OSPF Interview question and answer - Rare asked but to remember to crack fully!!!

1. What is OSPF? Why and on what environment do we implement this?

Ans:- OSPF is open shortest path first. It’s a link state protocol and hierarchical in nature. It uses area to separate the network and control the excessive traffic. OSPF is generally implemented in big companies with many small subsidiaries recently merged. It’s a kind of not so easy protocol to manage.

2. Why it’s known as Link state protocol? How it’s different than EIGRP? What algorithm does it use?

Ans:- OSPF is a link state protocol because it keeps database of the complete topology and has information about the links. OSPF uses SPF algorithm while EIGRP uses DUAL and its hybrid in nature.

                                                                             

3. What is OSPF’s metric? How is it identified in routing table? Does it support redistribution?

Ans:- It’s metric is Cost. Formula is (100000000/Bandwidth in bps). It is identified as keyword “O”. every routing protocol does support redistribution.

4. What are the states used in OSPF when forming neighborship?

Ans:-   DOWN state – when neighborship is down initially

            INIT – Both neighbors send hello packets to each other

            2-way – Both will reply or acknowledge to each other Hello and share their router-id . Just similar to we shake handshakes and introduce ourselves. DR/BDR election also happens in  this state. Neighborship is formed now

            Exstart – Each neighbor will want to be Master so that it does not have to start first to share the information. It will be decided on the basis of Highest Router-ID.

            Exchange – Master will receive the database descriptor (DBD) information in the form of LSU ( Link state updates). Once done, Master will share its database table with neighbor.

            Loading – If there is better information, either side will make request using LSR ( Link state Request) and reply with come in the form of LSA( Link state advertisement). LSAck will be sent for acknowledgement.

             Full – OSPF database is synced.

5. What are the packet types does OSPF use? Explain Hello, Dead Timers, LSA, LSU, LSR packets.

Ans:- Hello is a handshake packet tells about the alive status of OSPF interface.

Dead is the time it will wait after which it will break the neighborship and inform routing table for removing the route or connected route.

LSA- Link state advertisement.  It contains the information of the network  or update.

LSU – It contains multiple updates at one go.

LSR – It’s a request packet when router running OSPF needs better and updated information.

6. What will be Hello/Dead timers in network having link capacity of >T1 and <=T1?

Ans:- For link above T1 i.e. 1.544Mbps Hello = 10 seconds, Dead is 40 seconds

For link = or below Hello = 30 seconds, Dead = 120 Seconds.

Dead interval is exact four times than Hello and its value can’t be set manually.

7. What is the algorithm used by OSPF?

Ans:- OSPF uses SPF (Shortest Path First) algorithm for calculating the best path and preparing OSPF database.

8. What is OSPF Router ID and how is it elected?

Ans:-  Router id is 32 bit number similar to IP address. It is elected as highest IP Address of Loopback interfaces or Highest IP of the active physical interfaces if loopback is not present. It can also be set manually but it will take effect when OSPF process is reset completely or device is reloaded. There should be only one router-id per device running ospf to avoid database issues.

9. What is the DR/BDR in OSPF? How these are elected and in what scenario?

Ans:- DR/BDR election happens in Shared Multi access network to avoid full mesh scenario. In shared environment, every OSPF router will try to form neighborship with other router, so DR/BDR comes in to action and act as single point of contact. Neighborship will only form with DR and Backup DR. DR router will be the highest priority router and BDR will be second highest priority. If there is a tie, then Highest router-id will be used to make the decision. Router ID is elected as the highest numerical Loopback IP or the highest Physical Active IP address or the Interface IP which comes up first.

10. What will happen if we make priority of a OSPF router as “zero”?

Ans:- OSPF Router with Priority zero will not participate in DR/BDR election.

11. What are the multicast IPs used by OSPF? What multicast IP does DR/BDR router and non-DR use?

Ans:- OSPF Routers use 224.0.0.5 multicast IP address. However, in DR/BDR it uses 224.0.0.5 and 224.0.0.6. DR/BDR will use 224.0.0.6 while replying to OSPF routers in Broadcast Multi access network.

12. Does DR/BDR election happen in OSPF Point to Point Network

Ans:- No election happens in point to point network

                                                       

13. What problems can cause OSPF neighborship in “down” state?

Ans:-  Steps in forming neighborship in OSPF is almost similar to EIGRP’s. Two ospf devices should be connected back to back with cables connected on correct interfaces. IP and mask should be correct. Then run the ospf process if not run already and advertise the interface segments on both routers. Hello, Dead timers and MTU size should be same at both ends. There should not be any access-list or transparent firewall blocking the OSPF Packets or IP addresses. Neighborship should establish now. Area number and authentication parameters must match. If issues are still there, then check the router logs and try to analyze the problem.

