Data Link Layer

Overview

The data link layer gives the upper layers access to the system media. It is in charge of controlling access to the media, epitomizing parcels into properly measured casings for the media utilized, physical tending to, the trading of edges between hubs on the neighborhood system, and mistake recognition.

Media Access Control 

Layer 2, Local Area Network Technologies
and their Media Access Control Characteristics

Ethernet Wireless Ethernet	Token Ring FDDI
Contention Based (first come first serve)	Deterministic Control Based
Collisions	No Collisions
Send anytime (non-deterministic)	Wait for your turn (deterministic)
Physical Star Topology Logical Multi-Access or Bus Topology	Token Ring = Physical Star, Logical Ring Topology FDDI = Physical Dual Ring, Logical Ring
efficient use of bandwidth (send anytime)	Inefficient use of bandwidth (you have to wait your turn)
CSMA/CD (ethernet) CSMA/CA (wireless ethernet)	Token Passing

Control Based Access - Controlled access implies that gadgets or hubs alternate in succession. It is deterministic in that there is booked access of the medium. On the off chance that one gadget is putting information on the system then no other gadget can. Decently requested and unsurprising throughput, can be a wasteful utilization of transfer speed, as a gadget needs to hold its horses.
Contention Based Access - Contention based access is likewise called non-deterministic. This implies that the gadgets on the system don't have to alternate utilizing imparted media. Notwithstanding, to maintain a strategic distance from aggregate turmoil, a Carrier Sense Multiple Access (CSMA) procedure is utilized to verify the media is not being used before a gadget starts to transmit. In spite of the fact that gadgets endeavor to verify the media is not occupied, information crashes still happen with discord based access. Additionally, as more hubs are added to the system, the likelihood of impacts increments.

CSMA-CD (Carrier Sense Multi-Access with Collision Detection) is a media access strategy in which an ethernet host identifies if a sign is being transmitted. In the event that no sign is recognized on the wire, then the host will transmit. There does exist the likelihood that two or more has may sense the unlucky deficiency of a sign and transmit in the meantime. On the off chance that this happens, there is a crash of signs.

CSMA-CA (Carrier Sense Multi-Access with Collision Avoidance) remains for Collision Sense Multiple Access with Collision Avoidance. This is utilized for remote media access control. It utilizes a send and answer like the TCP three way hand shake, thusly it claims all authority to send before sending. After every message is sent the hosts related to the remote access point run a randomization calculation which sets an irregular need on who gets to send next. That alongside numerous control fields help to relieve a portion of the impedances and other radio related remote issues.

Network Topologies

Physical Topology versus Logical Topology - The physical topology is the game plan of gadgets (hubs) and how they are physically joined with the system.

The legitimate topology is the way information is exchanged starting with one gadget (hub) then onto the next paying little heed to how the gadgets are physically associated. It is additionally identified with how every host sees different has on the system and how every host gets to the media. A system's consistent topology is not so much the same as its physical topology. For example, in an Ethernet system, PCs are frequently associated with a switch or center point shaping a physical star topology, yet coherently the way the information voyages is a transport or multi-access topology. In a Token Ring system, PCs are associated with a MAU multistation access unit, structuring a physical star, yet legitimately data makes a trip clockwise from host to host in a ring topology. In FDDI, the physical topology is a double ring (lavish) and sensibly it is additionally a ring. For extra data see: http://en.wikipedia.org/wiki/Network_topology

Point to Point Topology - specifically interfaces two hubs. All edges are put on the media by one hub and taken off by the other. It can be both a physical and consistent topology. Physically it is two hubs specifically joined. Consistently it is two hubs for all intents and purposes associated straightforwardly, yet going through a system. It does exclude alternate gadgets in partitioned areas, that the information goes through. Thusly it structures a virtual circuit between the two hubs. A virtual Circuit is an intelligent association between two hubs and end clients don't recognize the intermedate gadgets.

Multi Access Topology - implies that the hubs are conveying on the same imparted media. One and only hub can utilize the media at once, and each hub sees each edge on the medium. Obviously, just the hub to which the casing is tended to really forms the edge. At the point when imparting media, CSMA/CD and token passing are utilized to lessen impacts.

Ring Topology - In a physical ring topology every gadget is associated with two neighboring gadgets making a physical ring practically like a physical transport. In a consistent ring topology every hub gets an edge thus, and if the casing is not tended to that hub, it passes it on. In a Token Ring system, a hub can't send information on the system unless it has the token, the token is then gone to the following hub thus on in a sensible ring. For more data see: http://en.wikipedia.org/wiki/Ring_network

Data Link Layer Sub Layers

LLC - Logical Link Control sub layer - Helps interface with the upper layers significance the Network layer. Intelligent Link Control (LLC) places data in the edge that recognizes which Network layer convention is being utilized. This data permits numerous Layer 3 conventions, for example, IP, IPX, Apple Talk, and DECNet, to use the diverse sorts of neighborhood media and interfaces, in the same way as Ethernet, Token Ring, distinctive WAN serial conventions and interfaces, for example, PPP, HDLC, and so forth .

MAC - Media Access Control sub layer - Media Access Control gives information connection layer tending to with source and destination MAC addresses. These locations are 48 bit physical locations, generally written in hexadecimal arrangement and smoldered into the NIC. Media Access Control is additionally in charge of denoting the starting and the completion of an edge with a begin of-casing and an end-of-edge delimiter. For more data see: http://en.wikipedia.org/wiki/Media_Access_Control

Layer 2 Frames

Layer 2 edge qualities are like different layers. There is a header, the information payload, and the trailer. The specifics of the edge vary concerning the kind of casing being referred to. There are LAN layer 2 innovation outlines (Ethernet, Token Ring) and WAN layer 2 innovation outlines (PPP, HDLC). One of the primary contrasts is that ethernet edges have source and destination MAC addresses in their casing headers and serial innovations like PPP and HDLC don't.