Data Link Layer

Layer 2 of the OSI Reference Model is the data link layer. This layer is responsible for defining the format of layer-2 frames as well as the mechanics of how devices communicate with each other over the physical layer. Here are the components the data link layer is responsible for:

■ Defining the Media Access Control (MAC) or hardware addresses
■ Defining the physical or hardware topology for connections
■ Defining how the network layer protocol is encapsulated in the
data link layer frame
■ Providing both connectionless and connection-oriented services

Normally, the data link layer does not provide connection-oriented services (ones that do error detection and correction). However, in environments that use SNA (Systems Network Architecture) as a data link layer protocol, SNA can provide sequencing and flow control to ensure the deliver of data link layer frames. SNA was developed by IBM to help devices communicate in LAN networks (predominantly Token Ring) at the data link layer. In most instances, it will be the transport layer that provides for reliable connections.

Make sure to remember that the primary function of the data link layer is to regulate how two networking devices connected to the same media type communicate with each other. If the devices are on different media types, the network layer typically plays a role in the communication of these devices.

Data Link Layer Addressing

The data link layer uses MAC, or hardware, addresses for communication. For LAN communications, each machine on the same connected media type needs a unique MAC address. A MAC address is 48 bits in length and is represented as a hexadecimal number. Represented in hex, it is 12 characters in length. To make it easier to read, the MAC address is represented in a dotted hexadecimal format, like this: FFFF.FFFF.FFFF. Since the MAC addresses uses hexadecimal numbers, the values used range from 0–9 and A–F, giving you a total of 16 values for a single digit. For example, a hexadecimal value of A would be 10 in decimal. There are other types of data link layer addressing besides MAC addresses. For instance, Frame Relay uses Data Link Connection Identifiers (DLCIs).

The first six digits of a MAC address are associated with the vendor, or maker, of the NIC. Each vendor has one or more unique sets of six digits. These first six digits are commonly called the organizationally unique identifier (OUI). For example, one of Cisco’s OUI values is 0000.0C. The last six digits are used to uniquely represent the NIC within the OUI value. Theoretically, each NIC has a unique MAC address. In reality, however, this is probably not true. What is important for your purposes is that each of your devices has a unique MAC address on its NIC within the same physical or logical segment. A logical segment is a virtual LAN (VLAN) and is referred to as a broadcast domain. Some devices allow you to change this hardware address, while others won’t.

Each data link layer frame contains two MAC addresses: a source MAC address of the machine creating the frame and a destination MAC address for the device or devices intended to receive the frame. There are three general types of addresses at the data link layer, shown in Table 2-4. A source MAC address is an example of a unicast address—only one device can create the frame.

However, destination MAC addresses can be any of the addresses listed in Table 2-4. The destination MAC address in the data link layer frame helps the other NICs connected to the segment to figure out if they need to process the frame when they receive it or to ignore it. The following sections covers each of these address types in more depth.