14. What is “Process id”? Does it have local or global significance? Does it need to be same on OSPF neighbor routers?

Ans:- Process id is used similarly to Autonomous system but it has local significance only. It can be different on neighbors running OSPF.

15. What is the concept of “Area” in OSPF? What is Backbone area, stub area, Totally Stubby Area, Not So Stuby area (NSSA)?

Ans:- OSPF is hierarchical in nature and therefor it uses areas to control and manage its LSA or database traffic. The concept of area brings stability in the environment and suppresses the unnecessary convergence updates within the area unless meant for other areas. Backbone area is “area 0” – this backbone area is must be physically or logically connected to other areas to communicate. No two areas can share information if one of the areas is not backbone area. Stub area blocks the LSA 4 and 5 i.e. External information which is sent by ABR ( Area Border Router)- It does so to reduce the external traffic. However, connectivity can happen using Default route injection done by ABR as an alternate way.  Any router wants to send packets external network from stub area will have default route in its routing table.

Totally Stubby Area – It blocks external LSA and summary LSA i.e. LSA 3, 4 and 5. Default route will be injected by ABR to routers of area.

Not So Stubby area –

16. Can area 1 and area 2 exchange or advertise networks if they are not connected to area 0 (Backbone area)? What we need to do make it happen?

Ans:- Every area should be connected to area 0 else there won’t be exchange of inter area routes. If it is not possible then use the concept of virtual link

17. What is the concept of Virtual-link in OSPF? Can virtual link be formed thru stub area?

Ans:- Per OSPF Rule, Inter area communication will happen when atleast one area should be area 0 or backbone area. If this is not possible, then we can use virtual-link to form a logical association. Virtual link is not possible thru Stub area.

18. Explain LSA and its all Types such as LSA 1, 2, 3,4,5,7. When do we use LSA type 6

Ans:-  LSA Type 1 :- Every OSPF router within the area speaks this LSA.

LSA Type 2 :- This LSA will be used in Shared multi-access network in DR/BDR election.

LSA Type 3 :- This LSA helps in exchange of inter area summary information. It will be used by ABR( Area Border Router)

LSA Type 4 :- This LSA is meant for external communication i.e. with other protocols. This LSA gets the information about position of ASBR.

LSA Type 5 :- This will be generated by ASBR and helps in exchange of external information of routes.

LSA Type 7:- It is used in special area i.e. NSSA Not So Stubby Area. It gets converted in to Type 5 by ABR.

OSPF Frequently Asked Questions

19. What LSA is used for inter area communication? What LSA type is used for inter protocol communication in OSPF?

Ans:-  Type 3 LSA Is used for inter area communication. For communication with other protocols or external routes , type 4 and 5 will be used.

                                                                                             

20. Explain the concept of Area Border Router (ABR) and Autonomous System Border Router (ASBR). Can router sitting between area 2 and 3 be called as ABR?

Ans:- ABR is the router which sits between Backbone area i.e area 0 and non Backbone area. ASBR is the router which sits between OSPF routing protocol and non-OSPF routing protocol. A Router in OSPF can only be called ABR when it has atleast one interface in area 0. Router between area 2 and 3 can not be known as ABR.

21. Which area blocks external route? Which area blocks Summarization?

Ans:-  Totally Stubby area blocks external LSAs i.e. Type 4 and 5 and summarization i.e. type 3. Stub area also blocks Type 4 and 5 LSA

22. What is the concept of “auto cost reference” and in what scenarios we should use it?
23. How OSPF inter area, external routes are denoted or represented in routing table?

Ans:-  Inter area as O IA , External as O E1 or E2.

24. What is O E1, O E2 letters in OSPF? Which is default external type 1 or external type 2?
25. Where does the summarization happen in OSPF? Which command is used for summarization at ABR and at ASBR?

Ans:- Summarization happens at Boundaries in OSPF i.e. either at ABR ( Boundary of two areas with one area as Backbone) OR at ASBR ( Autonomous System Border Router). ABR uses “ area range” command while ASBR uses “summary-address” command

OSPF Interview question - Simple but effective to crack 1

1: What is an OSPF neighbor?

