network etiketine sahip kayıtlar gösteriliyor. Tüm kayıtları göster
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2 Ekim 2014 Perşembe

CCNA Discovery Notes

1   
Which adapter enables a computer system to exchange information with other systems on a local network?
    #network interface card

2   
What measurement is commonly associated with computer processing speed?
    #hertz
   
3   
Which computer component is considered the nerve center of the computer system and is responsible for processing all of the data within the machine?
    #CPU
   
4   
Why do servers often contain duplicate or redundant parts?
    #Servers should be accessible at all times.
   
5   
What are two benefits of connecting a laptop computer to a docking station? (Choose two.)
    #An external monitor can be used.
    #Alternate connectivity options may be available.

6   
What characteristic of word processing software would make it a local application?
    #The software is stored on the local hard drive.
   
7   
A user plans to run multiple applications simultaneously on a computer. Which computer component is essential to accomplish this?
    #RAM
   
8   
What are two advantages of purchasing a preassembled computer? (Choose two.)
    #usually a lower cost
    #adequate for performing most general applications
   
9   
Applications can be grouped into general use software or industry specific software. What are two examples of industry specific software? (Choose two.)
    #an educational tool
    #medical practice management software
   
10   
What is the main storage drive used by servers, desktops, and laptops?
    #hard drive
   
11   
A user reports that a peripheral device that was installed correctly last week has not been functioning since the PC was booted today. All other PC functions are working properly. What are three things a service technician should do to solve the problem? (Choose three.)
    #Use the testing functionality on the peripheral itself, if available.
    #Disconnect all cables connected to the computer except those connected to the peripheral.
    #Ensure that the peripheral is powered on.
   
12   
What two functions does a UPS provide that a surge protector does not? (Choose two.)
    #It provides backup power from an internal battery.
    #It gives the user time to safely shut down the computer if the power fails.
   
13   
Which component is designed to remove high-voltage spikes and surges from a power line so that they do not damage a computer system?
    #surge suppressor
   
14   
Because of the potentially dangerous voltage levels, which two devices should you not open unless you have been specifically trained to work on them? (Choose two.)
    #monitor
    #power supply

15   
What can be used to prevent electrostatic discharge (ESD)?
    #grounding strap

16   
How many unique values are possible using a single binary digit?
    #2
   
17   
Administrators at a multicampus college need access to their schedules wherever the administrators go. Which computing device would be the most appropriate?
    #PDA
   
18   
Which two steps should be performed when installing a peripheral device? (Choose two.)
    #Download and install the most current driver.
    #Connect the peripheral using an appropriate cable or wireless connection.
   
19   
How is a server different from a workstation computer?
    #The server provides services to clients.
   
20   
Which three terms describe different types of computers? (Choose three.)
    #laptop
    #desktop
    #mainframe

21   
In newer operating systems, how are system resources assigned by default when components are installed?
    #dynamically assigned between the component and the operating system


1   
Which method of interacting with an operating system involves typing commands at a command prompt?
    #CLI
   
2    
What occurs when computers are configured to receive their network configurations dynamically?
    #Each computer requests configuration information from a server.
    
3    
An operating system is severely damaged. All data on the partition needs to be deleted, and all application software must be reinstalled. Which installation method should be used to correct the problem?
    #clean install
    
4    
Which Windows file system is more likely to be used in an enterprise-level environment?
    #NTFS
    
5    
What are two ways that a user can interact with an operating system shell? (Choose two.)
    #CLI
    #GUI
    
6    
Windows XP has a known problem with specific Intel processors. What is the best course of action to take in this situation?
    #Download and install the Windows patch that fixes this problem.
    
7    
The Lesson TaskMaster application from ChalkBoard has a problem. Every time the software is installed and loaded on a Windows XP-based Dell computer, and the moment something is typed followed by a tab, two tabs are inserted instead of one. What would be the best solution to this problem?
    #Download, install, and apply a patch from ChalkBoard.

8    
Which two statements are true about drive partitions? (Choose two.)
    #Partitions are necessary in multi-boot installations.
    #A hard drive can be divided into an operating system partition and a data partition.
    
9    
What is the purpose of a default gateway?
    #identifies the device that allows local network computers to communicate with devices on other networks

10    
What Explorer menu option is used to determine the Windows XP version installed on a computer?
    #Help

11    
Which three resource specifications are given by the manufacturer to ensure that an operating system performs as designed? (Choose three.)
    #required hard disk space
    #processor type and speed
    #recommended amount of RAM

12    
Which three pieces of information must be specified on a computer to allow that computer to send and receive information across networks? (Choose three.)
    #IP address
    #subnet mask
    #default gateway
    
13    
Which Windows XP Automatic Update option allows you to select the time when a Windows update is executed?
    #Automatic (recommended).
    
