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I

1. In a distributed system, a collection of independent computers appears to its users as a single coherent system.2. Client-server model: 2 processes are involved: one on the client one on the server machine. Communication takes the form

of the client process sending a message over the network to the server process, the client process waits for a reply

message, then the server process gets the request and performs the work.

3. Peer-to-peer: individuals who form a loose group can communicate with one or more people; there is no fixed division intoclients and servers.

4. LAN char.: Privately owned networks within a single building or campus of up to a few km in size. Classification criterion:a) their size: The worst case transmission time is bounded b) the transmission technology: A cable to witch all machines

are attached c) their topology: Bus Ring

5. Internetworks. A collection of interconnected networks is called an internetwork or internet. Expl: a collection of LANsconnected by a WAN

6.

7. An autonomous systemis an internet connected by homogeneous routers; generally the routers are under theadministrative control of a single entity.

8. An interior router protocol (IRP) passes writing info between routers within an autonomous system.9. The protocol used to pass routing info between routers in different autonomous systems is referred to as an exterior router

protocol (ERP).

10. A network is a series of points or nodes interconnected by communication paths.11. A protocolis an agreement between the communicating parties on how communications is to proceed.12. A computer network is a collection of autonomous computers interconnected by a single technology13. A list of protocols used by a certain system, one protocol per layer, is called a protocol stack

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II

1. def. A WAN spans a large geographical

area, often a country or continent. It contains

a collection of machines intended for running

user programs. Relation between hosts on

LANs and the subnet

Separation of the pure communication

aspects of the network from the

applications aspects (the hosts) greatly

simplifies the network design. The subnet

elements:

- transmission lines (copper wire, optical fibre,

radiotrans. lines)

- switching elementsspecialized computers. The switching computers are commonly called routers.

The collection of communication lines and routers form the subnet. Subnet: collection of routers and communication lines that

move packets from source host to destination.

2. Protocol Hierarchies.

To reduce their design complexity, most networks are organized as a stack of layers and levels. The purpose of each layer is to offer

certain services to the higher layers, shielding those layers from the details of how the offered services are actually implemented. In

a sense each layer is a kind of virtual machine, offering certain services to the layer above it.

A protocol is an agreement between the communicating parties on how communication is to proceed.

Each layer passes data and control information to the layer immediately below it until the lowest layer is reached. Below layer 1 is

the physical medium in which actual communication occurs. Between each pair of adiacent layers is an interface (defines which

primitives and services the lower layer makes available to the upper one).

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Defining clean interfaces between layers is important. Each layer performs a specific collection of well understood functions. Well

designed interfaces make simple to replace the implementation of one layer

Def: A set of layers and protocols is called network architecture

Specifications of architecture:

- to write the program

- to build the hardware for each layer

A list of protocols used by a certain system, one protocol per layer is called a protocol stack.

3. Layers can offer 2 different types of services to the layers above them: connection oriented and connectionless.

C-O service is modelled after the telephone system: the service user first establishes a connection and then releases the connection.C-less service is modelled after the postal system: each message carries the full destination address and each one is routed through

the system independent of all others.

Some services are reliable in sense that they never lose data. A reliable service is implemented by having a receiver acknowledge

the receipt of each message so the sender is sure that it arrived. Reliable connection service has 2 minor variations: message

sequences (the message boundaries are preserved), byte streams (the connection is simply a stream of bytes). Unreliable

connectionless service is called: datagram service.

4. OSI model.

The model is a first step towards international standardization of the protocols used in various layers. The model is called OSI

reference model because it deals with connecting open systems (systems that can open for communication with other systems). It

has 7 layers.

Principles to apply to arrive at the seven layers: 1) a layer should be created where a different abstraction is needed 2) each layer

should perform a well-defined function 3)the function of each layer should be chosen with an eye toward defining internationally

standardized protocols 4) the layer boundaries should be chosen to minimize the information flow across the interfaces 5) the

number of layers should be large enough that distinct functions need not be thrown together in the same layer.

The OSI model is not a network architecture because it does not specify exact services and protocols to be used in each layer. It tellswhat each layer should do.

5. The Physical LayerIs concern with transmitting bits over a communication channel.

Design issues: have to do with making sure that where one side sends a 1 bit it is received by the other side as a 1 bit not as a 0 bit.

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6. Network Layer

Controls the operation of the subnet. Design issue: determining how packets are routed from source to destination. Routers can be:

-based on static tables that are wised into the network and rarely damaged

- determined at the start of each conversation

- highly dynamic, being determined for each packet to reflect the network load

If too many packets are presented in the subnet at the same time, they will get in anothers way by forming bottlenecks.

The quality of service provided (delay, transit time, etc) is also a network layer issue. When a packet has to travel from one network

to another to get to its destination, many problems can arise.: the addressing used by the second network may be different from

the first one; the second one may not accept the packet at all because it is to large; the protocols may differ.

It is up to the network layer to overcome all the problems, to allow heterogeneous networks to be interconnected.In broadcast network the routing problem is simple, so the network layer is often thin or non existent.

7. Transport LayerIts basic function is to accept data from above (session layer), split it up into smaller units, pass these to the network layer to ensure

that the all the pieces arrive correctly at the other end. All this must be done efficiently and in a way that isolates the upper layers

from the changes in the hardware technology. The transport layer determines what type of service to provide to the session layer

and the users of the network. The most popular type of transport connection is an error free point-to-point channel that delivers

messages or bytes in the order in which they were sent.

The type of service is determined where the connection is established.The transport layer is a true end-to-end layer, all the way from the source to the destination.