*** From the perspective of an OSPF router, a neighbor is another OSPF router
that is attached to one of the first router’s directly connected links.

2: What is an OSPF adjacency?

***An OSPF adjacency is a conceptual link to a neighbor over which LSAs can
be sent.

3: What is an LSA? How does an LSA differ from an OSPF Update packet?

***A router originates a link state advertisement to describe one or more
destinations. An OSPF Update packet transports LSAs from one neighbor to
another. Although LSAs are flooded throughout an area or OSPF domain,
Update packets never leave a data link.

4: What is a link state database? What is link state database
synchronization?

***The link state database is where a router stores all the OSPF LSAs it knows
of, including its own. Database synchronization is the process of ensuring that all
routers within an area have identical link state databases.

5: What is the default HelloInterval?

***The default OSPF HelloInterval is 10 seconds.

6: What is the default RouterDeadInterval?

***The default RouterDeadInterval is four times the HelloInterval.

7: What is a Router ID? How is a Router ID determined?

***A Router ID is an address by which an OSPF router identifies itself. It is either
the numerically highest IP address of all the router’s loopback interfaces, or if no
loopback interfaces are configured, it is the numerically highest IP address of all
the router’s LAN interfaces.

8: What is an area?

***An area is an OSPF sub-domain, within which all routers have an identical link
state database.

9: What is the significance of area 0?

***Area 0 is the backbone area. All other areas must send their inter-area traffic
through the backbone.

10: What is MaxAge?

***MaxAge, 1 hour, is the age at which an LSA is considered to be obsolete.

11: What are the five OSPF packet types? What is the purpose of each type?

*** The five OSPF packet types, and their purposes, are:

Hellos, which are used to discover neighbors, and to establish and maintain
adjacencies

Updates, which are used to send LSAs between neighbors

Database Description packets, which a router uses to describe its link state
database to a neighbor during database synchronization

Link State Requests, which a router uses to request one or more LSAs from a
neighbor’s link state database

Link State Acknowledgments, used to ensure reliable delivery of LSAs

12:What are LSA types 1 to 5 and LSA type 7? What is the purpose of each
type?

*** The most common LSA types and their purposes are:

Type 1 (Router LSAs) are originated by every router and describe the originating
router, the router’s directly connected links and their states, and the router\xd5 s
neighbors.

Type 2 (Network LSAs) are originated by Designated Routers on multiaccess
links and describe the link and all attached neighbors.

Type 3 (Network Summary LSAs) are originated by Area Border Routers and
describe inter-area destinations.

Type 4 LSAs (ASBR Summary LSAs) are originated by Area Border Routers to
describe Autonomous System Boundary Routers outside the area.

Type 5 (AS External LSAs) are originated by Autonomous System Boundary
Routers to describe destinations external to the OSPF domain.

Type 7 (NSSA External LSAs) are originated by Autonomous System Boundary
Routers within not-so-stubby areas.

13: What are the four OSPF router types?

***The four OSPF router types are:

# Internal Routers, whose OSPF interfaces all belong to the same area

# Backbone Routers, which are Internal Routers in Area 0

# Area Border Routers, which have OSPF interfaces in more than one area

# Autonomous System Boundary Routers, which advertise external routes into
the OSPF domain
14: What are the four OSPF path types?

***The four OSPF path types are:

Intra-area paths

Inter-area paths

Type 1 external paths

Type 2 external paths
15: What are the five OSPF network types?

*** The five OSPF network types are:

i)Point-to-point networks

ii) Broadcast networks

iii) Non-broadcast multi-access (NBMA) networks

iv) Point-to-multipoint networks

v) Virtual links
16: What is a Designated Router?

***A Designated Router is a router that represents a multiaccess network, and
the routers connected to the network, to the rest of the OSFP domain.
17: How does a Cisco router calculate the outgoing cost of an interface?

***Cisco IOS calculates the outgoing cost of an interface as 108/BW, where BW
is the configured bandwidth of the interface.
18: What is a partitioned area?

***An area is partitioned if one or more of its routers cannot send a packet to the
area’s other routers without sending the packet out of the area.
19: What is a virtual link?

*** A virtual link is a tunnel that extends an OSPF backbone connection through
a non-backbone area.
20: What is the difference between a stub area, a totally stubby area, and a
not-so-stubby area?

***A stub area is an area into which no type 5 LSAs are flooded. A totally stubby
area is an area into which no type 3, 4, or 5 LSAs are flooded, with the exception
of type 3 LSAs to advertise a default route. Not-so-stubby areas are areas
through which external destinations are advertised into the OSPF domain, but
into which no type 5 LSAs are sent by the ABR.
21: What is the difference between OSPF network entries and OSPF router
entries?

