NOTES Flashcards
The Data Link Layer
The data link layer of the OSI model (Layer 2), as shown in the figure, prepares network data for the physical network.
WHAT IS A RESPONSIBLE OF DATA LINK LAYER?
The data link layer is responsible for network interface card (NIC) to network interface card communications.
WHAT DATA LINK LAYER FOLLOW?
The data link layer does the following:
Enables upper layers to access the media. The upper layer protocol is completely unaware of the type of media that is used to forward the data.
Accepts data, usually Layer 3 packets (i.e., IPv4 or IPv6), and encapsulates them into Layer 2 frames.
Controls how data is placed and received on the media.
Exchanges frames between endpoints over the network media.
Receives encapsulated data, usually Layer 3 packets, and directs them to the proper upper-layer protocol.
Performs error detection and rejects any corrupt frame.
WHAT A NODE DO?
In computer networks, a node is a device that can receive, create, store, or forward data along a communications path. A node can be either an end device such as a laptop or mobile phone, or an intermediary device such as an Ethernet switch.
WHAT HAPPEN WITHOUT DATA LINK?
Without the data link layer, network layer protocols such as IP, would have to make provisions for connecting to every type of media that could exist along a delivery path. Additionally, every time a new network technology or medium was developed IP, would have to adapt.
IEEE 802 LAN/MAN Data Link Sublayers?
The IEEE 802 LAN/MAN data link layer consists of the following two sublayers:
LLC - LOGICAL LINK CONTROL
MAC - MEDIA ACCESS CONTROL
DESCRIBES LLC?
Logical Link Control (LLC) - This IEEE 802.2 sublayer communicates between the networking software at the upper layers and the device hardware at the lower layers. It places information in the frame that identifies which network layer protocol is being used for the frame. This information allows multiple Layer 3 protocols, such as IPv4 and IPv6, to use the same network interface and media.
DESCRIBES MAC?
Media Access Control (MAC) – Implements this sublayer (IEEE 802.3, 802.11, or 802.15) in hardware. It is responsible for data encapsulation and media access control. It provides data link layer addressing and it is integrated with various physical layer technologies.
WHAT IS THE DIFERENCE IN LLC AND MAC?
The LLC sublayer takes the network protocol data, which is typically an IPv4 or IPv6 packet, and adds Layer 2 control information to help deliver the packet to the destination node.
The MAC sublayer controls the NIC and other hardware that is responsible for sending and receiving data on the wired or wireless LAN/MAN medium.
HOW MAC PROVIDES ENCAPSULATION?
The MAC sublayer provides data encapsulation:
Frame delimiting - The framing process provides important delimiters to identify fields within a frame. These delimiting bits provide synchronization between the transmitting and receiving nodes.
Addressing - Provides source and destination addressing for transporting the Layer 2 frame between devices on the same shared medium.
Error detection - Includes a trailer used to detect transmission errors.
The MAC sublayer also provides media access control, allowing multiple devices to communicate over a shared (half-duplex) medium. Full-duplex communications do not require access control.
WHAT MAC SUBLAYER RESOLVES?
Each network environment that packets encounter as they travel from a local host to a remote host can have different characteristics. For example, an Ethernet LAN usually consists of many hosts contending for access on the network medium. The MAC sublayer resolves this. With serial links the access method may only consist of a direct connection between only two devices, usually two routers. Therefore, they do not require the techniques employed by the IEEE 802 MAC sublayer.
HOW ROUTER ENCAPSULATES THE PACKET?
Router interfaces encapsulate the packet into the appropriate frame. A suitable media access control method is used to access each link. In any given exchange of network layer packets, there may be numerous data link layers and media transitions.
At each hop along the path, a router performs the following Layer 2 functions:
Accepts a frame from a medium
De-encapsulates the frame
Re-encapsulates the packet into a new frame
Forwards the new frame appropriate to the medium of that segment of the physical network
WHAT ARE DATA LINK STANDARS
Data link layer protocols are generally not defined by Request for Comments (RFCs), unlike the protocols of the upper layers of the TCP/IP suite. The Internet Engineering Task Force (IETF) maintains the functional protocols and services for the TCP/IP protocol suite in the upper layers, but they do not define the functions and operation of the TCP/IP network access layer.
Engineering organizations that define open standards and protocols that apply to the network access layer (i.e., the OSI physical and data link layers) include the following:
Institute of Electrical and Electronics Engineers (IEEE)
International Telecommunication Union (ITU)
International Organization for Standardization (ISO)
American National Standards Institute (ANSI)
WHAT IS THE DIFERENCE OF TOPOLOGIE PHISICAL AND LOGICAL?
Physical topology – Identifies the physical connections and how end devices and intermediary devices (i.e, routers, switches, and wireless access points) are interconnected. The topology may also include specific device location such as room number and location on the equipment rack. Physical topologies are usually point-to-point or star.
Logical topology - Refers to the way a network transfers frames from one node to the next. This topology identifies virtual connections using device interfaces and Layer 3 IP addressing schemes.
DESCRIBE TOPOLOGIES FOR A WAN?
POINT-TO-POINT - This is the simplest and most common WAN topology. It consists of a permanent link between two endpoints.;
HUB AND SPOKES- This is a WAN version of the star topology in which a central site interconnects branch sites through the use of point-to-point links. Branch sites cannot exchange data with other branch sites without going through the central site.;
MESH - This topology provides high availability but requires that every end system is interconnected to every other system. Therefore, the administrative and physical costs can be significant. Each link is essentially a point-to-point link to the other node.
WHAT IS TOPOLOGIE POINT-TO-POINT?