14    
Which two items must be unique to each computer and cannot be duplicated on a network? (Choose two.)
    #computer name
    #IP address
    
15    
Which two statements should be included in a pre-installation checklist when upgrading an operating system? (Choose two.)
    #Verify that the hardware resources meet or exceed the published minimum requirements.
    #Complete a full backup of all important files and data.
    
16    
What two items are available from the About Windows option through the Windows Explorer Help menu? (Choose two.)
    #the end-user license agreement
    #the operating system version number
    
17    
What are three characteristics of an operating system that is released under the GPL (GNU Public License)? (Choose three.)
    #full access to source code
    #software often available free
    #support normally user-based and often free

18    
Which statement is true about installing an operating system on computers in a networked environment?
    #The computer name must be unique for all computers on the same network.
    
19    
A computer in the Advanced Technology Center computer room on the second floor, room number 217, has the computer name ATC_217_7. What fact is true about the computer name?
    #The name makes it easier for users to reference the device when connecting to it to share resources.

20    
Which operating system patch installation method is used when a user is notified of a security update for remotely accessing a computer, but wishes not to install this patch?
    #prompt for permission


1
Which two statements concerning networking standards are true? (Choose two.)
    #provides consistent interconnections across networks
    #simplifies new product development

2
Which two items are included in a network logical map? (Choose two.)
    #naming scheme
    #IP addressing scheme

3
What is a reason for disabling simple file sharing?
    #It enables the user to set more specific security access levels.

4
Which address does an NIC use when deciding whether to accept a frame?
    #destination MAC address

5
If the default gateway is configured incorrectly on the host, what is the impact on communications?
    #The host can communicate with other hosts on the local network, but is unable to communicate with hosts on remote networks.

6    

Refer to the graphic. What does the router do after it determines that a data packet from Network 1 should be forwarded to Network 2?
    #It reassembles the frame with different MAC addresses than the original frame.

7
What is the function of the FCS field in an Ethernet frame?
    #detects transmission errors

8
Host A needs to learn the MAC address of Host B, which is on the same LAN segment. A message has been sent to all the hosts on the segment asking for the MAC address of Host B. Host B responds with its MAC address and all other hosts disregard the request. What protocol was used in this scenario?
    #ARP

9
What is a benefit of having a router within the distribution layer?
    #keeps broadcasts contained within a local network

10
Which term is used to describe the process of placing one message format into another format so that the message can be delivered across the appropriate medium?
    #encapsulation

11
Which table does a router use to make decisions about the interface through which a data packet is to be sent?
    #routing table

12
Which two networking devices are used to connect hosts to the access layer? (Choose two.)
    #hub
    #switch

13
What does the 100 mean when referencing the 100BASE-T Ethernet standard?
    #speed of transmission

14
What device is typically used as the default gateway for a computer?
    #the router interface closest to the computer

15    

Refer to the graphic. Five PCs are connected through a hub. If host H1 wants to reply to a message from host H2, which statement is true?
    #H1 sends a unicast message to H2, but the hub forwards it to all devices.


16
Which type of address is used in an Ethernet frame header?
    #MAC addresses only

17
A switch receives a frame with a destination MAC address that is currently not in the MAC table. What action does the switch perform?
    #It floods the frame out of all active ports, except the origination port.

18
Which device accepts a message on one port and always forwards the message to all other ports?
    #hub

19
What type of route allows a router to forward packets even though its routing table contains no specific route to the destination network?
    #default route

20
An integrated router can normally perform the functions of which two other network devices? (Choose two.)
    #switch
    #wireless access point

21
What is the purpose of logical addresses in an IP network?
    #They are used to determine which host device accepts the frame. 




