CompTIA Network+ exam Chapter 2
Seven layers of the OSI model and their functions:
Application: File, Print, Message, database and application services Presentation: Data encryption, compression, and translation services Session: Dialog control Transport: End-to-end connection Network: Routing Data Link: Framing Physical: Physical Topology Please Do Not Throw Sausage Pizza Away
Layer 2 of the OSI model is the ____.
Data Link layer
When data is encapsulated, which is the correct order?
Data, segment, packet, frame, bits
Which of the following OSI model layers identifies the topology used in a network?
Physical
Which of the following specifications of IEEE (Institute of Electrical and Electronic Engineers) defines the standard for fiber-optic cables?
802.8 Fiber-Optic LANs and MANs
At which of the following OSI layers does the hub operate?
Physical
Of the following, which is the highest layer in the OSI model?
Presentation
Which of the following devices work at the data link layer of the OSI reference model?
Switches Bridges
What are two purposes for segmentation with a bridge? (Choose two.)
To create more collision domains To add more bandwidth for users
At which of the following layers does a bridge handle frames?
Data link
Hardware address
A Data Link layer address assigned to every NIC at the MAC sublayer. The address is in the format xx:xx:xx:xx:xx:xx. Each xx is a two-digit hexadecimal number.
Logical address
A Network layer address that defines how data is sent from one network to another. Examples of logical addresses are IP and IPv6.
Data frame
A frame is the Protocol Data Unit encapsulation at the Data Link layer of the OSI reference model. A data frame encapsulates packets from the Network layer and prepares the data for transmission on a network medium.
Open Systems Interconnect (OSI)
A model defined by the ISO ( International Organization for Standardization) to categorize the process of communication between computers in terms of seven layers. The seven layers are Application, Presentation, Session, Transport, Network, Data Link, and Physical.
Routed protocol
A protocol (such as IP or IPv6) used to transmit user data through an internetwork. By contrast, routing protocols (such as RIP, IGRP, and OSPF) are used to update routing tables between routers.
HTTP, FTP, and Telnet work at which layer of the OSI model?
Application HTTP, FTP and Telnet use TCP at the Transport layer; however, they are all Application‐layer protocols, so the Application layer is the best answer for this question.
What is full duplex and half duplex?
Full duplex: The capacity to transmit information between a sending station and a receiving unit at the same time. Half duplex: The capacity to transfer data in only one direction at a time between a sending unit and receiving unit.
Which of the following organizations is responsible for developing network standards?
IEEE
At which layer of OSI is the Frame Check Sequence for error detection performed?
Layer 2
Which of the following layers of the OSI model is responsible for establishing a communication session between the processes running on different communication entities in a network?
Layer 5 (session layer)
The following four devices are the most popular at the Physical layer of the OSI model
Network Interface Cards (NICs) Transceivers Repeaters Hubs
Which OSI layer is responsible for packet forwarding?
Network layer
Which layer manages the transmission of data across a physical link and is primarily concerned with physical addressing and the ordered delivery of frames?
Protocol Data Units (PDUs) at the Data Link layer are called frames. As soon as you see the word frame in a question, you know the answer.
Which of the following devices operates at the network layer of the OSI model?
Router
Host 1 sent a SYN packet to Host 2. What will Host 2 send in response?
SYN/ACK A connection-oriented session is set up using what is called a three-way handshake. The transmitting host sends a SYN packet, the receiving host sends a SYN/ACK, and the transmitting host replies with the last ACK packet. The session is now set up.
Know the sublayers of the Data Link layer.
The Data Link layer has two sublayers: LLC and MAC. The LLC sublayer is responsible primarily for the multiplexing of Network layer protocols. The MAC sublayer is responsible for physical addressing and determining the appropriate time to place data on the network.
Which layer combines bits into bytes and bytes into frames and uses MAC addressing?
The Data Link layer is responsible for the framing of data packets.
In the figure, mark the layer of the OSI model which is responsible for connectionless communication, addressing and routing data, and message forwarding by clicking on it.
The Network layer is responsible for connectionless communication, addressing and routing data, and message forwarding
Which layer specifies voltage, wire speed, and connector pin-outs and moves bits between devices?
The Physical layer is responsible for the electrical and mechanical connections between devices.
Which layer is responsible for converting frames from the Data Link layer into electrical signals?
