OSI Model Concepts

2013. mjus 12., 13:05

The standard model for networking protocols and distributed applications is the International Standard Organization's Open System Interconnect (ISO/OSI) model. It defines seven network layers.

Short forOpen System Interconnection, an ISO standard for worldwide communications that defines a networking framework for implementing protocols in seven layers. Control is passed from one layer to the next, starting at the application layer in one station, proceeding to the bottom layer, over the channel to the next station and back up the hierarchy.

At one time, most vendors agreed to support OSI in one form or another, but OSI was too loosely defined and proprietary standards were too entrenched. Except for the OSI-compliant X.400 and X.500 e-mail and directory standards, which are widely used, what was once thought to become the universal communications standard now serves as the teaching model for all other protocols.

Control is passed from one layer to the next, starting at the application layer in one station, proceeding to the bottom layer, over the channel to the next station and back up the hierarchy.Read How to use the OSI Model to Troubleshoot Networks for more info.

Layer 1 - Physical

Physical layer defines the cable or physical medium itself, e.g., thinnet, thicknet, unshielded twisted pairs (UTP). All media are functionally equivalent. The main difference is in convenience and cost of installation and maintenance. Converters from one media to another operate at this level.

Layer 2 - Data Link

Data Link layer defines the format of data on the network. A network data frame, aka packet, includes checksum, source and destination address, and data. The largest packet that can be sent through a data link layer defines the Maximum Transmission Unit (MTU). The data link layer handles the physical and logical connections to the packet's destination, using a network interface. A host connected to an Ethernet would have an Ethernet interface to handle connections to the outside world, and a loopback interface to send packets to itself.

Ethernet addresses a host using a unique, 48-bit address called its Ethernet address or Media Access Control (MAC) address. MAC addresses are usually represented as six colon-separated pairs of hex digits, e.g., 8:0:20:11:ac:85. This number is unique and is associated with a particular Ethernet device. Hosts with multiple network interfaces should use the same MAC address on each. The data link layer's protocol-specific header specifies the MAC address of the packet's source and destination. When a packet is sent to all hosts (broadcast), a special MAC address (ff:ff:ff:ff:ff:ff) is used.

Layer 3 - Network

NFS uses Internetwork Protocol (IP) as its network layer interface. IP is responsible for routing, directing datagrams from one network to another. The network layer may have to break large datagrams, larger than MTU, into smaller packets and host receiving the packet will have to reassemble the fragmented datagram. The Internetwork Protocol identifies each host with a 32-bit IP address. IP addresses are written as four dot-separated decimal numbers between 0 and 255, e.g., 129.79.16.40. The leading 1-3 bytes of the IP identify the network and the remaining bytes identifies the host on that network. The network portion of the IP is assigned by InterNIC Registration Services, under the contract to the National Science Foundation, and the host portion of the IP is assigned by the local network administrators. For large sites, the first two bytes represents the network portion of the IP, and the third and fourth bytes identify the subnet and host respectively.

Even though IP packets are addressed using IP addresses, hardware addresses must be used to actually transport data from one host to another. The Address Resolution Protocol (ARP) is used to map the IP address to it hardware address.

Layer 4 - Transport

Transport layer subdivides user-buffer into network-buffer sized datagrams and enforces desired transmission control. Two transport protocols, Transmission Control Protocol (TCP) and User Datagram Protocol (UDP), sits at the transport layer. Reliability and speed are the primary difference between these two protocols. TCP establishes connections between two hosts on the network through 'sockets' which are determined by the IP address and port number. TCP keeps track of the packet delivery order and the packets that must be resent. Maintaining this information for each connection makes TCP a stateful protocol. UDP on the other hand provides a low overhead transmission service, but with less error checking. NFS is built on top of UDP because of its speed and statelessness. Statelessness simplifies the crash recovery.

Layer 5 - Session

The session protocol defines the format of the data sent over the connections. The NFS uses the Remote Procedure Call (RPC) for its session protocol. RPC may be built on either TCP or UDP. Login sessions uses TCP whereas NFS and broadcast use UDP.