1
Which definition describes the term Internet?
    #a network of networks that connects countries around the world

2
What type of connection point is a point of presence (POP)?
    #between an ISP and a home-based LAN

3
What is the term for the group of high-speed data links that interconnect ISPs?
    #Internet backbone

4
What are three characteristics of business class ISP service? (Choose three.)
   # fast connections
   # extra web space
   # additional e-mail accounts

5
What does the tracert command test?
    #the network path to a destination

6
What is a major characteristic of asymmetric Internet service?
    #Upload speeds and download speeds are different.

7
Which three elements are required to successfully connect to the Internet? (Choose three.)
    #an IP address
    #a network connection
    #access to an Internet service provider

8
Which two places are most appropriate to use UTP cabling? (Choose two.)
   # in a home office network
   # inside a school building

9
Which device can act as a router, switch, and wireless access point in one package?
    #ISR

10
What type of end-user connectivity requires that an ISP have a DSLAM device in their network?
    #digital subscriber line technology

11
Why would an ISP require a CMTS device on their network?
    #to connect end users using cable technology

12 

   
Refer to the graphic. What type of cabling is shown?
    #fiber

13  

  
Refer to the graphic. What type of cabling is shown?
    #UTP

14
Which two characteristics describe copper patch panels? (Choose two.)
    #uses RJ-45 jacks
    #allows quick rearrangements of network connections

15
What term describes each router through which a packet travels when moving between source and destination networks?
    #hop

16
What does adherence to cabling standards ensure?
   # reliable data communications

17
What connector is used to terminate Ethernet unshielded twisted pair (UTP) cabling?
   # RJ-45

18    

Refer to the graphic. What type of cable is shown?
    #crossover

19
What are two advantages of cable management? (Choose two.)
    #aids in isolation of cabling problems
    #protects cables from physical damage

20
What are two common causes of signal degradation when using UTP cabling? (Choose two.)
    #having improper termination
    #using low quality cables or connectors

21
What are three commonly followed standards for constructing and installing cabling? (Choose three.)
    #pinouts
    #cable lengths
    #connector types




1
Which two statements describe packets that are sent through a Linksys integrated router using NAT? (Choose two.)
    #Packets that are sent to a destination outside the local network need to be translated.
    #Packets that are sent between hosts on the same local network do not need to be translated.

2
A PC obtains its IP address from a DHCP server. If the PC is taken off the network for repair, what happens to the IP address configuration?
    #The address is returned to the pool for reuse when the lease expires.


What is the destination MAC address in a multicast Ethernet frame?
    #an address that begins with 01-00-5E in hexadecimal
 

How many bits are available for Class B host IP addresses using a default subnet mask?
    #16
 

Which default subnet mask provides the most host bits?
    #255.0.0.0
 

6   


Refer to the graphic. NAT and DHCP are installed on the Linksys integrated router. Which IP address is most likely to be assigned to the local computer, Host1?
    #10.0.0.17
 
7    


Refer to the graphic. The host requests an IP configuration from the Linksys integrated router which is configured to provide DHCP services. In addition to the host IP address, which additional IP address is provided that will allow the host to access the ISP and the Internet?
    #internal IP address of the integrated router that connects to the local network

8    


Refer to the graphic. A user at the workstation cannot connect to the server. All cables have been tested and are working and all devices have IP addressing. However, the user cannot ping the server. What is causing the problem?
    #The workstation and server are on different logical networks.
 

Which part of an IP address identifies a specific device on a network?
    #host portion
 
10 
Which type of server dynamically assigns an IP address to a host?
    #DHCP
 
11 
What is one of the purposes of NAT?
    #prevents external users from detecting the IP addresses used on a network
 
12 
Assuming a default mask, which portion of the IP address 175.124.35.4 represents the host?
    #35.4
 
13 
How large are IPv4 addresses?
    #32 bits
 
14 
Which statement is true concerning private IP addresses?
    #solves the issue of a finite number of available public IP addresses
 
15 
Yvonne is talking to her friend on the phone. What type of message is this?
    #unicast


16 
Which of the following are private IP addresses? (Choose three.)
    #10.1.1.1
    #172.16.4.4
    #192.168.5.5
 
17 
Which three statements describe a DHCP Discover message? (Choose three.)
    #The destination IP address is 255.255.255.255.
    #The message comes from a client seeking an IP address.
    #All hosts receive the message, but only a DHCP server replies.
 
18 
What destination IP address is used in a unicast packet?
    #a specific host
 
19 
How many usable hosts are available given a Class C IP address with the default subnet mask?
    #254
 
20 
What information must be included within a unicast message for it to be delivered on an Ethernet network?
    #MAC and IP addresses that correspond to a specific destination host

21 
A host with the IP address 172.32.65.13 and a default mask belongs to what network?
    #172.32.0.0


1
What is the correct order of the layers of the OSI reference model, starting at the lowest layer and working up the model?
    #physical, data link, network, transport, session, presentation, application


What three items are contained in an Ethernet header and trailer? (Choose three.)
    #source MAC address
    #destination MAC address
    #error-checking information


Which protocols are TCP/IP application layer protocols? (Choose two.)
    #FTP
    #SMTP
 
4    


Refer to the graphic. Which protocol is used to access this website?
    #HTTP
 

What is an advantage of the use of layers in the OSI reference model?
    #It prevents changes in one layer from affecting other layers.
 

