UNIT III - NETWORK LAYER

1. What are the network support layers and the user support layers?

Network support layers:

The network support layers are Physical layer, Data link layer and

Network layer. These deals with electrical specifications, physical connection,

transport timing and reliability.

User support layers:

The user support layers are: Session layer, Presentation layer,

Application layer. These allow interoperability among unrelated software system.

2. With a neat diagram explain the relationship of IEEE Project to the OSI model?

The IEEE has subdivided the data link layer into two sub layers:

* Logical link control (LLC)

* Medium access control (MAC)

LLC is non-architecture specific. The MAC sub layer contains a number

of distinct modules ,each carries proprietary information specific to the

LAN product being used.

3. What are the functions of LLC?

The IEEE project 802 model takes the structure of an HDLC frame and divides it

into 2 sets of functions. One set contains the end user portion of the HDLC frame - the

logical address, control information, and data. These functions are handled by the IEEE

802.2 logical link control (LLC) protocol.

4. What are the functions of MAC?

MAC sub layer resolves the contention for the shared media. It contains

synchronization, flag, flow and error control specifications necessary to move

information from one place to another, as well as the physical address of the next station

to receive and route a packet.

5. What is protocol data unit?

The data unit in the LLC level is called Protocol Data Unit (PDU). It contains four

fields.

 Destination Service Point

Address (DSAP)

• Source Service Access Point

Other layers

Network

Data link

Physical

Other layers

Network

Logical Link Control

Media Access Control

Physical

DSAP SSAP Control Information

• Control field
• Information field

6. What are headers and trailers and how do they get added and removed?

The control data added to the beginning of a data is called headers. The

control data added to the end of a data is called trailers. At the sending machine, when

the message passes through the layers each layer adds the headers or trailers. At the

receiving machine, each layer removes the data meant for it and passes the rest to the

next layer.

7. What are the responsibilities of network layer?

The network layer is responsible for the source-to-destination delivery of

packet across multiple network links. The specific responsibilities of network layer

include the following:

• Logical addressing. 
• Routing.

8. What is a virtual circuit?

A logical circuit made between the sending and receiving computers. The

connection is made after both computers do handshaking. After the connection, all

packets follow the same route and arrive in sequence.

9. What are data grams?

In datagram approach, each packet is treated independently from all others. Even

when one packet represents just a place of a multipacket transmission, the network treats

it although it existed alone. Packets in this technology are referred to as datagram.

10. What are the two types of implementation formats in virtual circuits?

Virtual circuit transmission is implemented in 2 formats. 

• Switched virtual circuit
•  Permanent virtual circuit.

11. What is meant by switched virtual circuit?

Switched virtual circuit format is comparable conceptually to dial-up line in

circuit switching. In this method, a virtual circuit is created whenever it is needed and

exits only for the duration of specific exchange.

12. What is meant by Permanent virtual circuit?

Permanent virtual circuits are comparable to leased lines in circuit switching.

In this method, the same virtual circuit is provided between two uses on a continuous

basis. The circuit is dedicated to the specific uses.

13. Define Routers.

Routers relay packets among multiple interconnected networks. They Route

packets from one network to any of a number of potential destination networks on

internet routers operate in the physical, data link and network layer of OSI model.

14. What is meant by hop count?

The pathway requiring the smallest number of relays, it is called hop-count

routing, in which every link is considered to be of equal length and given the value one.

15. How can the routing be classified?

The routing can be classified as,

• Adaptive routing 
• Non-adaptive routing.

16. What is time-to-live or packet lifetime?

As the time-to-live field is generated, each packet is marked with a lifetime,

usually the number of hops that are allowed before a packet is considered lost and

accordingly, destroyed. The time-to-live determines the lifetime of a packet.

17. What is meant by brouter?

A brouter is a single protocol or multiprotocol router that sometimes act as a

router and sometimes act as a bridge.

18. Write the keys for understanding the distance vector routing.

The three keys for understanding the algorithm are

• Knowledge about the whole networks
• Routing only to neighbors
•  Information sharing at regular intervals

19. Write the keys for understanding the link state routing.

The three keys for understanding the algorithm are

• Knowledge about the neighborhood.

• Routing to all neighbors.

• Information sharing when there is a range.

20. How the packet cost referred in distance vector and link state routing?

In distance vector routing, cost refer to hop count while in case of link state

routing, cost is a weighted value based on a variety of factors such as security levels, traffic

or the state of the link.

21. How the routers get the information about neighbor?

A router gets its information about its neighbors by periodically sending them a

short greeting packets. If the neighborhood responds to the greeting as expected, it is

assumed to be alive and functioning. If it dose not, a change is assumed to have occurred

and the sending router then alerts the rest of the network in its next LSP.

22. What are the four internetworking devices?

The four inter networking devices are,

• Repeaters
• Bridges
• Routers
• Gateway

23. Define IP address.

IP address is the 3-bit number for representing a host or system in the network.

One portion of the IP address indicates a networking and the other represents the host in

a network.

24. What is Token Bus?

Token Bus is a physical bus that operates as a logical ring using tokens.

Here stations are logically organized into a ring. A token is passed among stations. If a

station wants to send data, it must wait and capture the token. Like Ethernet, station

communicates via a common bus.

25. What is token passing?

Stations may attempt to send data multiple times before a transmission

makes it onto a link. This redundancy may create delays of indeterminable length if the

traffic is heavy. Token ring resolves this uncertainty by requiring that stations take turns

sending data. Each station may transmit only during its turn and may send only one frame

during each turn. The mechanism that coordinates this rotation is called token passing.

26. Define Masking?

Masking is the process that extracts the address of the physical network

from an IP address.

27. What are the rules of boundary-level masking?

The rules of boundary-level masking

•  The bytes in the IP address that corresponds to 255 in the maskwill be repeated in the subnetwork address
•  The bytes in the IP address that corresponds to 0 in the mask will change to 0 in the subnetwork address

28. What are the rules of nonboundary-level masking?

•  The bytes in the IP address that corresponds to 255 in the mask will be repeated in the subnetwork address

• The bytes in the IP address that corresponds to 0 in the mask will change to 0 in the subnetwork address For other bytes, use the bit-wise AND operator

29. Define Gateway.

A device used to connect two separate networks that we different

communication protocols.

30. What is LSP?

In link state routing, a small packet containing routing information sent by

a router to all other router by a packet called link state packet.