The Physical layer takes frames from the Data Link layer and encodes the 1s and 0s into a digital signal for transmission on the network medium.
Which layer defines how data is formatted, presented, encoded, and converted?
The Presentation layer makes sure that data is in a readable format for the Application layer.
The Session Layer
The Session layer is responsible for setting up, managing, and then tearing down sessions between Presentation layer entities. This layer also provides dialog control between devices, or nodes. It coordinates communication between systems and serves to organize their communication by offering three different modes: simplex, half duplex, and full duplex. To sum up, the Session layer basically keeps applications' data separate from other applications' data.
Which layer is responsible for creating, managing, and terminating sessions between applications?
The Session layer sets up, maintains, and terminates sessions between applications.
Internet Architecture Board (IAB)
The committee that oversees management of the Internet. It is made up of two subcommittees: the Internet Engineering Task Force (IETF) and the Internet Research Task Force (IRTF).
Session layer
The fifth layer of the OSI model, which determines how two computers establish, use, and end a session. Security authentication and network-naming functions required for applications occur here. The Session layer establishes, maintains, and breaks dialogs between two stations.
Introduction to Encapsulation
When a host transmits data across a network to another device, the data goes through encapsulation: It's wrapped with protocol information at each layer of the OSI model. Each layer communicates only with its peer layer on the receiving device.
Which of the following units of data does the data-link layer send from the network layer to the physical layer of the OSI model?
Data frames
At which of the following layers of the OSI model does a bridge operate?
Data link
Which of the following layers provides mechanisms to transport data between network devices through error checking, service addressing, and segmentation?
Transport
Advantages we gain by using the OSI layered model
• The OSI model divides network communication processes into smaller and simpler components, thus aiding component development, design, and troubleshooting. • It allows multiple-vendor development through the standardization of network components. • It encourages industry standardization by defining the specific functions that occur at each layer of the model. • It allows various types of network hardware and software to communicate. • It prevents changes in one layer from affecting other layers, facilitating development, and making application programming much easier.
TCP and UDP reside at which layer of the OSI model?
4 TCP and UDP are Transport layer protocols. The Transport layer is Layer 4 of the OSI model.
Drag the IEEE 802 standards to match them with their areas.
802.2 Logical Link Control 802.4 Token Passing Bus 802.5 Token Passing Ring 802.11 Wireless LAN 802.3 CSMA/CD
Which of the following provides protection against network eavesdropping and signal interference and works at the physical layer?
Shielded twisted pair (STP)
What is the function of TDM (time-division multiplexing)?
Transmits and receives independent signals over a common signal path by means of synchronized network devices
Which of the following are functions of the presentation layer of the OSI model?
Data encryption Character-code translation Data compression
Which Layer 1 devices can be used to enlarge the area covered by a single LAN segment? (Choose two.)
Hub Repeater
Which of the following mnemonic devices can you use to remember the first letter of the name of each layer of the OSI model in the proper order?
Please Do Not Throw Sausage Pizza Away.
At which layer is routing implemented, enabling connections and path selection between two end systems?
The Network layer provides routing through an internetwork and logical addressing.
Packet
The basic division of data sent over a network.
Drag the OSI model layers to match them with their features.
1. Data Link ==> Synchronizing transmissions 2. Network ==> Logical addressing 3. Session ==> Tearing down a session 4. Application ==> Service advertisement 5. Physical ==> Synchronizing bits 6. Presentation ==> Encryption 7. Transport ==> Windowing
Local Area Network (LAN)
A network that is restricted to a single building, a group of buildings, or even a single room. A LAN can have one or more servers. LANs are defined by the Data Link protocols they run. For example, Ethernet networks are LANs, but PPP networks are not. They are WAN links.
Internet Research Task Force (IRTF)
An international organization that works under the Internet Architecture Board to research new Internet technologies.
Function of the three top layers of the OSI
Application: Provides a user interface Presentation: Presents data - Handles processing such as encryption Session: Keeps different applications' data separate
Your company's network is facing a continuous problem of busy network during peak working hours. To resolve this issue, you decide to break the company's busy network into segments. Drag the interconnectivity device that you will use from the bottom onto its matching location on the top.
