S1 / 06 MODULE 6 / DATA LINK LAYER
Deliver the frame.
Control the local link.
The data link layer moves packets across an individual link. It packages network-layer data into frames and coordinates how devices use the local medium.
What the data link layer does
Layer 2 provides node-to-node delivery over a local medium. It accepts a network-layer packet, encapsulates it into a frame, adds link-layer addressing and error-detection information, and controls access to the medium. The data link layer is concerned with the local link; routers can remove one frame and create a different frame for the next link while the IP packet continues toward its destination.
IMG-0404Separate LLC and MAC responsibilities
The Logical Link Control (LLC) sublayer provides an interface to upper-layer protocols and identifies which network-layer protocol is carried. In IEEE 802.2 LLC, the DSAP and SSAP fields identify service access points. Ethernet II instead uses its Type field to identify the encapsulated protocol; do not confuse the two formats. LLC is implemented in software. The Media Access Control (MAC) sublayer handles data encapsulation, frame addressing, and access to the physical medium. MAC behavior depends on the link technology, such as Ethernet or wireless LAN.
IMG-0455 · IMG-0530Recognize link topologies
A point-to-point topology connects two nodes over a dedicated link. Multi-access media allow multiple nodes to share the same medium. A logical topology describes how devices access the medium and how frames move, which can differ from the physical cable layout. The topology and medium determine how devices coordinate transmission and what happens when two devices attempt to transmit at once.
IMG-0402Understand media access control
On shared media, devices need rules for deciding when they can transmit. Contention-based methods allow devices to compete for access. Classic shared Ethernet used CSMA/CD to detect collisions; modern switched full-duplex Ethernet links do not use collision detection. Wireless LANs use CSMA/CA because a radio cannot reliably detect a collision while transmitting in the same way. Controlled-access methods coordinate which device may transmit.
Media access control methodsRead the frame structure
A frame carries the packet plus Layer 2 control information. An Ethernet frame includes destination and source MAC addresses, a Type/Length field, payload, and a Frame Check Sequence (FCS). The FCS allows the receiver to detect transmission errors; it does not repair a damaged frame. The exact frame format and addressing fields depend on the Layer 2 technology.
Data link frame · LLC · MACRecognize standards and link protocols
Several organizations publish networking standards. IEEE develops the 802 family, including 802.3 Ethernet and 802.11 wireless LAN. ITU, ISO, and ANSI also publish standards used across communications and information technology. Different links use different data-link protocols: Ethernet is common on LANs, while WAN examples include PPP, HDLC, and Frame Relay. Select a protocol that matches the link technology and interoperability requirements. PPP is a standardized, vendor-neutral option for point-to-point serial links; Cisco HDLC is vendor-specific.
IEEE · ITU · ISO · ANSI · Ethernet · PPP · HDLC · Frame RelayCHECK YOUR UNDERSTANDING
Trace packet-to-frame encapsulation.
Practice LLC versus MAC, local delivery, frame fields, and media access.