What term is used to describe how TCP/IP protocols are layered and interact to format, address, and transmit information across a network?
    #protocol stack
 

Whenever e-mail clients send letters, what device is used to translate the domain names into their associated IP addresses?
    #DNS server


An Internet server is running both FTP and HTTP services. How does the server know which of these applications should handle an incoming segment?
    #The segment destination port number identifies the application that should handle it.
 

You are creating a network-based video game. What influences your decision about which transport protocol to use for the application?
    #UDP will not disrupt the game to retransmit dropped packets.
 
10 
Which port number is used by SMTP?
    #25
 
11 
Which application is most likely used to translate www.cisco.com to 198.133.219.25?
    #DNS
 
12 
Which of the following are layers of the TCP/IP model? (Choose three.)
    #Application
    #Internet
    #Network Access
 
13 
Which protocol is used by e-mail servers to communicate with each other?
    #SMTP
 
14 
What client software enables logged in users to communicate with other logged in users in real time?
    #instant messaging

15 
What type of server would use IMAP?
    #e-mail

16 
What resolves a website URL to an IP address?
    #DNS server
 
17 
What information is contained in an IP header?
    #source and destination IP addresses
 
18 
Which protocol is used by FTP to transfer files over the Internet?
    #TCP
 
19 
A client has decoded a frame and started the de-encapsulation process. In which order does the de-encapsulation process occur?
 
    1) remove Ethernet header and trailer
    2) remove IP header
    3) remove TCP header
    4) pass data to the application

 

20 
A device receives an Ethernet frame and recognizes the MAC address as its own. What does the device do to the message to get to the encapsulated data?
    #removes the Ethernet header and trailer

21 
Cabling issues are associated with which OSI layer?
    #1
 
22 
A network client in a corporate environment reboots. Which type of server would most likely be used first?
    #DNS



1
Complete the following sentence: WEP is used to ______ , and EAP is used to _____ wireless networks.
    #encrypt; authenticate users on


What are three advantages of wireless over wired technology? (Choose three.)
    #anytime, anywhere connectivity
    #ease of adding additional devices


A technician has been asked to provide wireless connectivity to the wired Ethernet network of a building. Which three factors affect the number of access points needed? (Choose three.)
    #the size of the building
    #the number of solid interior walls in the building
    #the presence of microwave ovens in several offices
 

Which statement is true concerning wireless bridges?
    #connects two networks with a wireless link
 
5   

Refer to the graphic. In the Wireless menu option of a Linksys integrated router, what does the Network Mode option Mixed mean?
    #The router supports 802.11b, 802.11g, and 802.11n devices.
 

Why is IEEE 802.11 wireless technology able to transmit further distances than Bluetooth technology?
    #has higher power output
 

Which statement is true concerning an ad-hoc wireless network?
    #created by connecting wireless clients in a peer-to-peer network
 

What is the difference between using open authentication and pre-shared keys?
    #Pre-shared keys require an encrypted secret word. Open authentication does not require a secret word.
 

Which statement is true about open authentication when it is enabled on an access point?
    #requires no authentication
 
10 
Which WLAN component is commonly referred to as an STA?
    #wireless client

11 
Which two statements about a service set identifier (SSID) are true? (Choose two.)
    #tells a wireless device to which WLAN it belongs
    #all wireless devices on the same WLAN must have the same SSID
 
12 
Why is security so important in wireless networks?
    #Wireless networks broadcast data over a medium that allows easy access.
 
13 
What term describes the encoding of wireless data to prevent intercepted data from being read by a hacker?
    #encryption
 
14 
What type of authentication do most access points use by default?
    #Open
 
15 
What are two authentication methods that an access point could use? (Choose two.)
    #EAP
    #pre-shared keys

16 
What are two benefits of wireless networking over wired networking? (Choose two.)
    #mobility
    #reduced installation time
 
17   

Refer to the graphic. In the Wireless menu of a Linksys integrated router, what configuration option allows the presence of the access point to be known to nearby clients?
    #SSID Broadcast

18 
What are the two WEP key lengths? (Choose two.)
    #64 bit
    #128 bit

19 
Which two statements characterize wireless network security? (Choose two.)
    #With SSID broadcast disabled, an attacker must know the SSID to connect.
    #Using the default IP address on an access point makes hacking easier.
 
