Networking - The TCP/IP Five-Layer Model - The Data Link Layer

The Data Link layer in the TCP/IP model combines many of the functions of the Data Link layer in the OSI model. It is responsible for moving frames across a single physical link and for identifying devices on that link.

In particular, the Data Link layer is responsible for:

• Framing bits from the Physical layer into meaningful units called frames,
• Providing local (link level) addressing so that devices on the same medium can be distinguished, and
• Detecting errors that may have occurred on the physical link.

Typical examples of Data Link layer technologies in a TCP/IP network include:

• Ethernet MAC and LLC sublayers,
• Wireless LAN MAC protocols such as 802.11, and
• Point-to-Point Protocol (PPP) used on serial and tunnel links.

The Data Link layer is responsible for several processes, each of which is responsible for a series of methods:

1. Media Access and Logical Topology
   • Contention-based access (as in Ethernet CSMA/CD and modern switched Ethernet), and
   • Controlled access methods such as token passing or polling on some legacy media.
2. Link-Level Addressing
   • Assigning and using hardware addresses (MAC addresses) to identify individual interfaces, and
   • Learning and maintaining tables that map addresses to switch ports.
3. Error Detection and Limited Recovery
   • Using checksums or cyclic redundancy checks (CRCs) to detect damaged frames, and
   • In some technologies, requesting retransmission of corrupted frames.

Let us move on to layer 3, the Network Layer.

Nah, I want to skip around:

• Introduction
• Physical (Layer 1)
• Data Link (Layer 2)
• Network (Layer 3)
• Transport (Layer 4)
• Application (Layer 5)
• Conclusion

Copyright 1999, Marc Elliot Hall, DBA Sensation! Services