Layer 6 - Presentation

External Data Representation (XDR) sits at the presentation level. It converts local representation of data to its canonical form and vice versa. The canonical uses a standard byte ordering and structure packing convention, independent of the host.

Layer 7 - Application

Provides network services to the end-users. Mail, ftp, telnet, DNS, NIS, NFS are examples of network applications.

OSI Model Reference Table

Introduction to the OSI Model

2013. mjus 12., 13:13

The Open System Interconnection Reference Model (OSI) is a seven layer model that was developed as part of the effort to standardize networking that was started in the late 1970's as part of the Open Systems Interconnection (OSI) initiative.

OSI Model's place in the CCNA and Network+ exams

Both the 640-802 CCNA exam and the Network+ exam test for some of the common knowledge of the OSI Model.

For the CCNA 640-802 exam this information is tested as part of the "Describe how a network works" domain as part of the following subtopics:

Use the OSI and TCP/IP models and their associated protocols to explain how data flows in a network

Describe the purpose and basic operation of the protocols in the OSI and TCP models

Select the components required to meet a network specification

Beyond these subtopics you'll need to have at least a general understanding of the model across some of the other exam topics as well but this is the primary area of focus.

For the Network+ exam most of this relates to subtopic 4.1 Explain the function of each layer of the OSI model.

For both certification exams you'll need to know where in the model certain protocols function as well as knowing at what layers hardware devices such as routers, switches, bridges, et cetera work. Additionally, you'll need to have a good understanding of how security of data is handled through the devices and what security features are offered to the data in transit and at which levels of the OSI model offer what types of security.

NOTES FROM THE FIELD -TrainSignal offers in depth training for both theCisco CCNA 640-802exam as well asCompTIA's Network+examThe Seven Layers of the OSI Model

In summary the four layers of the OSI model are broken as follows:

ThePhysical Layerdefines the electrical and physical properties and the operating specifications for the devices and media in use. The main job of the Physical Layer is the physical "connection" or attachment of given media and how it is configured (e.g. Token Ring cable, size of cable used, termination in place etc.). In some instances, there may be secondary responsibilities of this layer depending on the device for things such as flow control, modulation/demodulation and so forth. The protocol data unit in use at this level of the OSI model is referred to as a "bit."

TheData Link Layerprovides the practical means to transfer data between network nodes as its main job is to transfer data between network nodes in a wide area network or between nodes on the same local area network segment/subnet. It has the secondary responsibility to detect and correct errors (as permissible) that may take place at the Physical Layer. The protocol data unit in use at this level of the OSI model is referred to as a "frame."

TheNetwork Layerhandles the forwarding and routing of data along logical paths between network connected nodes. In addition to routing and forwarding functions of this layer of the model is also performs addressing, error handling, quality of service control, congestion control and packet sequencing. The protocol data unit in use at this level of the OSI model is referred to as a "packet."

TheTransport Layeris responsible for the reliable, end to end transfer, recovery and flow control of the segments between the nodes. The protocol data unit in use at this level of the OSI model is referred to as a "segment."

TheSession Layeraddresses the build up and tear down of the connection sessions between nodes on a network. The protocol data unit in use at this level (and all of the subsequent levels) of the OSI model is referred to simply as "data."

ThePresentation Layeris responsible for taking the data from applications at the application layer and breaking it down for use on the session layer as well as the reverse. It also has the task of formatting the data so that it can be sent to other nodes.

TheApplication Layerhandles the initial connection of a given application to the network. It is where applications and application type activities such as browsing the web, sending and receiving email and performing file transfers take place. There are applications that wholly reside at the level such as Telnet and FTP.

Protocol Use at each of the TCP/IP Model Layers

At each layer of the OSI Model there are associated protocols that are in use.

These are not fully comprehensive lists but are examples of the more common protocols that are functioning at these different levels of the OSI Model.

