- •7 Data Link Layer
- •7.0 Chapter Introduction
- •7.0.1 Chapter Introduction Page 1:
- •7.1 Data Link Layer - Accessing the Media
- •7.1.1 Data Link Layer - Supporting & Connecting to Upper Layer Services Page 1:
- •7.1.2 Data Link Layer - Controlling Transfer across Local Media Page 1:
- •7.1.3 Data Link Layer - Creating a Frame Page 1:
- •7.1.4 Data Link Layer - Connecting Upper Layer Services to the Media Page 1:
- •7.1.5 Data Link Layer - Standards Page 1:
- •7.2 Media Access Control Techniques
- •7.2.1 Placing Data on the Media Page 1:
- •7.2.2 Media Access Control for Shared Media Page 1:
- •7.2.3 Media Access Control for Non-Shared Media Page 1:
- •7.2.4 Logical Topology vs Physical Topology Page 1:
- •7.2.5 Point-to-Point Topology Page 1:
- •7.2.6 Multi-Access Topology Page 1:
- •7.2.7 Ring Topology Page 1:
- •7.3 Media Access Control Addressing and Framing Data
- •7.3.1 Data Link Layer Protocols - The Frame Page 1:
- •7.3.2 Framing - Role of the Header Page 1:
- •7.3.3 Addressing - Where the Frame Goes Page 1:
- •7.3.4 Framing - Role of the Trailer Page 1:
- •7.3.5 Data Link Layer Protocols - The Frame Page 1:
- •7.4 Putting it All Together
- •7.4.1 Follow Data Through an Internetwork Page 1:
- •7.5 Labs and Activities
- •7.5.1 Investigating Layer 2 Frame Headers Page 1:
- •7.6 Chapter Summary
- •7.6.1 Summary and Review Page 1:
- •7.7 Chapter Quiz
- •7.7.1 Chapter Quiz Page 1:
7.3.2 Framing - Role of the Header Page 1:
As shown in the figure, the frame header contains the control information specified by the Data Link layer protocol for the specific logical topology and media used.
Frame control information is unique to each type of protocol. It is used by the Layer 2 protocol to provide features demanded by the communication environment.
Typical frame header fields include:
-
Start Frame field - Indicates the beginning of the frame
-
Source and Destination address fields - Indicates the source and destination nodes on the media
-
Priority/Quality of Service field - Indicates a particular type of communication service for processing
-
Type field - Indicates the upper layer service contained in the frame
-
Logical connection control field - Used to establish a logical connection between nodes
-
Physical link control field - Used to establish the media link
-
Flow control field - Used to start and stop traffic over the media
-
Congestion control field - Indicates congestion in the media
The field names above are nonspecific fields listed as examples. Different Data Link layer protocols may use different fields from those mentioned. Because the purposes and functions of Data Link layer protocols are related to the specific topologies and media, each protocol has to be examined to gain a detailed understanding of its frame structure. As protocols are discussed in this course, more information about the frame structure will be explained.
7.3.2 - Framing - Role of the Header The diagram depicts a simple frame structure with a focus on the frame header role. The following components are shown from left to right. Header: - Start frame field - Tells other devices on the network that a frame is coming along the medium. - Address field - Stores the source and destination data-link addresses. - Type/Length field - Optional field used by some protocols to state either what type of data is coming or possible the length of the frame. Data FCS Stop Frame
7.3.3 Addressing - Where the Frame Goes Page 1:
The data Link layer provides addressing that is used in transporting the frame across the shared local media. Device addresses at this layer are referred to as physical addresses. Data Link layer addressing is contained within the frame header and specifies the frame destination node on the local network. The frame header may also contain the source address of the frame.
Unlike Layer 3 logical addresses that are hierarchical, physical addresses do not indicate on what network the device is located. If the device is moved to another network or subnet, it will still function with the same Layer 2 physical address.
Because the frame is only used to transport data between nodes across the local media, the Data Link layer address is only used for local delivery. Addresses at this layer have no meaning beyond the local network. Compare this to Layer 3, where addresses in the packet header are carried from source host to destination host regardless of the number of network hops along the route.
If the packet in the frame must pass onto another network segment, the intermediate device - a router - will decapsulate the original frame, create a new frame for the packet, and send it onto the new segment. The new frame will use source and destination addressing as necessary to transport the packet across the new media.
Addressing Requirements
The need for Data Link layer addressing at this layer depends on the logical topology.
Point-to-point topologies, with just two interconnected nodes, do not require addressing. Once on the medium, the frame has only one place it can go.
Because ring and multi-access topologies can connect many nodes on a common medium, addressing is required for these typologies. When a frame reaches each node in the topology, the node examines the destination address in the header to determine if it is the destination of the frame.
7.3.3 - Addressing - Where the Frame Goes The diagram depicts addressing factors for a logical point-to-point topology and a logical multi-access topology. Logical Multi-Access Topology: The diagram shows multiple PC's sharing a common bus-style media. A multi-access frame has many possible destinations. Data Link Layer addresses are required. Logical Point-to-Point Topology: The diagram shows two routers connected to a network cloud via serial WAN links. A point-to-point frame has only one possible destination. Data Link Layer addresses are not required.