WEEK 5-1.docx

**WEEK 5 - 1** **Data Link Layer** **Purpose of the Data Link Layer** - The Data Link layer is responsible for communications between end-device network interface cards. - It allows upper layer protocols to access the physical layer media and encapsulates Layer 3 packets (IPv4 and IP...

**WEEK 5 - 1** **Data Link Layer** **Purpose of the Data Link Layer** - The Data Link layer is responsible for communications between end-device network interface cards. - It allows upper layer protocols to access the physical layer media and encapsulates Layer 3 packets (IPv4 and IPv6) into Layer 2 Frames. - It also performs error detection and rejects corrupts frames. A computer diagram with orange arrows and a black box Description automatically generated with medium confidence **IEEE 802 LAN/MAN Data Link Sublayers** - IEEE 802 LAN/Man standards are specific to the type of network (Ethernet, WLAN,WPAN,etc). The Data Link Layer consists of two sublayers. - **Logical Link Control (LLC)** - LLC sublayer communicates between the networking software at the upper layers and the device hardware at the lower layers. - **Media Access Control (MAC)** - The MAC sublayer is responsible for data encapsulation and media access **Providing Access to Media** Packets exchanged between nodes may experience numerous data link layers and media transitions. At each hop along the path, a router performs four basic Layer 2 functions: - Accepts a frame from the network medium. - De-encapsulates the frame to expose the encapsulated packet. - Re-encapsulates the packet into a new frame. - Forwards the new frame on the medium of the next network segment. **Data Link Layer Standards** Data link layer protocols are defined by engineering organizations: - IEEE - ITU (International Telecomm. Union) - ISO(International Organizations for Standardization (ISO) - American National Standards Institute (ANSI) **Physical and Logical Topologies** The topology of a network is the arrangement and relationship of the network devices and the interconnections between them. There are two types of topologies used when describing networks: - **Physical topology** - shows physical connections and how devices are interconnected - **Logical topology** - identifies the virtual connections between devices using device interfaces and IP addressing schemes **\ ** **WAN Topologies** **There are three common physical WAN topologies:** - **Point-to-point** - **the simplest and most common WAN topology. Consists of a permanent link between two endpoints.** - **Hub and spoke** - **similar to a star topology where 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.** **Point-to-Point WAN Topology** - **Physical point-to-point topologies directly connect two nodes.** - **The nodes may not share the media with other hosts.** - **Because all frames on the media can only travel to or from the two nodes, Point-to-Point WAN protocols can be very simple** **LAN Topologies** **End devices on LANs are typically interconnected using a star or extended start topology. Star and extended start topologies are easy to install, very scalable and easy to troubleshoot** **Early Ethernet and Legacy Token Ring technologies provide two additional topologies:** - **Bus** - **All end systems chained together and terminated on each end.** - **Ring** - **Each end system is connected to its respective neighbors to from a ring.** **\ ** Several computer diagrams showing different types of computer networks Description automatically generated with medium confidence **Half and Full Duplex Communication** - **Half-duplex communication** - **Only allows one device to send or receive at a time on a shared medium.** - **Used on WLANs and legacy bus topologies with Ethernet hubs.** - **Full-duplex communication** - **Allows both devices to simultaneously transmit and receive on a shared medium.** ** Ethernet switches operate in full-duplex mode.** **\ ** **Access Control Methods** - **Contention-based access** - **Controlled access** **Contention-Based Access -- CSMA/CD** - **CSMA/CD** - **CSMA/CD collision detection process:** **\ ** **Contention-Based Access -- CSMA/CA** - **CSMA/CA** - **CSMA/CA collision avoidance process:** **Data Link Frame** **The Frame** **Data is encapsulated by the data link layer with a header and a trailer to form a frame.** **A data link frame has three parts:** ** Header** ** Data** ** Trailer** **The fields of the header and trailer vary according to data link layer protocol.** **The amount of control information carried with in the frame varies according to access control information and logical topology.** **\ Frame Fields**  **Field** **Description** -------------------------- ---------------------------------------------- **Frame Start and Stop** **Identifies beginning and end of frame** **Addressing** **Indicates source and destination nodes** **Types** **Identifies encapsulated Layer 3 protocol** **Control** **Identifies flow control service** **Data** **Contains the frame payload** **Error Detection** **Used for determine transmission errors** **LAN and WAN Frames** **The logical topology and physical media determine the data link** **protocol used:** ** Ethernet** ** 802.11 Wireless** ** Point-to-Point (PPP)** ** High-Level Data Link Control (HDLC)** ** Frame-Relay** **Each protocol performs media access control for specified logical** **topologies.** **\ ** **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.** A diagram of a computer Description automatically generated