In this scenario, you will use a bridge to resolve the problem of busy network during peak working hours. A bridge is a device used to divide a busy network into segments and reduce network traffic. It is an interconnectivity device that is used to connect two LANs (local area networks) using the same communication protocols, and provides address filtering between them. A bridge operates at the data link layer of the OSI model. In this scenario, you cannot use hubs and switches, as they do not break a busy network into segments to reduce traffic. Hubs and switches are simply connecting devices that connect hosts to resources on a LAN.
Modulation Techniques
Modulation is the process of varying one or more properties of a waveform, called the carrier signal, with a signal that typically contains information to be transmitted. Modulation of a waveform transforms a baseband (Ethernet or wireless) message signal into a passband signal (a passband [a band-pass filtered signal] is the range of frequencies or wavelengths that can pass through a filter without being attenuated). In current networks, modulation takes a digital or analog signal and puts it in another signal that can be physically transmitted.
At which layer of the OSI model would you find IP?
Network IP is a Network layer protocol. TCP is an example of a Transport layer protocol, Ethernet is an example of a Data Link layer protocol, and T1 can be considered a Physical layer protocol.
What Network devices operate at all seven layers of the OSI model?
Network management stations (NMSs) Web and application servers Gateways (not default gateways) Network hosts
Which layer of the OSI model is responsible for code and character-set conversion as well as recognizing data formats?
Presentation The Presentation layer makes data "presentable" for the Application layer.
Drag the OSI layers to match them with their descriptions.
1. Datalink ==> Packages bits of data from the physical layer into frames, transfers them from one computer to another, and receives acknowledgement from the addressed computer 2. Application ==> Allows programs on a network node (computer) to access network services, such as file transfers, electronic mail, and database access 3. Transport ==> Divides long communications into smaller data packages, and handles error recognition and correction, and acknowledges the correct receipt of data 4. Network ==> Addresses data messages and translates logical addresses into physical ones, and routes data to addresses on the network 5. Physical ==> Transmits bits from one computer to another and regulates the transmission stream over a medium 6. Session ==> Establishes and controls data communication between applications operating on two computers 7. Presentation ==> Translates application layer data to an intermediate form that provides security, encryption, and compression for the data
Steps in the Connection-Oriented session or "The Three-Way Handshake" of the Transport Layer
1. The first "connection agreement" segment is a request for synchronization. (SYN) 2. The next segments acknowledge the request and establishes connection parameters—the rules—between hosts. These segments request that the receiver's sequencing is synchronized here as well so that a bidirectional connection is formed. (SYN/ACK) 3. The final segment is also an acknowledgment. It notifies the destination host that the connection agreement has been accepted and that the connection has been established. Data transfer can now begin. (ACK)
At a transmitting device, the data-encapsulation method works like this:
1. User information is converted to data for transmission on the network. 2. Data is converted to segments, and a reliable connection is set up between the transmitting and receiving hosts. 3. Segments are converted to packets or datagrams, and a logical address is placed in the header so each packet can be routed through an internetwork. A packet carries a segment of data. 4. Packets or datagrams are converted to frames for transmission on the local network. Hardware (Ethernet) addresses are used to uniquely identify hosts on a local network segment. Frames carry packets. 5. Frames are converted to bits, and a digital encoding and clocking scheme is used.
One of the major components of the Data Link layer is the result of the IEEE's 802 subcommittees and their work on standards for local area and metropolitan area networks (LANs/MANs). The committee met in February 1980, so they used the 80 from 1980 and the 2 from the second month to create the name Project 802. The designation for an 802 standard always includes a dot (.) followed by either a single or a double digit. These standards are:
802.1 LAN/MAN Management (and Media Access Control Bridges) 802.2 Logical Link Control 802.3 CSMA/CD (Ethernet) 802.4 Token Passing Bus 802.5 Token Passing Ring 802.6 Distributed Queue Dual Bus (DQDB) Metropolitan Area Network (MAN) 802.7 Broadband Local Area Networks 802.8 Fiber-Optic LANs and MANs 802.9 Isochronous LANs 802.10 LAN/MAN Security 802.11 Wireless LAN 802.12 Demand Priority Access Method 802.15 Wireless Personal Area Network 802.16 Wireless Metropolitan Area Network (also called WiMAX) 802.17 Resilient Packet Ring 802.18 LAN/MAN Standards Committee Just remember that 802.3 calls out anything having to do with Ethernet and 802.11 is anything wireless.