20 
What does the Wi-Fi logo indicate about a wireless device?
    #The device is interoperable with other devices of the same standard that also display the Wi-Fi logo.
 
21 
What access-point feature allows a network administrator to define what type of data can enter the wireless network?
    #traffic filtering



1
Identify three techniques used in social engineering. (Choose three.)
    #vishing
    #phishing
    #pretexting


A flood of packets with invalid source-IP addresses requests a connection on the network. The server busily tries to respond, resulting in valid requests being ignored. What type of attack occurred?
    #SYN flooding


While surfing the Internet, a user notices a box claiming a prize has been won. The user opens the box unaware that a program is being installed. An intruder now accesses the computer and retrieves personal information. What type of attack occurred?
    #Trojan horse
 

What is a major characteristic of a Worm?
    #exploits vulnerabilities with the intent of propagating itself across a network


During a pretexting event, how is a target typically contacted?
    #by phone
 

What term is used to describe a dedicated hardware device that provides firewall services?
    #appliance-based

7    

Refer to the graphic. In the Linksys Security menu, what does the SPI Firewall Protection option Enabled provide?
    #It requires that packets coming into the router be responses to internal host requests.
 

Many best practices exist for wired and wireless network security. The list below has one item that is not a best practice. Identify the recommendation that is not a best practice for wired and wireless security.
    #Disable the wireless network when a vulnerability analysis is being performed.


What environment would be best suited for a two-firewall network design?
    #a large corporate environment
 
10 
Which statement is true about port forwarding within a Linksys integrated router?
    #Only external traffic that is destined for specific internal ports is permitted. All other traffic is denied.
 
11 
Which two statements are true concerning anti-spam software? (Choose two.)
    #When anti-spam software is loaded, legitimate e-mail may be classified as spam by mistake.
    #Even with anti-spam software installed, users should be careful when opening e-mail attachments.
 
12 
What type of advertising is typically annoying and associated with a specific website that is being visited?
    #popups
 
13 
To which part of the network does the wireless access point part of a Linksys integrated router connect?
    #internal
 
14 
What statement is true about security configuration on a Linksys integrated router?
    #Internet access can be denied for specific days and times.

15 
Which abbreviation refers to an area of the network that is accessible by both internal, or trusted, as well as external, or untrusted, host devices?
    #DMZ

16 
Which statement is true regarding anti-virus software?
    #Only after a virus is known can an anti-virus update be created for it.
 
17 
What is one function that is provided by a vulnerability analysis tool?
    #It identifies missing security updates on a computer.
 
18 
What part of the security policy states what applications and usages are permitted or denied?
    #acceptable use
 
19 
What best practice relates to wireless access point security?
    #a change of the default IP address
 
20 
What is a widely distributed approach to marketing on the Internet that advertises to as many individual users as possible via IM or e-mail?
    #spam



1
A technician is unsuccessful in establishing a console session between a PC and a Linksys integrated router. Both devices have power, and a cable is connected between them. Which two troubleshooting steps could help to diagnose this problem? (Choose two.)
    #Ensure the correct cable is used.
    #Ensure the link status LED on the integrated router is lit.

2
Identify two physical-layer network problems. (Choose two.)
    #hardware failure
    #loose cable connections


Only one workstation on a particular network cannot reach the Internet. What is the first troubleshooting step if the divide-and-conquer method is being used?
    #Check the workstation TCP/IP configuration.
 

Which two troubleshooting techniques are suitable for both home networks and large corporate networks? (Choose two.)
    #documenting the troubleshooting process
    #keeping a record of system upgrades and software versions
 

What two items could be checked to verify connectivity between the router and the ISP? (Choose two.)
    #router status page
    #connectivity status as indicated by LEDs


When acting as a DHCP server, what three types of information can an ISR provide to a client? (Choose three.)
    #default gateway
    #dynamic IP address
    #DNS server address


A customer called the cable company to report that the Internet connection is unstable. After trying several configuration changes, the technician decided to send the customer a new cable modem to try. What troubleshooting technique does this represent?
    #substitution
 

Which troubleshooting method begins by examining cable connections and wiring issues?
    #bottom-up
 

What command is used to determine the location of delay for a packet traversing the Internet?
    #tracert

10 
A technician suspects that a Linksys integrated router is the source of a network problem. While troubleshooting, the technician notices a blinking green activity LED on some of the ports. What does this indicate?
    #The ports are operational and are receiving traffic.
 