*** OSPF network entries are entries in the route table, describing IP
destinations. OSPF router entries are entries in a separate route table that record
only routes to ABRs and ASBRs.
22: Why is type 2 authentication preferable over type 1 authentication?

***Type 2 authentication uses MD5 encryption, whereas type 1 authentication
uses clear-text passwords.
23: Which three fields in the LSA header distinguish different LSAs? Which
three fields in the LSA header distinguish different instances of the same
LSA?

***The three fields in the LSA header that distinguish different LSAs are the
Type, Advertising Router, and the Link State ID fields. The three fields in the LSA
header that distinguish different instances of the same LSA are the Sequence
Number, Age, and Checksum fields

OSPF Interview Tough question and Answers - Krack !!

1: Which command in OSPF shows the network LSA information?

** The command show ip ospf [process-id area-id] database network displays the network link-state information.

2: What command would you use to create a totally stubby area?

** The command area area-id stub no-summary will create a totally stubby area. This is a subcommand to the router ospf process-id command. It is necessary only on the ABR, but all the other routers in the area must be configured as stub routers.

3: What is a virtual link, and what command would you use to create it?

** A virtual link is a link that creates a tunnel through an area to the backbone (Area 0). This allows an area that cannot connect directly to the backbone to do so virtually. The command to create the link is area area-id virtual-link router-id. Note that the area-id that is supplied is that of the transit area, and the router-id is that of the router at the other end of the link. The command needs to be configured at both ends of the tunnel.

4: Where would you issue the command to summarize IP subnets? State the command that is used.

** Summarization is done at area boundaries. The command to start summarization is the area range command, with the syntax area area-id range address mask. To summarize external routes, use the summary-address command on the ASBRs.

5: How would you summarize external routes before injecting them into the OSPF domain?

** The command summary-address address mask is the command that you would use.

6: When is a virtual link used?

** A virtual link is used when an area is not directly attached to the backbone area (Area 0). This may be due to poor design and a lack of understanding about the operation of OSPF, or it may be due to a link failure. The most common cause of an area separating from the backbone is link failure, which can also cause the backbone to be segmented. The virtual link is used in these instances to join the two backbone areas together. Segmented backbone areas might also be the result of two companies merging.

7: Give the command for defining the cost of a default route propagated into an area.

** The command to define the cost of a default route propagated into another area is area area-id default-cost cost.

8: Give an example of when it would be appropriate to define a default cost.

** It is appropriate to define a default cost for the default route when a stub area has more than one ABR. This command allows the ABR or exit point for the area to be determined by the network administrator. If this link or the ABR fails, the other ABR will become the exit point for the area.

9: On which router is the area default cost defined?

** The default cost for the default route is defined on the ABR. The ABR will then automatically generate and advertise the route cost along with the default route.

10: Give the command to configure a stub area and state on which router it is configured.

** The command syntax to configure a stub area is area area-id stub. This command is configured on the ABR connecting to the area and on all the routers within the area. Once the configuration is completed, the Hellos are generated with the E bit set to 0. All routers in the area will only form adjacencies with other routers that have the E bit set.

11: What is the purpose of the area range command, and why is it configured on the ABR?

** The area range command is configured on an ABR because it dictates the networks that will be advertised out of the area. It is used to consolidate and summarize the routes at an area boundary.

12: Give the commands to configure a router to place subnets 144.111.248.0 through to 144.111.255.0 in Area 1 and to put all other interfaces into Area 0.
** The commands are as follows:


network 144.111.248.0 0.0.7.255 area 1



network 0.0.0.0 255.255.255.255 area 0


13: Give the syntax to summarize the subnets 144.111.248.0 to 144.111.254.255 into another autonomous system.
** The syntax is as follows:


summary-address 144.111.248.0 255.255.248.0


14: Explain briefly the difference between the area range command and the summary-address command.

** The area range command is used to summarize networks between areas and is configured on the ABR. The summary-address command is used to summarize networks between autonomous systems and is configured on the ASBR.

15: Explain the following syntax and what it will achieve: area 1 stub no-summary.

** The command area 1 stub no-summary creates a totally stubby area. The number after the word area indicates the area that is being defined as a totally stubby area. This is necessary because the router might be an ABR with connections to many areas. Once this command is issued, it prevents summarized and external routes from being propagated by the ABR into the area. To reach the networks and hosts outside the area, routers must use the default route advertised by the ABR into the area.

