layers, the layer append or pre-append certain information with the data coming from upper layer as a result the size at each down layer increases. The additional instructions represent overhead to the existing data and are recognized as instructions to the layer at which the instructions were applied. To other layers, the encapsulated instructions are not distinguished from the original data. The final appending of instructions is performed as part of the lower layer protocol standards (such as the IEEE 802.3 Ethernet standard) before being carried as an encoded signal over a physical medium.

Communication between two end stations


As part of the IEEE 802.3 Ethernet standard, data is encapsulated with instructions in the form of a header and a trailer before it can be propagated over physical media on which Ethernet is supported. Each stage of encapsulation is referred to by a protocol data unit or PDU, which at the data link layer is known as a frame.

Ethernet frames contain instructions that govern how and whether data can be transmitted over the medium between two or more points. Ethernet frames come in two general formats, the selection of which is highly dependant on the protocols that have been defined prior to the framing encapsulation

Frame Formats

Two types of frame Ethernet-II and IEEE802.3. They are identified from the field value

  • Two frame formats are recognized as standard for Ethernet based networks. The DIX version 2 frame type standard was originally developed during the early 1980’s, where today it is recognized as the Ethernet II frame type. Ethernet II was eventually accepted and integrated into the IEEE 802 standards, highlighted as part of section 3.2.6 of the IEEE 802.3x-1997 standards documentation. The IEEE 802.3 Ethernet standard was originally developed in 1983, with key differences between the frame formats including a change to the type field that is designed to identify the protocol to which the data should be forwarded to once the frame instructions have been processed. In the IEEE 802.3 Ethernet format, this is represented as a length field which relies on an extended set of instructions referred to as 802.2 LLC to identify the forwarding protocol.
  • Ethernet II and IEEE 802.3 associate with upper layer protocols that are distinguished by a type value range, where protocols supporting a value less than or equal to 1500 (or 05DC in Hexadecimal) will employ the IEEE 802.3 Ethernet frame type at the data link layer. Protocols represented by a type value greater than or equal to 1536 (or 0600 in Hexadecimal) will employ the Ethernet II standard, and which represents the majority of all frames within Ethernet based networks.
  • Other fields found within the frame include the destination and source MAC address fields that identify the sender and the intended recipient(s), as well as the frame check sequence field that is used to confirm the integrity of the frame during transmission.

Ethernet II Frame


The Ethernet II frame references a hexadecimal type value which identifies the upper layer protocol. One common example of this is the Internet Protocol (IP) which is represented by a hexadecimal value of 0x0800. Since this value for IP represents a value greater than 0x0600, it is determined that the Ethernet II frame type should be applied during encapsulation. Another common protocol that relies on the Ethernet II frame type at the data link layer is ARP, and is represented by the hexadecimal value of 0x0806.

IEEE 802.3 Frame

  • For the IEEE 802.3 frame type, the type field is contained as part of the SNAP extension header and is not so commonly applied the protocols in today’s networks, partially due to the requirement for additional instructions which results in additional overhead per frame. Some older protocols that have existed for many years but that are still applied in support of Ethernet networks are likely to apply the IEEE 802.3 frame type. One clear example of this is found in the case of the Spanning Tree Protocol (STP) that is represented by a value of 0x03 within the type field of the SNAP header.

Frame Forwarding


Ethernet based networks achieve communication between two end stations on a local area network using Media Access Control (MAC) addressing that allows end systems within a multi access network to be distinguished. The MAC address is a physical address that is burned into the network interface card to which the physical medium is connected. This same MAC address is retrieved and used as the destination MAC address of the intended receiver by the sender, before the frame is transferred to the physical layer for forwarding over the connected medium

MAC Address

Figure 2.6 MAC Address
  • Each MAC address is a 48 bit value commonly represented in a hexadecimal format and comprised of two parts that attempt to ensure that every MAC address is globally unique. First part is an organizationally unique identifier that is vendor specific, and he second part of the MAC address is a value that is incrementally and uniquely assigned to each product (e.g. a Network Interface Card or similar product supporting port interfaces for which a MAC is required).

Type of link Layer addressing

There are three type of link layer address

  1. Unicast address
  2. Broadcast
  3. Multicast address
  4. Unicast Address: In this addressing the 8th bit of 1st octet of the MAC address is always set to 0. This signifies that any frame containing this MAC address in the destination MAC address field is intended for a single destination only.

Where hosts exist within a shared collision domain, all connected hosts will receive the unicast transmission but the frame will be generally ignored by all hosts where this MAC does not match. Unicast transmissions are only forwarded from a single physical interface to the intended destination, even in cases where multiple interfaces may exist.

Example : 12:23:34:FA: CB: FE

  1. Broadcast Address-In order to allow traffic to be broadcasted to all hosts within a local area network, the destination MAC address field of the frame is populated with a value that is defined in hexadecimal as FF:FF:FF:FF:FF:FF, and which specifies that all recipients of a frame with this address defined should accept receipt of this frame and process the frame header and trailer.
  2. Multicast addressing- Since there is no relative distinction between unicast MAC addresses and multicast MAC address formats, the multicast address is differentiated using the 8th bit of the first octet. Where this bit value represents a value of 1, it identifies that the address is part of the multicast MAC address range, as opposed to unicast MAC addresses where this value is always 0.

Carrier Sense

Figure 2.7 Carrier Sense
  • As traffic is prepared to be forwarded over the physical network, it is necessary for hosts in shared collision domains to determine whether any traffic is currently occupying the transmission medium. Transmission media such as in the case of 10Base2 provides a shared medium over which CSMA/CD must be applied to ensure collisions are handled should they occur. If the transmission of a frame is detected on the link, the host will delay the forwarding of its own frames until such time as the line becomes available, following which the host will begin to forward frames from the physical interface towards the intended destination.
  • Where two hosts are connected over a medium capable of supporting full duplex transmission as in the case of media such as 10BaseT, it is considered not possible for transmitted frames to suffer collisions since transmission and receipt of frames occurs over separate wires and therefore there is no requirement for CSMA/CD to be implemented.

Frame Processing

Figure 2.8 Frame Processing
  • Once a frame is forwarded, If the destination MAC address and the MAC address of the host are not the same, or the destination MAC address is not a MAC broadcast or multicast address to which the host is listening for, the frame will be ignored and discarded. For the intended destination, the frame will be received and processed, initially by confirming that the frame is intended for the hosts physical interface. The host must also confirm that the integrity of the frame has been maintained during transmission by taking the value of the frame check sequence (FCS) field and comparing this value with a value determined by the receiving host. If the values do not match, the frame will be considered as corrupted and will be subsequently discarded.
  • For valid frames, the host will then need to determine the next stage of processing by analyzing the type field of the frame header and identify the protocol to which this frame is intended. In this example the frame type field contains a hexadecimal value of 0x0800 that identifies that the data taken from the frame should be forwarded to the Internet Protocol, prior to which, the frame header and trailer are discarded.

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