Sem 2 Networking Programming Week 5-1 Data Link Layer

Network Topologies

• Star and extended star topologies are easy to install, scalable, and easy to troubleshoot.

Bus and Ring Topologies

• Bus topology: all end systems are chained together and terminated on each end.
• Ring topology: each end system is connected to its respective neighbors to form a ring.

Communication Modes

• Half-duplex communication: only allows one device to send or receive at a time on a shared medium.
• Full-duplex communication: allows both devices to simultaneously transmit and receive on a shared medium.

Access Control Methods

• Contention-based access: CSMA/CD and CSMA/CA.
• Controlled access: used on WLANs and legacy bus topologies with Ethernet hubs.

Data Link Frame

• A data link frame consists of a header, data, and trailer.
• The fields of the header and trailer vary according to the data link layer protocol.

Frame Fields

• Frame Start and Stop: identify the beginning and end of a frame.
• Addressing: indicates source and destination nodes.
• Types: identifies the encapsulated Layer 3 protocol.
• Control: identifies flow control services.
• Data: contains the frame payload.
• Error Detection: used to detect transmission errors.

LAN and WAN Frames

• LAN and WAN frames use different data link layer protocols, such as Ethernet, 802.11 Wireless, Point-to-Point (PPP), High-Level Data Link Control (HDLC), and Frame-Relay.

Layer 2 Addresses

• Also referred to as a physical address.
• Contained in the frame header.
• Used only for local delivery of a frame on the link.
• Updated by each device that forwards the frame.

Data Link Layer

• Responsible for communications between end-device network interface cards.
• Allows upper layer protocols to access the physical layer media and encapsulates Layer 3 packets into Layer 2 frames.
• Performs error detection and rejects corrupt frames.

IEEE 802 LAN/MAN Data Link Sublayers

• Consists of two sublayers: Logical Link Control (LLC) and Media Access Control (MAC).
• LLC sublayer communicates between networking software and device hardware.
• MAC sublayer is responsible for data encapsulation and media access.

Providing Access to Media

• Routers perform four basic Layer 2 functions: accepting a frame, de-encapsulating, re-encapsulating, and forwarding.

Data Link Layer Standards

• Defined by engineering organizations: IEEE, ITU, ISO, and ANSI.

Physical and Logical Topologies

• Physical topology: shows physical connections and interconnections between devices.
• Logical topology: identifies virtual connections between devices using device interfaces and IP addressing schemes.

WAN Topologies

• Point-to-point: a permanent link between two endpoints.
• Hub and spoke: a central site interconnects branch sites through point-to-point links.
• Mesh: provides high availability but requires every end system to be connected to every other end system.