16: Why would you configure the routing process to log adjacency changes as opposed to turning on debug for the same trigger?

** The reason to configure the router process to log adjacency changes to syslog as opposed to running debug is an issue of resources. It takes fewer router and administrator resources to report on a change of state as it happens than to have the debugger running constantly. The debug process has the highest priority and thus everything waits for it.

17: Give some of the common reasons that neighbors fail to form an adjacency.

** Many OSPF problems stem from adjacency problems that propagate throughout the network. Many problems are often traced back to neighbor discrepancies.

If a router configured for OSPF routing is not seeing an OSPF neighbor on an attached network, do the following:

- Make sure that both routers are configured with the same IP mask, MTU, Interface Hello timer, OSPF Hello interval, and OSPF dead interval.

- Make sure that both neighbors are part of the same area and area type.

- Use the debug and show commands to trace the problem.

18: When configuring a virtual link, which routers are configured?

** The configuration is between the ABRs, where one of the ABRs resides in Area 0 and the other in the area that is disconnected from the backbone. Both of the ABRs are also members of the transit area. Having created the virtual link, both ABRs are now members of Area 0, the disconnected area, and the transit area.

19: What does the command area 1 default-cost 15 achieve?

** The command area 1 default-cost 15 will assign a cost of 15 to the default route that is to be propagated into the stub area. This command is configured on the ABR attached to the stub area.

20: Explain what is placed in the parameters area-id and router-id for the command area area-id virtual-link router-id.

** The parameter area-id is the area ID of the transit area. So if the ABR in Area 0 is creating a virtual link with the ABR in Area 3 through Area 2, the area ID stated in the command is Area 2. The router ID is the router ID of the router with whom the link is to be formed and a neighbor relationship and adjacency established.

OSPF advantage and disadvantage

OSPF Definition:

Open Shortest Path First (OSPF),

It is a routing protocol used to determine the correct route for packets within IP networks. It was designed by the Internet Engineering Task Force to serve as an Interior Gateway Protocol replacing RIP.

Advantages of OSPF:
Changes in an OSPF network are propagated quickly.
OSPF is heirarchical, using area 0 as the top as the heirarchy.
OSPF is a Link State Algorithm.
OSPF supports Variable Length Subnet Masks (VLSM).
OSPF uses multicasting within areas.
After initialization, OSPF only sends updates on routing table sections which have changed, it does not send the entire routing table.
Using areas, OSPF networks can be logically segmented to decrease the size of routing tables. Table size can be further reduced by using route summarization.
OSPF is an open standard, not related to any particular vendor. Disadvantages of OSPF:
OSPF is very processor intensive.
OSPF maintains multiple copies of routing information, increasing the amount of memory needed.
Using areas, OSPF can be logically segmented (this can be a good thing and a bad thing).
OSPF is not as easy to learn as some other protocols.
In the case where an entire network is running OSPF, and one link within it is "bouncing" every few seconds, OSPF updates would dominate the network by informing every other router every time the link changed state (I've done this). OSPF routers check the status of other routers on the network by sending a small hello packet at regular intervals. If a router does not respond to the hello packet, it is assumed dead, and routing updates are sent to every other router by using a multicast address.
In the case where there are no network changes, OSPF will use very little bandwidth (only sending hello packets). As soon as there is an outage, however, OSPF will flood the network as the change is sent to every router (and then every router notifies every other router about the change). This system of near silence when possible and flooding when necessary ensures that routing information gets propagated throughout the network as quickly as possible 

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interview question for network administrator 12

Explain stop and wait flow control. - Here transmitter sends one message and waits for acknowledgement(ACK) - After receiving ACK it transmits next message. - Its main advantage is it is very simple and easy to understand. - Its main disadvantage is its speed is very slow and time is wasted. - Here it takes twice as long to transmit data and receive ACK.

Explain Poll/select. - It is a line discipline used for centrally controlled networks. - Here primary station controls data and secondary station responds to instructions. - Primary polls to secondary to determine if it has data to transmit. - If secondary has data it responds to poll with a message and acts as a transmitter. - If primary selects secondary, it identifies secondary as the receiver and checks if it is ready to receive data.

Explain the function of line discipline. - Line discipline is used in the data link layer which coordinates hop to hop data delivery. - Here the hop can be a computer or a network controller. - It also coordinates the half duplex transmission on data communication network. - Acknowledgement discipline is used for simple networks. - Poll/select is used for centrally controlled data networks.