At the Application layer you can find many but some of the more common ones include:

DHCP - Dynamic Host Configuration Protocol

FTP - File Transfer Protocol

HTTP - HyperText Transfer Protocol

IMAP - IMAP4, Internet Message Access Protocol (version 4)

LDAP - Lightweight Directory Access Protocol

LPD - Line Printer Daemon Protocol

MIME (S-MIME) - Multipurpose Internet Mail Extensions and Secure MIME

NFS - Network File System

NNTP - Network News Transfer Protocol

NTP - Network Time Protocol

POP - POP3, Post Office Protocol (version 3)

RDP - Remote Desktop Protocol

RPC - Remote Procedure Call

SMTP - Simple Mail Transfer Protocol

SNMP - Simple Network Management Protocol

SNTP - Simple Network Time Protocol

SSH - Secure Shell

TELNET - Terminal Emulation Protocol of TCP/IP

TFTP - Trivial File Transfer Protocol

At the Presentation layer you can find these common protocols:

MIME - Multipurpose Internet Mail Extensions

SSL - Secure Sockets Layer

TLS - Transport Layer Security

XDR - eXternal Data Representation

At the Session layer you can find socket driven connections and session establishment in Transmission Control Protocol (TCP), Session Initiation Protocol (SIP), and Real-time Transport Protocol (RTP).

You can also find Named Pipe sessions, a protocol in the Server Message Block (SMB) suite as well as the NetBIOS (Network Basic Input/Output System) application Programming Interface (since NetBIOS is not formally a true networking protocol).

Session Announcement Protocol (SAP) is a protocol for broadcasting multicast session information and it is also found at the Session layer.

At the Transport layer you can find these common protocols:

SPX - Sequenced Packet Exchange

TCP - Transmission Control Protocol

UDP - User Datagram Protocol

SCTP - Stream Control Transmission Protocol

At the Network Layer you can find these common protocols:

ATP - AppleTalk Transaction Protocol

IPv4 - Internet Protocol v4

IPv6 - Internet Protocol v6

IPX - Internetwork Packet Exchange

ICMP - Internet Control Message Protocol

IGMP - Internet Group Management Protocol

OSPF - Open Shortest Path First

At the Data Link Layer you can find these common protocols:

PPP - Point-to-Point Protocol

PPTP - Point-to-Point Tunneling Protocol

SLIP - Serial Line Internet Protocol

L2TP - Layer 2 Tunneling Protocol

Since the Physical Layer is really for defining the physical "connection" or attachment of given media and how it is configured as well as the electrical and physical properties and the operating specifications for the devices and media in use there are no actual TCP/IP common protocols that are in use.

You can find certain combinations of media and standards at this layer such as RS-232 (Recommended Standard 232) which is the standard for data and control signals connecting between a DTE (Data Terminal Equipment) and a DCE (Data Circuit-terminating Equipment) and Digital Subscriber Line (DSL) which provides digital data transmission over local telephone lines.

In this article we reviewed the tie in of the OSI Model to the CCNA and Network+ exams as well as took a look at the breakdown of the seven layers of the OSI Model

We wrapped up with a quick look at some of the protocols that are in use at each of the OSI Model Layers

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How to use the OSI Model to Troubleshoot Networks

2013. mjus 12., 13:07

In a previous article, we explained the OSI model and how it works. In that article, we went over how the OSI model is a standard theoretical model for how networking hardware, software, protocols work together. What we didnt discuss in that article is how the OSI model can help you in your day to day life.

Some of you might be thinking theoretical models dont help me or the OSI model is just some engineering thing for the nerds. However, in reality, it is quite the opposite. The OSI model can help you. Let me show you how.

Using the OSI model

First off, I want you to have a visual image of the OSI mode. It looks like this:-

On the left hand side is a user. On the right hand side, you could have a server. Every requestANDresponse has to travel from the left, down every layer, to the physical layer, across the physical layer, up the layers on the right, and up to the server on the top right hand corner.

By understanding this information and having a visual image of how the OSI model works, you have a very valuable troubleshooting tool. If you can visualize the different pieces that complete the path of the data, you will have much less trouble resolving a networking issue. You can think of the OSI model as a map for the path that your data takes. If your data is not completing the path, you can check each waypoint on the map until you find the area that is causing the problem.