Which IEEE standard specifies the protocol for CSMA/CD?
802.3 The 802.3 standard, commonly associated with Ethernet, specifies the media-access method used by Ethernet, which is known as Carrier Sense Multiple Access with Collision Detection (CSMA/CD).
What is a modem?
A modulator is a device that performs modulation of a signal and a demodulator is a device that performs demodulation, the inverse of modulation. We typically just call these modems (from modulator-demodulator), which can perform both operations.
Media Access Control (MAC)
A sublayer of the Data Link layer that controls the way multiple devices use the same media channel. It controls which devices can transmit and when they can transmit.
Logical Link Control (LLC)
A sublayer of the Data Link layer that provides an interface between the MAC sublayer and the Network layer.
Data packet
A unit of data sent over a network. A packet includes a header, addressing information, and the data itself. A packet is treated as a single unit as it is sent from device to device. Also known as a datagram.
Institute of Electrical and Electronics Engineers (IEEE)
An international organization that sets standards for various electrical and electronics issues.
What is frequency-division multiplexing (FDM)?
Analog and digital modulation use something called frequency-division multiplexing (FDM), where several low-pass information signals are transferred simultaneously over the same shared physical network, using separate passband channels (several different frequencies). The digital baseband modulation methods found in our Ethernet networks, and also known as line coding, are used to transfer a digital bit stream over a baseband channel. Baseband means that the signal being modulated used the complete available bandwidth.
You are connected to a server on the Internet and you click a link on the server and receive a time-out message. What layer could be the source of this message?
Application If the remote server is busy or does not respond to your web browser request, this is an Application layer problem.
Which layer of the OSI model provides a user interface in the form of an entry point for programs to access the network infrastructure?
Application The top layer of the OSI model gives applications access to the services that allow network access.
The OSI model has seven layers:
Application (Layer 7) Presentation (Layer 6) Session (Layer 5) Transport (Layer 4) Network (Layer 3) Data Link (Layer 2) Physical (Layer 1) (Please Do Not Throw Sausage Pizza Away) (All People Seem to Need Data Processing )
At which layers of the OSI model do bridges, hubs, and routers primarily operate, respectively?
Data Link, Physical, Network Bridges, like switches, are Data Link layer devices. Hubs, like repeaters, are Physical layer devices. Routers are Network layer devices.
The Presentation Layer
It presents data to the Application layer and is responsible for data translation and code formatting. A successful data-transfer technique is to adapt the data into a standard format before transmission. Computers are configured to receive this generically formatted data and then convert it back into its native format for reading—for example, from EBCDIC to ASCII. By providing translation services, the Presentation layer ensures that the data transferred from one system's Application layer can be read and understood by the Application layer on another's. Tasks like data compression, decompression, encryption, and decryption are all associated with this layer.
Which layer of the OSI model is responsible for converting data into signals appropriate for the transmission medium?
Physical The Physical layer's job is to convert data into impulses that are designed for the wired or wireless medium being used on the attached segment.
Acknowledgments, sequencing, and flow control are characteristic of which OSI layer?
Layer 4 A reliable Transport layer connection uses acknowledgments to make sure all data is received reliably. A reliable connection is defined by the use of acknowledgments, sequencing, and flow control, which is characteristic of the Transport layer (Layer 4).
The IEEE (Institute of Electrical and Electronics Engineers) Ethernet Data Link layer has two sublayers:
Media Access Control (MAC): Defines how packets are placed on the media. Contention media access is "first come, first served" access, where everyone shares the same bandwidth—hence the name. Physical addressing is defined here, as are logical topologies. What's a logical topology? It's the signal path through a physical topology. Line discipline, error notification (not correction), ordered delivery of frames, and optional flow control can also be used at this sublayer. Logical Link Control (LLC): Responsible for identifying Network layer protocols and then encapsulating them, an LLC header tells the Data Link layer what to do with a packet once a frame is received. It works like this: A host receives a frame and looks in the LLC header to find out where the packet is destined—say, the IP protocol at the Network layer. The LLC can also provide flow control and sequencing of control bits.
Routers perform routing at which OSI layer?
Network
The OSI Reference Model
One of the greatest functions of the OSI specifications is to assist in data transfer between disparate hosts regardless if they're Unix-Windows-or Mac-based. But keep in mind that the OSI model isn't a physical model; it's a conceptual and comprehensive yet fluid set of guidelines, which application developers utilize to create and implement applications that run on a network. It also provides a framework for creating and implementing networking standards, devices, and internetworking schemes.
