Principles of The Data Link Layer: The Framing Problem

Limitations Imposed Upon The Data Link Layer

Limitations Imposed by The Data Link Layer

The data link layer uses the service provided by the physical layer. Although there are many different implementations of the physical layer from a technological perspective, they all provide a service that enables the data link layer to send and receive bits between directly connected devices.

Most data link layer technologies impose limitations on the size of the frames:

1. Some technologies only impose a maximum frame size.
2. Others enforce both minimum and maximum frame sizes.
3. Finally, some technologies only support a single frame size. In this case, the data link layer will usually include an adaptation sub-layer to allow the network layer to send and receive variable-length packets. This adaptation layer may include fragmentation and reassembly mechanisms.

Limitations Imposed By The Physical Layer

The physical layer service facilitates the sending and receiving of bits, but it’s usually far from perfect:

• The physical layer may change the value of a bit being transmitted due to any reason, e.g., electromagnetic interferences.
• The Physical layer may deliver more bits to the receiver than the bits sent by the sender.
• The Physical layer may deliver fewer bits to the receiver than the bits sent by the sender.

The Framing Problem

The data link layer must allow end systems to exchange frames containing packets despite all of these limitations.

On point-to-point links and Local Area Networks, the first problem to be solved is how to encode a frame as a sequence of bits so that the receiver can easily recover the received frame despite the limitations of the physical layer. This is the framing problem. It can be defined as: "How does a sender encode frames so that the receiver can efficiently extract them from the stream of bits that it receives from the physical layer?” Several solutions have been proposed and are used in practice in different data link layer technologies.

Solution #1: Idle Physical Layer

A first solution to solve the framing problem is to require the physical layer to remain idle for some time after the transmission of each frame. These idle periods can be detected by the receiver and serve as a marker to indicate frame boundaries.

Unfortunately, this solution is not sufficient for two reasons:

1. First, some physical layer implementations can’t remain idle and always need to transmit bits.
2. Second, inserting an idle period between frames decreases the maximum bandwidth that can be achieved by the data link layer.

Solution #2: Multi-symbol Encodings

Some physical layer implementations provide an alternative to this idle period. All physical layer types are able to send and receive physical symbols that represent values $0$ and $1$. Also, several physical layer types are able to exchange other physical symbols as well. Some technologies use these other special symbols as markers for the beginning or end of frames. For example, the Manchester encoding used in several physical layers can send four different symbols. Apart from the encodings for $0$ and ...