My experience with the OSI model

In my time spent as a network administrator, I would use the OSI model daily. Let me explain how.

When a get a call from a user that explains a problem they are having, I would immediately visualize the OSI model. The user might say that they cant bring up a graphic they are trying to download from the Internet. That graphic is brought up in an application. (layer 7 of the OSI model). I could either start at the top or the bottom, depending on what I suspected was the problem. I would usually start at the bottom (termed the bottom up approach). At the bottom of the OSI model is the physical layer (layer 1). So, I would proceed to ask them questions like this:

Is your network cable plugged in? (physical)

Is there a link light on the Ethernet switch and Ethernet NIC? (data-link)

Do you have an IP address? (network)

Can you ping your default gateway? (network, testing LAN IP connectivity)

Do you have DNS server information?

Can you ping your DNS server? (network, testing IP connectivity)

Do you have a firewall configured? (network on up to application)

Can you ping the host you are trying to get to by name? (application, DNS and network WAN IP connectivity)

What format is the graphic in? Do you have a viewer for that format? (presentation)

Can your web browser open up another website? (basic application troubleshooting)

It may turn out that the graphic they were trying to bring up was a .TIFF file and they didnt have a decoder for that type of file. Thus, this would have been a presentation error issue as the presentation layer deals with formats of graphics & files, as well as compression and encryption.

Methods of using the OSI model

I just gave you an example for using the OSI model with a bottom up approach to troubleshooting. There are three different ways to use the OSI model:

Bottom up troubleshooting by going from the physical layer (layer 1) up to the application layer (layer 7)

Top down- troubleshooting by going from the application layer (layer 7) down to the physical layer (layer 1)

Divide and Conquer in this method, you start with whatever layer you feel is most likely the cause of the problem, then move in whatever direction you feel is the more likely cause of the issue (either up or down the OSI model)

In Summary

Now you may not be a network administrator but the OSI model can help anyone troubleshoot any networking problem. I hope that this example gave you some ideas as to how you can use the OSI model, everyday, to troubleshoot your networking problems.

CCNA OSI Model Questions

2011. november 15., 13:00

Question 1Which of the following correctly describe steps in the OSI data encapsulation process? (Choose two)

A. The transport layer divides a data stream into segments and may add reliability and flow control information.

B. The data link layer adds physical source and destination addresses and an FCS to the segment.

C. Packets are created when the network layer encapsulates a frame with source and destination host addresses and protocol-related control information.

D. Packets are created when the network layer adds Layer 3 addresses and control information to a segment.

E. The presentation layer translates bits into voltages for transmission across the physical link.

Answer:A DExplanationThe transport layer segments data into smaller pieces for transport. Each segment is assigned a sequence number, so that the receiving device can reassemble the data on arrival.

The transport layer also use flow control to maximize the transfer rate while minimizing the requirements to retransmit. For example, in TCP, basic flow control is implemented by acknowledgment by the receiver of the receipt of data; the sender waits for this acknowledgment before sending the next part.

-> A is correct.The data link layer adds physical source and destination addresses and an Frame Check Sequence (FCS) to the packet (on Layer 3), not segment (on Layer 4) -> B is not correct.

Packets are created when network layer encapsulates a segment (not frame) with source and destination host addresses and protocol-related control information. Notice that the network layer encapsulates messages received from higher layers by placing them into datagrams (also called packets) with a network layer header -> C is not correct.

The Network layer (Layer 3) has two key responsibilities. First, this layer controls the logical addressing of devices. Second, the network layer determines the best path to a particular destination network, and routes the data appropriately.

-> D is correct.

The Physical layer (presentation layer) translates bits into voltages for transmission across the physical link -> E is not correct.

Question 2Which layer of the OSI reference model uses the hardware address of a device to ensure message delivery to the proper host on a LAN?

A. physical

B. data link

C. network

D. transport

Answer:BExplanationThe hardware address of a device or the Media Access Control (MAC) address is added in the Data Link layer. An Ethernet MAC address is a 48-bit binary value expressed as 12 hexadecimal digits (for example: 00:15:A4:CB:03:CA).