Which of the following layers synchronizes data exchange between applications on separate devices?
Session
The Physical Layer
The Physical layer does two important things: it sends bits and receives bits. Bits come only in values of 1 or 0. The Physical layer communicates directly with the various types of actual communication media. This layer is also where you identify the interface between the data terminal equipment (DTE) and the data communication equipment (DCE). (Some older phone company employees still call DCE data circuit-terminating equipment.) The DCE is usually located at the customer whereas the DTE is the attached device. The services available to the DTE are most often accessed via the DCE device, which is a modem or channel service unit/data service unit (CSU/DSU). The Physical layer specifies the layout of the transmission media, otherwise known as its topology. A physical topology describes the way the cabling is physically laid out (Bus, Star, Ring, and Mesh), as opposed to the logical topology that we just talked about in "The Data Link Layer" section.
The Transport Layer
The Transport layer segments and reassembles data into a data stream. Services located in the Transport layer handle data from upper-layer applications and unite it onto the same data stream. They provide end-to-end data transport services and can establish a logical connection between the sending host and destination host on an internetwork. Transmission Control Protocol (TCP) and User Datagram Protocol (UDP) work at the Transport layer. TCP is a reliable service and UDP is not. The Transport layer can be connectionless or connection-oriented Port Addresses are in Transport Layer
What's the purpose of digital modulation?
The purpose of digital modulation is to transfer a digital bit stream over an analog bandpass channel. (A good example would be data transmitting over the public switched telephone network, where a bandpass filter limits the frequency range to 300-3400 Hz, or over a limited radio frequency band.) The purpose of an analog modulation is to transfer an analog baseband (or lowpass) signal (for example, an audio signal, wireless network, or TV signal) over an analog bandpass channel at a different frequency.
What is Time-division multiplexing (TDM)?
Time-division multiplexing (TDM) is a method of transmitting and receiving many independent signals over a common signal path by means of synchronized network devices at each end of the transmission line so that each signal appears on the line only a fraction of time in an alternating pattern. The receiving end demultiplexes the signal back to its original form.
What is the purpose of flow control?
To provide a means for the receiver to govern the amount of data sent by the sender
On which OSI layer does the UDP protocol work?
Transport
In the figure, drag the functions of the OSI layer model from the right onto their respective layers.
Application Layer: Talks to application software Presentation Layer: Reformats, encrypts/decodes, compresses/decompresses Session Layer: Makes connection at both ends Transport Layer: Disassembles and reassembles segments Network Layer: Finds best possible route Data Link Layer: Error correction Physical Layer: Passes data on to media
Which layer chooses and determines the availability of communicating partners along with the resources necessary to make the connection, coordinates partnering applications, and forms a consensus on procedures for controlling data integrity and error recovery?
Application layer is responsible for finding the network resources broadcast from a server and adding flow control and error control (if the application developer chooses).
Connection-Oriented Communication protocol of the Transport Layer
Before a transmitting host starts to send segments down the model, the sender's TCP process contacts the destination's TCP process to establish a connection. The resulting creation is known as a virtual circuit. This type of communication is called connection-oriented. During this initial handshake, the two TCP processes also agree on the amount of information that will be sent in either direction before the respective recipient's TCP sends back an acknowledgment. With everything agreed on in advance, the path is paved for reliable communication to take place.
Two types of packets are used at the Network layer:
Data Packets: These are used to transport user data through the internetwork. Protocols used to support data traffic are called routed protocols. Two examples of routed protocols are Internet Protocol (IP) and Internet Protocol version 6 (IPv6). Route-Update Packets: These are used to update neighboring routers about the networks connected to all routers within the internetwork. Protocols that send route-update packets are called routing protocols, and some common ones are Routing Information Protocol (RIP), RIPv2, Enhanced Interior Gateway Routing Protocol (EIGRP), and Open Shortest Path First (OSPF). Route-update packets are used to help build and maintain routing tables on each router.
A receiving host has failed to receive all the segments that it should acknowledge. What can the host do to improve the reliability of this communication session?