In this arrangement, two nodes do not have to share the media with other hosts. Additionally, when using a serial communications protocol such as Point-to-Point Protocol (PPP), a node does not have to make any determination about whether an incoming frame is destined for it or another node. Therefore, the logical data link protocols can be very simple, as all frames on the media can only travel to or from the two nodes. The node places the frames on the media at one end and those frames are taken from the media by the node at the other end of the point-to-point circuit.
LANS TOPOLOGIES?
In multiaccess LANs, end devices (i.e., nodes) are interconnected using star or extended star topologies.
In this type of topology, end devices are connected to a central intermediary device, in this case, an Ethernet switch.
EXTENDED STAR AND STAR?
An extended star extends this topology by interconnecting multiple Ethernet switches. The star and extended topologies are easy to install, very scalable (easy to add and remove end devices), and easy to troubleshoot. Early star topologies interconnected end devices using Ethernet hubs.
POINT-TO-POINT LAN?
At times there may be only two devices connected on the Ethernet LAN. An example is two interconnected routers. This would be an example of Ethernet used on a point-to-point topology.
Legacy LAN Topologies
Early Ethernet and legacy Token Ring LAN technologies included two other types of topologies:
Bus - All end systems are chained to each other and terminated in some form on each end. Infrastructure devices such as switches are not required to interconnect the end devices. Legacy Ethernet networks were often bus topologies using coax cables because it was inexpensive and easy to set up.
Ring - End systems are connected to their respective neighbor forming a ring. The ring does not need to be terminated, unlike in the bus topology. Legacy Fiber Distributed Data Interface (FDDI) and Token Ring networks used ring topologies.
Half-duplex communication
Both devices can transmit and receive on the media but cannot do so simultaneously. WLANs and legacy bus topologies with Ethernet hubs use the half-duplex mode. Half-duplex allows only one device to send or receive at a time on the shared medium.
Full-duplex communication
Both devices can simultaneously transmit and receive on the shared media. The data link layer assumes that the media is available for transmission for both nodes at any time. Ethernet switches operate in full-duplex mode by default, but they can operate in half-duplex if connecting to a device such as an Ethernet hub.
NOTES HALF-DUPLEX AND FULL-DUPLEX
In summary, half-duplex communications restrict the exchange of data to one direction at a time. Full-duplex allows the sending and receiving of data to happen simultaneously.
It is important that two interconnected interfaces, such as a host NIC and an interface on an Ethernet switch, operate using the same duplex mode. Otherwise, there will be a duplex mismatch creating inefficiency and latency on the link.
Access Control Methods
Ethernet LANs and WLANs are examples of multiaccess networks. A multiaccess network is a network that can have two or more end devices attempting to access the network simultaneously.
Some multiaccess networks require rules to govern how devices share the physical media. There are two basic access control methods for shared media:
Contention-based access
Controlled access
Contention-based access
In contention-based multiaccess networks, all nodes are operating in half-duplex, competing for the use of the medium. However, only one device can send at a time. Therefore, there is a process if more than one device transmits at the same time. Examples of contention-based access methods include the following:
Carrier sense multiple access with collision detection (CSMA/CD) used on legacy bus-topology Ethernet LANs
Carrier sense multiple access with collision avoidance (CSMA/CA) used on Wireless LANs
Controlled access
In a controlled-based multiaccess network, each node has its own time to use the medium. These deterministic types of legacy networks are inefficient because a device must wait its turn to access the medium. Examples of multiaccess networks that use controlled access include the following:
Legacy Token Ring
Legacy ARCNET
Contention-Based Access - CSMA/CD
Examples of contention-based access networks include the following:
Wireless LAN (uses CSMA/CA)
Legacy bus-topology Ethernet LAN (uses CSMA/CD)
Legacy Ethernet LAN using a hub (uses CSMA/CD)
These networks operate in half-duplex mode, meaning only one device can send or receive at a time. This requires a process to govern when a device can send and what happens when multiple devices send at the same time.
If two devices transmit at the same time, a collision will occur. For legacy Ethernet LANs, both devices will detect the collision on the network. This is the collision detection (CD) portion of CSMA/CD. The NIC compares data transmitted with data received, or by recognizing that the signal amplitude is higher than normal on the media. The data sent by both devices will be corrupted and will need to be resent.
Contention-Based Access - CSMA/CA
Another form of CSMA used by IEEE 802.11 WLANs is carrier sense multiple access/collision avoidance (CSMA/CA).
CMSA/CA uses a method similar to CSMA/CD to detect if the media is clear. CMSA/CA uses additional techniques. In wireless environments it may not be possible for a device to detect a collision. CMSA/CA does not detect collisions but attempts to avoid them by waiting before transmitting. Each device that transmits includes the time duration that it needs for the transmission. All other wireless devices receive this information and know how long the medium will be unavailable.
In the figure, if host A is receiving a wireless frame from the access point, hosts B, and C will also see the frame and how long the medium will be unavailable.
The Frame
The data link layer prepares the encapsulated data (usually an IPv4 or IPv6 packet) for transport across the local media by encapsulating it with a header and a trailer to create a frame.
WHAT IS A TYPE OF A FRAME?
The data link protocol is responsible for NIC-to-NIC communications within the same network. Although there are many different data link layer protocols that describe data link layer frames, each frame type has three basic parts:
Header
Data
Trailer
Unlike other encapsulation protocols, the data link layer appends information in the form of a trailer at the end of the frame.
NOTES FIELDS FRAME?
All data link layer protocols encapsulate the data within the data field of the frame. However, the structure of the frame and the fields contained in the header and trailer vary according to the protocol.
There is no one frame structure that meets the needs of all data transportation across all types of media. Depending on the environment, the amount of control information needed in the frame varies to match the access control requirements of the media and logical topology. For example, a WLAN frame must include procedures for collision avoidance and therefore requires additional control information when compared to an Ethernet frame.