Question 3Which layer of the OSI reference model uses flow control, sequencing, and acknowledgments to ensure that reliable networking occurs?

A. data link

B. network

C. transport

D. presentation

E. physical

Answer:CQuestion 4Which layer in the OSI reference model is responsible for determining the availability of the receiving program and checking to see if enough resources exist for that communication?

A. transport

B. network

C. presentation

D. session

E. application

Answer:EQuestion 5Data transfer is slow between the source and destination. The quality of service requested by the transport layer in the OSI reference model is not being maintained. To fix this issue, at which layer should the troubleshooting process begin?

A. presentation

B. session

C. transport

D. network

E. physical

Answer:D

Question 6Which protocols are found in the network layer of the OSI reference model and are responsible for path determination and traffic switching?

A. LAN

B. routing

C. WAN

D. network

Answer:BQuestion 7Refer to the exhibit. An administrator pings the default gateway at 10.10.10.1 and sees the output as shown. At which OSI layer is the problem?-----------------------------------------------C:\> ping 10.10.10.1

Pinging 10.10.10.1 with 32 bytes of data:

Request timed out.

Request timed out.

Request timed out.

Request timed out.

Ping statistics for 10.10.10.1:

Packets: sent 4, Received = 0, Lost 4 (100% loss)

---------------------------------------------------------

A. data link layer

B. application layer

C. access layer

D. session layer

E. network layer

Answer:EExplanationThe Network layer is responsible for network addressing and routing through the internetwork. So a ping fails, you may have an issue with the Network layer (although lower layers like Data Link & Physical may cause the problem).

Question 8Which of the following are types of flow control? (Choose three)

A. buffering

B. cut-through

C. windowing

D. congestion avoidance

E. load balancing

Answer:A C DExplanationThree types of flow control are buffering, windowing & congestion avoidance:

+Buffering: If a device receives packets too quickly for it to handle then it can store them in a memory section called a buffer and proceed them later.

+Windowing: a window is the quantity of data segments that the transmitting device is allowed to send without receiving an acknowledgment for them. For example:

With the window size of 1, the sending device sends 1 segment and the receiving device must reply with 1 ACK before the sending device can send the next segment. This waiting takes some time.

By increasing the window size to 3, the sending device will send up to 3 segments before waiting an ACK -> helps reduce the waiting time.

+Congestion avoidance: lower-priority traffic can be discarded when the network is overloaded -> minimize delays.

Question 9A network administrator is verifying the configuration of a newly installed host by establishing an FTP connection to a remote server. What is the highest layer of the protocol stack that the network administrator is using for this operation?

A. application

B. presentation

C. session

D. transport

E. internet

F. data link

Answer:AExplanationFTP belongs to Application layer and it is also the highest layer of the OSI model.

Question 10A receiving host computes the checksum on a frame and determines that the frame is damaged. The frame is then discarded. At which OSI layer did this happen?

A. session

B. network

C. physical

D. data link

E. transport

Answer:DExplanationWhen using the term frame we can easily recognize it belongs to the Data Link layer. In this layer, an Frame Check Sequence (FCS) field is added to the frame to verify that the frame data is received correctly.

OSI Model Tutorial

2011. november 12., 4:22

Welcome to the most basic tutorial for networker! Understanding about OSI model is one of the most important tools to help you grasp how networking devices like router, switch, PC work.

Lets take an example in our real life to demonstrate the OSI model. Maybe you have ever sent a mai+l to your friend, right? To do it, you have to follow these steps:

1. Write your letter

2. Insert it into an envelope

3. Write information about sender and receiver on that envelope

4. Stamp it

5. Go to the post office and drop it into a mail inbox

From the example above, I want to imply we have to go through some steps in a specific order to complete a task. It is also applied for two PCs to communicate with each other. They have to use a predefined model, named OSI, to complete each step. There are 7 steps in this model as listed below:

This is also the well-known table of the OSI model so you must take time to learn by heart. A popular way to remember this table is to create a fun sentence with the first letters of each layer. For example:AllPeopleSeemToNeedDataProcessing or a more funny sentence sorted from layer 1 to layer 7:PleaseDoNotThrowSausagePizzaAway.