11 
Which three settings must match on the client and access point for a wireless connection to occur? (Choose three.)
    #SSID
    #authentication
    #encryption key
 
12 
What should a network administrator do first after receiving a call from a user who cannot access the company web server?
    #Ask the user what URL has been typed and what error message displays.

13    

Refer to the graphic. What configuration is incorrect in the network shown?
    #The wired connection is the wrong type of cable.
 
14 
A technician is troubleshooting a security breach on a new wireless access point. Which three configuration settings make it easy for hackers to gain access? (Choose three.)
    #broadcasting the SSID
    #using open authentication
    #using the default internal IP address
 
15    

Refer to the graphic. What configuration is incorrect in the network shown?
    #The host IP address is incorrect.

16 
Typically, help desk personnel assist end users in which two tasks? (Choose two.)
    #identifying when the problem occurred
    #implementing the solution to the problem
 
17 
Network baselines should be performed in which two situations? (Choose two.)
    #after the network is installed and running optimally
    #after major changes are implemented on the network
 
18 
Which two items should be added to the documentation following a troubleshooting event? (Choose two.)
    #final resolution
    #results of successful and unsuccessful troubleshooting steps

19 
A PC is plugged into a switch and is unable to connect to the network. The UTP cable is suspected. What could be the problem?
    #A crossover cable is being used.

20 
Which ipconfig command requests IP configuration from a DHCP server?
    #ipconfig /renew
 
21    

Refer to the graphic. The wireless host cannot access the Internet, but the wired host can. What is the problem?
    #The host WEP key is incorrect.
 
22 
How does remote-access software help in the troubleshooting process?
    #Diagnostics can be run without a technician being present at the site.
 
23 
What command is used to determine if a DNS server is providing name resolution?
    #nslookup

19 Eylül 2014 Cuma

Network Course Lecture Notes

                        -   
[   ] lecture1.ppt            02-Sep-2006 12:16  575K  
[   ] lecture2.ppt            02-Sep-2006 12:15  224K  
[   ] lecture3.ppt            02-Sep-2006 12:18  2.3M  
[   ] lecture4.ppt            02-Sep-2006 12:20  2.4M  
[   ] lecture5.ppt            03-Sep-2006 12:18  133K  
[   ] lecture6.ppt            03-Sep-2006 12:19  525K  
[   ] lecture7.ppt            03-Sep-2006 12:28  220K  
[   ] lecture8.ppt            03-Sep-2006 12:30  416K  
[   ] lecture9.ppt            03-Sep-2006 12:32  319K  
[   ] lecture10.ppt           03-Sep-2006 12:35   72K  
[   ] lecture11.ppt           06-Sep-2006 22:20  631K  
[   ] lecture12.ppt           06-Sep-2006 22:18  694K  
[   ] lecture13&14.ppt        06-Sep-2006 22:28  365K  
[   ] lecture15.ppt           07-Sep-2006 17:35  294K  
[   ] lecture16&17.ppt        07-Sep-2006 17:36  368K  

Introduction to OSI model

OSI Layers
The 7 layer model is used to allow multi vendor networking devices and releases to cooperate and developers to use a layered approach to plan and approach how applications will interact.

In the 1970's the Open Systems Interconnection or IBM solution was release to break the barrier between companies and organisations - on a software and hardware level.
The model was created to break the barrier between vendors to create interoperable network devices. The model describes how data moves from the application onto the network. All of the processes required to communicate are set as a blueprint.

Todd Lammel sets this out in an easy to understand way. Think of a company. A large company right through to a small company each has layers that must work together to perform an operation. Sales, HR, Shipping all work together to allow a company to perform numerous operations . If one fails the rest go down.
For example the company isn't doing so well - so they hire a new Sales consultant. HR pick the wrong one, research creates a bad product, more of finances money needs to go into sales, marketing needs to work twice as hard...well it goes on. Its a simple rippled effect but you get the idea.

Each layer therefore needs to work together to allow the lower layers and upper layers to complete their task.

So what are the layers?

7. Application
6. Presentation
5. Session
4. Transport
3. Network
2. Data Link
1. Physical

The upper layers (7-5) work to provide an interface to the users, handles and formats data and keeps application data separate.
The lower layers 4-1 then allow the data to be reliably transmitted and error checked. They will then create logical addressing to allow data to be correctly routed and then converting the packets into acceptable transmission types to then be physically sent over the link.
Once received it will then move from the bottom layers (1-4) to the upper layers (5-7). Data therefore moves form the upper to the lower across the media to the lower levels of the receiving devices to then move to the upper layers again to be presented.