Decrease the window size. A receiving host can control the transmitter by using flow control (TCP uses windowing by default). By decreasing the window size, the receiving host can slow down the transmitting host so the receiving host does not overflow its buffers.
Know the devices that operate at each layer of the OSI model
Hubs and repeaters see only bits, making them Layer 1 devices. Because all networking devices have physical connectivity to the network, they all operate at Layer 1, but hubs and repeaters operate only at this layer, whereas other devices work in higher layers. Nevertheless, we generally consider that a device operates at the highest layer it supports; that layer's functionality is the main reason we implement the device on the network. For example, switches and bridges are considered Layer 2 devices because they understand and make decisions based on Layer 2 addresses. Routers are Layer 3 devices for a similar reason; they deal with Layer 3 addresses. Networking devices, such as workstations that run applications, are said to operate at the Application layer (or you may hear that they operate at all layers) because they must include Application layer protocols that offer services to networked applications.
Here are the features of the OSI model layers:
Physical Layer: Synchronizing bits How bits are represented on the medium Wiring standards for connectors and jacks Physical topology Bandwidth usage Multiplexing strategy Data Link Layer: Synchronizing transmissions Physical addressing Logical topology Method of transmitting on the media Connection services Network Layer: Logical addressing Switching Route discovery and selection Connection services Bandwidth usage Multiplexing strategy Transport Layer: Windowing TCP/UDP Buffering Session Layer: Tearing down a session Setting up a session Maintaining a session Presentation Layer: Encryption Data formatting Application Layer: Service advertisement Application services
The Application Layer
The Application layer of the OSI model marks the spot where users actually communicate or interact with the computer. Technically, users communicate with the network stack through application processes, interfaces, or APIs that connect the application in use to the operating system of the computer. Take the case of Internet Explorer (IE). You could uninstall every trace of networking components from a system, such as TCP/IP, the network card, and so on, and you could still use IE to view a local HTML document without a problem. But things would definitely get messy if you tried to do something like view an HTML document that had to be retrieved using HTTP or nab a file with FTP or TFTP because IE responds to requests like those by attempting to access the Application layer. So what's happening is that the Application layer acts as an interface between the application program—which isn't part of the layered structure—and the next layer down by providing ways for the application to send information down through the protocol stack. In other words, IE doesn't reside within the Application layer—it interfaces with Application layer protocols when it needs to deal with remote resources. The Application layer is also responsible for identifying and establishing the availability of the intended communication partner and determining whether sufficient resources for the requested communication exist.
Remember the OSI layers
The Application, Presentation, and Session layers are upper layers and are responsible for communicating from a user interface to an application. The Transport layer provides segmentation, sequencing, and virtual circuits. The Network layer provides logical network addressing and routing through an internetwork. The Data Link layer provides framing and placing of data on the network medium. The Physical layer is responsible for taking 1s and 0s and encoding them into a digital signal for transmission on the network segment.
The Data Link Layer
The Data Link layer provides the physical transmission of the data and handles error notification, network topology, and flow control. This means the Data Link layer ensures that messages are delivered to the proper device on a LAN using hardware (MAC) addresses and translates messages from the Network layer into bits for the Physical layer to transmit. The Data Link layer formats the message into pieces, each called a data frame, and adds a customized header containing the destination and source hardware address. This added information forms a sort of capsule that surrounds the original message in much the same way that engines, navigational devices, and other tools were attached to the lunar modules of the Apollo project. These various pieces of equipment were useful only during certain stages of flight and were stripped off the module and discarded when their designated stage was complete. This is a great analogy for data traveling through networks because it works very similarly. It's important for you to understand that routers, which work at the Network layer, don't care about where a particular host is located. They're only concerned about where networks are located and the best way to reach them—including remote ones. The Data Link layer is responsible for the unique identification of each device that resides on a local network. For a host to send packets to individual hosts on a local network as well as transmit packets between routers, the Data Link layer uses hardware addressing. Each time a packet is sent between routers, it's framed with control information at the Data Link layer. However, that information is stripped off at the receiving router, and only the original packet is left completely intact. This framing of the packet continues for each hop until the packet is finally delivered to the correct receiving host. It's important to understand that the packet itself is never altered along the route; it's only encapsulated with the type of control information required for it to be properly passed on to the different media types. MAC Addresses are in the Data Layer
The Network Layer
The Network layer manages logical device addressing, tracks the location of devices on the network, and determines the best way to move data. This means that the Network layer must transport traffic between devices that aren't locally attached. Routers are Layer 3 devices that are specified at the Network layer and provide the routing services within an internetwork. A router can also be referred to as a Layer 3 switch. These terms are interchangeable. It happens like this: First, when a packet is received on a router interface, the destination IP address is checked. If the packet isn't destined for that particular router, the router looks up the destination network address in the routing table. Once the router chooses an exit interface, the packet is sent to that interface to be framed and sent out on the local network. If the router can't find an entry for the packet's destination network in the routing table, the router drops the packet. IP Addresses are in the Network Layer
Which layer provides logical addressing that routers use for path determination?