Give the difference between cable and voice modems. - Cable modem operates on higher frequencies up to 850MHz. - Cable modem uses more complex modulation and demodulation techniques than simple FSK modulation. - Cable modems connect subscribers to cable tv and voice modem connect subscriber to the telephone network. - Cable modem works on Bit rate of 30Mbps and voice modem works on 56000 bps. - Cable modem works on voice and video whereas voice modem work only on voice.

What are scramble and descramble circuits used for? - Scramble circuit detects the undesirable sequence of 0 and 1 and convert them into sequence more acceptable. - It randomizes the data to convert data back at the receiver. - For this different algorithms are used both at the transmitter and the receiver. - Descramble circuit generates the original sequence at the receiver. - Example: eight consecutive 0’s are converted to alternate 0’s and 1’s which is more acceptable to the clock recovery.

Explain adaptive equalizers. - These are located in the receiver section of the modem. - It automatically adjusts the gain and delay of the input received to compensate for the phase and amplitude impairments. - It determines the quality of the received signal. - It continuously vary its settings to achieve bandwidth characteristics for the circuit. - Because of their adjusting property they are called adaptive equalizers

Issues for data interfacing in physical layer. - Raw data is to be converted into binary form 0 and 1 to be sent to the next layer. - Different voltages are required for representing 0(0 to1.5 volts) and 1(3 to 5 volts). - How many nano seconds a bit should last. - How transmission may proceed from physical layer to data link layer or from communication channel to physical layer . - How many pins network connectors may use for serial communication.

Which physical property of information is changed for transmission? - Current or Voltage of signal is changed for transmitting it in the form of zeros and ones. - Signal broken into harmonics called frequency using Fourier transform. - Amplitude of signals due to Fourier transform may be reduced. - A filter is attached before the transmission medium which allows only required harmonics to pass through the medium. - Signal at the receiver end can be reconstructed from harmonics using inverse Fourier transform .

Explain resource sharing - Different resources are attached to a computer and if they can be used by other computers in the same network it is called resource sharing. - It is generally possible in local area networks confined to offices or institutes. - The host requires an operating system to access the resources of the server. - FTP called the file transfer protocol is used to provide the shared access between host and the server. - It is cheaper, faster and easier to maintain.

Which transmission medium radiates and how it can be avoided? - Twisted pair cable radiates energy during transmission. - Two wires present in the cable constitute an Antenna which radiates energy in all directions. - The wires are twisted like a DNA present in human blood. - Waves coming out from the two twisted wires are out of phase by 180 degree so they cancel out. - Leads to no radiations and also sheath could be provided outside the cable to provide insulation. Which category of twisted pair is mainly used. - There are up to 7 categories of twisted pair but Category 3 and 5 twisted pairs are mostly used. - Category 3 has 2 individually insulated wires twisted together . - 4 such twisted pairs are put together in a sheath to form the medium. - Category 5 is same as 3 with more number of twist per cm. More the number of twists the better the communication medium . - Leads to less cross talk and better signal transmission over longer distances. Which transmission medium works at 2 giga bits per second. - The fiber cable sends data at rate of 2 giga bites per second. - Cable consists of Hair like thin fiber which works on the principle of total internal reflection. - It transmits data with speed of light and works on bandwidth up to terahertz. - Transmitting medium is fiber of glass which is insulated on the outside. - Advantage:Attenuation of signal is minimum. Speed of transmission is very high. Can infrared be used for transmission - The infrared waves can be used for short range communication in networking (up to 45 km). - It is wireless communication so no physical medium is required to connect the transmitter and receiver . - Works on line of sight system i.e. the transmitting and receiving antennas should be in the sight of each other. - No government license is needed till now for this communication . - Example :used in television ,stereos. Compare satellite and cable medium for transmission. - Delay given by satellite during its transmission is 3 micro seconds per km. - Delay given by cable wire is 5 micro seconds per km. - Satellite is broadcast media placed in the geo stationary orbit around the earth. - Satellite is used in wireless communication system and cable is used in wired communication. - On security grounds cable is better than a satellite. Explain Store and forward Network. - Store and forward method is used in message switching. Example: SMS - The data sent by the sender is first stored at the server. - Data is later forwarded from the server to the receiver. - It may use time division multiple access for transmission i.e. the whole bandwidth is given to single user for smalltime slots. - No physical path is required for transmission of data from sender to the receiver.