Each layer in the OSI model provides specific functions:

7. Application - File, Print, Message, Database, and Application services
6. Presentation - Data Encryption, Compressions and Translation
5. Session - Dialog control and session establishment
4. Transport - End to End connection
3. Network - Routing
2. Data Link - Framing
1. Physical - Physical connections and topology

7 - Application Layer

The application layer is the first layer to be presented. It allows the user to communicate with the other lower layers of the model to allow data transmission.
Now the first thing you must understand is the in most cases the application itself is not a layer 7 item..
For example - take an internet browser. You can send data across the network using it but it is not a layer 7 device. If you uninstall networking properties and services the program will still allow you to look and view local HTML files.
However if you try and communicate accross the network - your data will never make it.

Without the entry to the OSI model data cannot be transmitted and established on the network.
However protocols such as FTP or TFTP are layer 7 protocols as they establish a method of communicating with the lower levels.

The Application layer - is responsible for identifying and establishing the availability of the intended communication partner and determining whether sufficient resources for the communication exists.
This layer acts as the interface for the user, levels and protocols.

6 - Presentation Layer
The presenation layer - suprisingly - presents data to the network. It is responsible for translating data and formatting The layer provides coding and conversions functions to allow data to move from the upper layers to the lower layers.
Without this data cannot be presented and understood by the other layers. Failures here will mean data cannot be used to determine paths and session leaving it unable to move over the network.

Computers receive and send formatted data and convert it into a language understood by itself. By translating data the presentation layer can be sure that data can be understood by the receiving end.
Data compression, decompression, encryption and decryption are associated with the presentation layer.

5 - Session Layer
The session layer has a simple action to perform. It allows numerous presentation sessions to be separated and to remain separate. The session layer sets up these sessions, manages them and them tears them down when no longer active.
The session layer makes use of different modes - half duplex, full duplex or simplex. The layer provides dialogue between devices or nodes.
Basically put - the layer keeps different applications data separate.
There is more but at this level - you don't really need it.

4 - Transport
The transport layer segments data into numerous data streams resembling them on the other side. This provides end to end transport services and allows a logical session to be created when hosts connect.
At this layer we see 2 very common connection protocols: TCP and UDP. Notice that's a connection protocol NOT a routing protocol.

TCP is a connection orientated protocol. This means that for every connection and set number of packets sent the receiving host must acknowledge the sending host. This connection orientated method allows more reliable but slower communication across the network - although as devices become much faster these time differences are quickly decreasing.
TCP uses a number of services such as Flow control, CSMA/CD and Windowing to make sure that frames arrive when they should in the correct order.

Although not openly a topic within the OSI model its a nice time to go into it all.

Flow Control
Flow control allows for data integrity over the medium. TCP uses flow control to make sure that any packets sent are acknowledged and the they have not been lost/dropped in the process. It also makes sure that any data sent isn't going to cause issues on the receiving hosts end especially where buffering is concerned.

When segments arrive at the receiving host they are acknowledged. This is the key concept. This maintained and established with a three way handshake. The host will communicate with each other to make sure that the connection is active and that packets are going to arrive when and where they are specified.
This three way handshake is a critical part of the Transport layer process and is called the virtual circuit. Throught commubnication the 2 hosts will check with each other to make sure the connection is still active and that the sender or reciever has not lost contact.

In the handshake there are 3 parts. Initially there is a connection agreement where the hosts will request synchronization. The second segment will then be used to negotiate the link and acknowledge the requests to establish connection parameters/rules between the hosts.
The third segment will then acknowledge the connection and parameters.

By using flow control the receiving host makes use of 2 district states - Not ready and Go/Ready states. When communication occurs the speed of processing and the link can cause packets to queue. Although devices have a buffer i.e memory to store packets not yet processed this is limited. When this is full packets can be queued and lost. However in a connection orientated flow controlled state these packets are simply stopped.

The device once busy and full will send out a Not Ready message to the sending host. This basically means stop for a moment and let me finish what I am doing. Once complete the device send a Go packet and the sending host will check the link and begin sending once again.