The Network layer provides logical addressing, typically IP addressing, and routing.
Which layer is used for reliable communication between end nodes over the network and provides mechanisms for establishing, maintaining, and terminating virtual circuits as well as controlling the flow of information?
The Transport layer uses virtual circuits to create a reliable connection between two hosts.
Data Link layer
The second layer of the OSI model. It describes the logical topology of a network, which is the way that packets move throughout a network. It also describes the method of media access. See also Open Systems Interconnect (OSI).
Which of the following is true regarding sequencing and acknowledgments? (Choose all that apply.)
The segments delivered are acknowledged back to the sender upon their reception. Any segments not acknowledged are retransmitted. Segments are sequenced back into their proper order upon arrival at their destination.
International Organization for Standardization (ISO)
The standards organization that developed the OSI model. This model provides a guideline for how communications occur between computers.
What is Encapsulation?
The technique used by layered protocols in which a layer adds header information to the Protocol Data Unit (PDU) from the layer above. As an example, in Internet terminology, a packet contains a header from the Data Link layer, followed by a header from the Network layer (IP), followed by a header from the Transport layer (TCP), followed by the application protocol data.
The OSI's seven layers are divided into two groups
The top three layers define the rules of how the applications working within host machines communicate with each other as well as with end users. The bottom four layers define how the actual data is transmitted from end to end.
The OSI's seven layers are divided into two groups.
The top three layers define the rules of how the applications working within host machines communicate with each other as well as with end users: Application, Presentation, Session The bottom four layers define how the actual data is transmitted from end to end: Transport, Network, Data Link, Physical
Protocol Data Units (PDUs)
To communicate and exchange information, each layer uses Protocol Data Units (PDUs). These hold the control information attached to the data at each layer of the model. They're usually attached to the header in front of the data field but can also be in the trailer, or end, of it.
Segmentation of a data stream happens at which layer of the OSI model?
Transport
The OSI reference model has seven distinct layers. Each layer performs functions that help applications to run on a network. Identify the layer responsible for end-to-end connection by clicking it.
Transport
Function of the four bottom layers of the OSI
Transport: Provides reliable or unreliable delivery - Performs error correction before re-transmit Network: Provides logical addressing, which routers use for path determination Data Link: Combines packets into bytes and bytes into frames - Provides access to media using MAC address - Perform error detection, not correction Physical: Moves bits between devices - Specifies voltage, wire speed and pin-out of cables
The OSI layers are described below:
• Physical: Transmits bits from one computer to another and regulates the transmission stream over a medium • Datalink: Packages bits of data from the physical layer into frames, transfers them from one computer to another, and receives acknowledgement from the addressed computer • Network: Addresses data messages, translates logical addresses into actual physical addresses, and routes data to addresses on the network • Transport: Divides long communications into smaller data packages, handles error recognition and correction, and acknowledges the correct receipt of data • Session: Establishes and controls data communication between applications operating on two computers • Presentation: Translates application layer data to an intermediate form that provides security, encryption, and compression for the data • Application: Allows programs on a network node (computer) access network services, such as file transfers, electronic mail, and database access
Here are some key points about routers that you really should commit to memory:
• Routers, by default, won't forward any broadcast or multicast packets. • Routers use the logical address in a Network layer header to determine the next-hop router to forward the packet to. • Routers can use access lists, created by an administrator, to control security on the types of packets that are allowed to enter or exit an interface. • Routers can provide Layer 2 bridging functions if needed and can simultaneously route through the same interface. • Layer 3 devices (routers, in this case) provide connections between virtual LANs (VLANs). • Routers can provide quality of service (QoS) for specific types of network traffic.
