All data sent out on a network is from a source and is going to a destination. After data is transmitted, the data link layer of the OSI model provides access to the networking media and physical transmission across the media, which enables the data to locate its intended destination on a network. In addition, the data link layer handles error notification, network topology, and flow control.

In this chapter, you will learn about LAN media and the IEEE model and you will learn how the data link layer provides reliable transit of data across a physical link by using the Media Access Control (MAC) addresses. In so doing, the data link layer is concerned with physical (as opposed to network, or logical) addressing, network topology, line discipline (how end systems will use the network link), error notification, ordered delivery of frames, and flow control. In addition, you will learn how the data link layer uses the MAC address to define a hardware or data link address in order for multiple stations to share the same medium and still uniquely identify each other.

Layer 1 involves media, signals, bit streams that travel on media, components that put signals on media, and various topologies. It performs a key role in the communication that takes place between computers, but its efforts, alone, are not enough. Each of its functions has its limitations. Layer 2 addresses these limitations.

For each limitation in Layer 1, Layer 2 has a solution. For example, Layer 1 cannot communicate with the upper-level layers; Layer 2 does that with Logical Link Control (LLC). Layer 1 cannot name or identify computers; Layer 2 uses an addressing (or naming) process. Layer 1 can only describe streams of bits; Layer 2 uses framing to organize or group the bits. Layer 1 cannot decide which computer will transmit binary data from a group that are all trying to transmit at the same time. Layer 2 uses a system called Media Access Control (MAC).

The Institute of Electrical and Electronic Engineers (IEEE) is a professional organization that defines network standards. The IEEE standards (including IEEE 802.3 and IEEE 802.5) are the predominant and best known LAN standards in the world today. IEEE 802.3 specifies the physical layer, Layer 1, and the channel-access portion of the data link layer, Layer 2. 

The OSI model has seven layers. IEEE standards involve only the two lowest layers, therefore the data link layer is broken into two parts:

• the technology-independent 802.2 LLC standard
• the specific, technology-dependent parts that incorporate Layer 1 connectivity

The IEEE divides the OSI data link layer into two separate sublayers. Recognized IEEE sublayers are:

• Media Access Control (MAC) (transitions down to media)
• Logical Link Control (LLC) (transitions up to the network layer)

These sublayers are active, vital agreements that make technology compatible and computer communication possible. Visit these sites:

The IEEE standard appears, at first glance, to violate the OSI model in two ways. First, it defines its own layer (LLC), including its own Protocol Data Unit (PDU), interfaces, etc. Second, it appears that the MAC layer standards, 802.3 and 802.5, cross over the Layer 2/Layer 1 interface. However, 802.3 and 802.5 define the naming, framing, and Media Access Control rules around which specific technologies were built.

Basically, the OSI model is an agreed-upon guideline; the IEEE came later to solve the problems that networks encountered after they had been built. The curriculum will continue to use the OSI model, but it is important to remember that LLC and MAC perform important functions in the OSI data link layer.

One other difference between the OSI model and the IEEE standards is that of the NIC. The NIC is where the Layer 2 MAC address resides, but in many technologies the NIC also has the transceiver (a Layer 1 device) built into it and connects directly to the physical medium. So it would be accurate to characterize the NIC as both a Layer 1 and a Layer 2 device.

IEEE created the logical link sublayer to allow part of the data link layer to function independently from existing technologies. This layer provides versatility in services to network layer protocols that are above it, while communicating effectively with the variety of technologies below it. The LLC, as a sublayer, participates in the encapsulation process. The LLC PDU is sometimes also called an LLC packet, but this is not a widely used term.

LLC takes the network protocol data, an IP packet, and adds more control information to help deliver the IP packet to its destination. It adds two addressing components of the 802.2 specification - the Destination Service Access Point (DSAP) and the Source Service Access Point (SSAP). This repackaged IP packet then travels to the MAC sublayer for handling by the required specific technology for further encapsulation and data. An example of this specific technology might be one of the varieties of Ethernet, Token Ring, or FDDI.

The LLC sublayer of the data link layer manages communications between devices over a single link on a network. LLC is defined in the IEEE 802.2 specification and supports both connectionless and connection-oriented services, used by higher-layer protocols. IEEE 802.2 defines a number of fields in the data link layer frames that enable multiple higher-layer protocols to share a single physical data link.

The Media Access Control (MAC) sublayer deals with the protocols that a host follows in order to access the physical media.

Layer 2 has four main concepts that you must learn:

1. Layer 2 communicates with the upper-level layers through Logical Link Control (LLC).
2. Layer 2 uses a flat addressing convention (Naming refers to the assignment of unique identifiers - addresses).
3. Layer 2 uses framing to organize or group the data.
4. Layer 2 uses Media Access Control (MAC) to choose which computer will transmit binary data, from a group in which all computers are trying to transmit at the same time.