The datagrams are received in the same sequence they are sent and the transmission will fail if this order is broken. To get around the problem of missing packets the host can acknowledge each packet that is sent.
The connection orientated service states the following characteristics:
  • A three way connection (virtual circuit) is established
  • Frames are sequenced
  • Acknowledgements are used
  • Flow control is established
Windowing and Acknowledgements come hand in hand. I realise this is getting pretty long so time to cut it down a bit.
Windowing is a method to set how many packets are sent before the host must send acknowledgments. Thats is. In a Window size of 4 the sender will send 4 packets before waiting for a response. If the host acknowledges the 5th packet (no thats not a typo) then connection resumes. Lets look at that again:

---> Send Segment 1 --->
---> Sned Segment 2 --->
Host 1 (Sender) ---> Send Segment 3 ---> Host 2 (Reciever)
---> Send Segment 4 --->
<--- Ack. Segment 5 ---

So the host will recieve the segments 1-4. Now its ready for 5. It isn't going to say ok I've got 4...because that could mean it didn't get 2 and 3. So instead it says simply I'm ready for 5 (acknowledge 5). If the host had a Window size of 3 then it would recieve segments 1,2 and 3..and acknowlege its ready for 4.

but what is a segment is missed?
Well the recieving host will acknowledge the missed segment...confisuing isn't it? Or maybe not!
So look at the below:

---> Send Segment 1 --->
---> Sned Segment 2 --->
Host 1 (Sender) ---> Send Segment 3 ERROR --\ Host 2 (Reciever)
---> Send Segment 4 --->
<--- Ack. Segment 3 ---
---> Send Segment 3 --->
<--- Ack Segment 5 --->

You see. Segment 3 was missed breaking the chain. Host 2 then realises the problem and acknowledges it - basically saying "Hey host 1 - something has messed up. I am going to admit (acknowledge) segment 3 was lost - please send it again". And there we have it. Segment 3 is send and then the host 2 will acknowledge that it has all the segments it needs and tells host 1 its ready for 5.

Hopefully that isn't to bad. You may initially fight it as I did at first - thinking well why not acknowledge you got the last frame?? but then you just accept it....

3 - Network Layer
Ok transport was big huh! It gets a little simpler now.
The Network layer is classed as Layer 3. Layer 3 works to find routes to networks. Packets passed around the network are recieved on the router/layer 3 switches interface. These packets are read finding the destination address of the network to which the network is being passed. Now there is a key point here. Layer 3 devices are concerned with the Networks - not the destination device. This is saved for the layer 2 devices.

Layer 3 devices use 2 main types of packets - Data Packets which transport data through networks and Route Packets which send routing information and updates to other layer 3 devices.
This layer concerns itself with routing information building routing tables to destination networks through routing protocols such as RIP or EIGRP etc.
This layer concerns more with the actual devices and due to the length I'm going to go into this layer in more detail in another post.

2 - Data Link Layer
The data link layer provides physical transmission of data through the network. This layers main jobs include Physical transmission of data, data error handling and notifications, network topology, and flow control.
This layer provides many of the main functions required to get data to its destination.
This layer formats the message into pieces called data frames. By working with data frames the physical layer provides further addressing to take it from the network layer network boundry to the actual device.

The unique identification of devices falls to Layer 2 using Logical Link Control (LLC) or Media Access Control (MAC).
MAC addressing is concerned with how packets are placed on the media. Using contention access - i.e first come first served access data is places on the media on a first come first served basis. Physical addressing is defined here as well as the logical topology to the devices and networks. The local topology defines how the signal path is generated to get from point A (the source) to point B (the destination)

LLC or Logical Link Control is used to determine the Network Layer Protocols being used by the packet. when a packet is recieved it is analysed to find where the packet is destined. eg IP at the network layer.
Switches are devices which run at layer 2 - these use mac addressessing to find devices. This is hardware based using ASIC as apposed to softaware based like a router which ses routing tables to determine routes.

1 - Physical Layer
Ok these last few layers are shorter as I am going to go into them in more depth later on.
The physical layer is a simple one - it converts the data frames to bits that are sent along the physical medium. By doing this you get from one device to another and complete the phyiscal routing of data.
The physical layer specifies the trasmission of data using 1s and 0s. This lays out the connection and specifies the electrical, mechanical, procedural and functional requirements for activating, maintaining and deactiviting a link.

Interfaces such as the DTE and DCE are determined here along with the end to end connectivity. Hubs work on this level as they provide the most simple form of connectivity - on that is exteremly rare in modern networks.

Once the data has moved down the 7 layers and is transmitted along the physical medium is arrives at the physical layer of the recieving host. It will not pass up the layers transerring from bits to frames through to packets and into the correct sessions.
From here it reaches the application without the users being any wiser about what has happened. 



Networking Essentials