There are two notices about this table:

1. First, the table is arranged from top to bottom (numbering from 7 to 1). Each step is called a layer so we have 7 layers (maybe we usually call them layers to make them more technical ^^).

When a device wants to send information to another one, its data must go from top to bottom layer. But when a device receives this information, it must go from bottom to top to decapsulate it. In fact, the reverse action at the other end is very natural in our life. It is very similar when two people communicate via mail. First, the writer must write the letter, insert it into an envelope while the receiver must first open the envelope and then read the mail. The picture below shows the whole process of sending and receiving information.

Note: The OSI model layers are often referred to by number than by name (for example, we refer saying layer 3 to network layer) so you should learn the number of each layer as well.

2. When the information goes down through layers (from top to bottom), a header is added to it. This is called encapsulation because it is like wrapping an object in a capsule. Each header can be understood only by the corresponding layer at the receiving side. Other layers only see that layers header as a part of data.

At the receiving side, corresponding header is stripped off in the same layer it was attached.

Understand each layer

Layer 7 Application layerThis is the closest layer to the end user. It provides the interface between the applications we use and the underlying layers. But notice that the programs you are using (like a web browser IE, Firefox or Opera) do not belong to Application layer. Telnet, FTP, email client (SMTP), HyperText Transfer Protocol (HTTP) are examples of Application layer.

Layer 6 Presentation layerThis layer ensures the presentation of data, that the communications passing through are in the appropriate form for the recipient. In general, it acts as a translator of the network. For example, you want to send an email and the Presentation will format your data into email format. Or you want to send photos to your friend, the Presentation layer will format your data into GIF, JPG or PNG format.

Layer 5 Session layerLayer 5 establishes, maintains and ends communication with the receiving device.

Layer 4 Transport layerThis layer maintains flow control of data and provides for error checking and recovery of data between the devices. The most common example of Transport layer is Transmission Control Protocol (TCP) and User Datagram Protocol (UDP).

Layer 3 Network layerThis layer provides logical addresses which routers will use to determine the path to the destination. In most cases, the logic addresses here means the IP addresses (including source & destination IP addresses).

Layer 2 Data Link LayerThe Data Link layer formats the message into adata frame, and adds a header containing the hardware destination and source address to it. This header is responsible for finding the next destination device on a local network.

Notice that layer 3 is responsible for finding the path to the last destination (network) but it dont care about who will be the next receiver. It is the Layer 2 that helps data to reach the next destination.

This layer is subdivide into 2 sub-layers: logical link control (LLC) and media access control (MAC).

The LLC functions include:

+ Managing frames to upper and lower layers

+ Error Control

+ Flow control

The MAC sublayer carries the physical address of each device on the network. This address is more commonly called a devices MAC address. MAC address is a 48 bits address which is burned into the NIC card on the device by its manufacturer.

Layer 1 Physical layerThe Physical Layer defines the physical characteristics of the network such as connections, voltage levels and timing.

To help you remember the functions of each layer more easily, I created a fun story in which Henry (English) wants to send a document to Charles (French) to demonstrate how the OSI model works.

Lastly, I summarize all the important functions of each layer in the below table (please remember them, they are very important knowledge you need to know about OSI model):

Note: In fact, OSI is just is a theoretical model of networking. The practical model used in modern networks is the TCP/IP model. You may think Hm, its just theoretic and has no use in real life! I dont care! but believe me, you will use this model more often than the TCP/IP model so take time to grasp it, you will not regret I promise :)

Let's practise CCNA: OSI & TCP/IP Model

2013. oktber 1., 13:59

Question 1Where does routing occur within the DoD TCP/IP reference model?

A. applicationB. internetC. networkD. transport

Answer:BExplanationThe picture below shows the comparison between TCP/IP model & OSI model. Notice that the Internet Layer of TCP/IP is equivalent to the Network Layer which is responsible for routing decision.

Question 2Refer to exhibit.

A network administrator cannot establish a Telnet session with the indicated router. What is the cause of this failure?

A. A Level 5 password is not set.B. An ACL is blocking Telnet access.C. The vty password is missing.D. The console password is missing.

Answer:CQuestion 3Before installing a new, upgraded version of the IOS, what should be checked on the router, and which command should be used to gather this information? (Choose two)

A. the amount of available ROMB. the amount of available flash and RAM memoryC. the version of the bootstrap software present on the routerD. show versionE. show processesF. show running-config

Answer:B DExplanationWhen upgrading new version of the IOS we need to copy the IOS to the Flash so first we have to check if the Flash has enough memory or not. Also running the new IOS may require more RAM than the older one so we should check the available RAM too. We can check both with the show version command.

Question 4Refer to the exhibit. An administrator pings the default gateway at 10.10.10.1 and sees the output as shown. At which OSI layer is the problem?

A. data link layerB. application layerC. access layerD. session layerE. network layer

Answer:EExplanationThe Network layer is responsible for network addressing and routing through the internetwork. So a ping fails, you may have an issue with the Network layer (although lower layers like Data Link & Physical may cause the problem).

Question 5At which layer of the OSI model does PPP perform?

A. Layer 2B. Layer 3C. Layer 4D. Layer 5

Answer:A

Question 6Which of the following correctly describe steps in the OSI data encapsulation process? (Choose two)

A. The transport layer divides a data stream into segments and may add reliability and flow control information.B. The data link layer adds physical source and destination addresses and an FCS to the segment.C. Packets are created when the network layer encapsulates a frame with source and destination host addresses and protocol-related control information.D. Packets are created when the network layer adds Layer 3 addresses and control information to a segment.E. The presentation layer translates bits into voltages for transmission across the physical link.

Answer:A DExplanationThe transport layer segments data into smaller pieces for transport. Each segment is assigned a sequence number, so that the receiving device can reassemble the data on arrival.

The transport layer also use flow control to maximize the transfer rate while minimizing the requirements to retransmit. For example, in TCP, basic flow control is implemented by acknowledgment by the receiver of the receipt of data; the sender waits for this acknowledgment before sending the next part.

-> A is correct.

The data link layer adds physical source and destination addresses and an Frame Check Sequence (FCS) to the packet (on Layer 3), not segment (on Layer 4) -> B is not correct.

Packets are created when network layer encapsulates a segment (not frame) with source and destination host addresses and protocol-related control information. Notice that the network layer encapsulates messages received from higher layers by placing them into datagrams (also called packets) with a network layer header -> C is not correct.

The Network layer (Layer 3) has two key responsibilities. First, this layer controls the logical addressing of devices. Second, the network layer determines the best path to a particular destination network, and routes the data appropriately.

-> D is correct.

The Physical layer (presentation layer) translates bits into voltages for transmission across the physical link -> E is not correct.

Question 7A network administrator is verifying the configuration of a newly installed host by establishing an FTP connection to a remote server. What is the highest layer of the protocol stack that the network administrator is using for this operation?

A. applicationB. presentationC. sessionD. transportE. internetF. data link

Answer:A

Explanation

FTP belongs to Application layer and it is also the highest layer of the OSI model.

Question 8

At which layer of the OSI model is RSTP used to prevent loops?

A. data linkB. networkC. physicalD. transport

Answer:A

Question 9

Which layer in the OSI reference model is responsible for determining the availability of the receiving program and checking to see if enough resources exist for that communication?

A. transportB. networkC. presentationD. sessionE. application

Answer:E

Question 10

A receiving host computes the checksum on a frame and determines that the frame is damaged. The frame is then discarded. At which OSI layer did this happen?

A. sessionB. networkC. physicalD. data linkE. transport

Answer:DExplanationWhen using the term frame we can easily recognize it belongs to the Data Link layer. In this layer, an Frame Check Sequence (FCS) field is added to the frame to verify that the frame data is received correctly.