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8. TCP/IP Reference Model. Internet Layer

It is the reference model used in the grandparent of all wide area networks , the ARPANET, and its successor, the worldwide

Internet.ARPANET-was a research network. It connected hundreds of universities and government institutions, using telephone lines. When

satellite and radio networks were added later, the existing protocols had trouble interworking with them, so a new reference

architecture was needed. This architecture later become known as the TCP/IP reference model after its two primary protocol. A

major goal was that the network be able to survive loss of subnet hardware, with existing conversation being broken off.

Internet Layer. All these requirements led choise of a packet-switching network based on a connectionless internetwork layer. This

layer is the linchpin that holds the whole architecture together. It s job: to permit hosts to inject packets into any network and have

travel independently to the destination.

The internet layer defines an official packet format and protocol called IP. The job of the Internet layer is to deliver IP packet

where they are supposed to go. Packet routing is clearly the major issue here, as is avoiding congestion.

9. TCP/IP Reference Model. Transport Layer

It is the reference model used in the grandparent of all wide area networks , the ARPANET, and its successor, the worldwide

Internet.

ARPANET-was a research network. It connected hundreds of universities and government institutions, using telephone lines. When

satellite and radio networks were added later, the existing protocols had trouble interworking with them, so a new reference

architecture was needed. This architecture later become known as the TCP/IP reference model after its two primary protocol. Amajor goal was that the network be able to survive loss of subnet hardware, with existing conversation being broken off.

The transport layer is designed to allow peer entities on the source and destination hosts to carry on a conversation, just as in the

OSI transport layer. Two end-to-end transport protocols have been defined here. 1. TCP (transmission control protocol)is a reliable

connection-oriented protocol that allows a byte stream originating on one machine to be delivered without error on any other

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machine in the internet. 2 UDP ( user datagram protocol) is an unreliable, connectionless protocol for applications that do not want

TCPs sequencing or flow control and wish to provide their own.

14. Architecture of internet

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The client calls his ISP over a dial-up telephone line. The modem is a card within a PC that concerts the digital signal the computer

produces to analog signals that can pass over the telephone system. These signals are transferred to the ISPs POP(point of

presence), where they removed from the telephone system and injected into the ISPs regional network. From this point the system

is fully digital and packet switched. The ISPs regional network consists of interconnected routers in the various cities the ISP servers.

If the packet is destinated for a host served directly by the ISP, the packet is delivered to the host. Otherwise it is handed over to the

ISP,s backbone operator. To allow packets to hop between backbone, all the NAPs ( a room full of routers at least one per

backbone. The large backbone also have many direct connections between their routers, a technique known as private peering.

Some companies have interconnected all their existing internal networks, using the same technique as the internet.

11. Relationship between Data Rate and Bandwidth

The concept of effective bandwidth is somewhat fuzzy one we have said that it is the band within which most of the signal energy isconfined. The important issues here is that although a given waveform may contain frequencies over a large range, as a practical

metter any transmission medium that is used will be able to accommodate only a limited band of frequencies. This limits the data

rate that can be carried on the transmission medium.

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12. Maximum data rate of a channel

Even a perfect channel has a finite transmission capacity. Nyquist derived an equation expressing the maximum data rate for a finite

bandwidth noiseless channel. He proved that if an arbitrary signal has been run through a low pass filter with bandwidth, thefiltered signal can be completely reconstructed by making only 2H samples per second. If the signal consists of V discrete levels, max

data rate=2H*logV bits/sec. The amount of thermal wise is measured by the ratio of the signal power to the noise power.

Shannons major result is that the maximum data rate of a channel whose bandwidth is H Hz, and whosesignalto-wise radio is S/N

is given by: max nr of bits/sec =H log(1+S/N). Shannons result was derived from information theory arguments and applies to any

channel subject to thermal noise.

13-Switching:

when you or your computer places a telephone call, the switching equipment within the telephone system seeks out a physical path

all the way from your telephone to the receiver's telephone .This technique is called circuit switching.

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the local loop: the main parts of the system are illustrated

Here we use the local loops ,the trunks , the toll offices and the end offices .Both of which contain switching equipment andswitches calls . An end office has up to 10.000 local loops .When a computer wishes to send digital data over an analog dial-up line ,

the data must first first be converted to analog form for transmission over the local loop by a modem .

At the telephone company end office data are converted to digital form for transmission over the long -hail trunks .

If the other end is a computer with a modem , the reverse conversion -digital to analog is needed to traverse the local loops at the

destination .the ISP1 has a bank of modems each connected to a different local loops .

Each of the six rectangles represent a carrier switching office (end office , toll office ...).In this example each office has 3 incoming

lines and 3 outgoing lines .when call passes through a switching office ,a physical connection is established between the line on

which the call came in and one of the output lines .

14- Packet switching techniques: comparison between CS and PS technique :

The alternative to circuit switching is packet switching . with this technology individual packs are sent as needed , with no dedicated

path being set up in advance .its up to each packet to find its way to the destination .An important property of circuit switching is

the need to set up an end to end path before any data can be send .Packet switching: Place a tight upper limit on block size allowing

packets to be buffered in rooter main memory instead of on disk .Packet switching is more tolerant than circuit switching .CS is

completely transparent ,the sender and receiver can use any bit rate format and framing method they wanted with PS the carrier

determins the basic parameters , a final difference is the charging algorithm , with CS charging has been based on distance and time

, with PS connect time is not an issue but the volume of traffics sometimes is.

15-Data link layer services :

it can be designed to offer many services : 1- unacknowledged connectionless service 2- Acknowledged connectionless service 3-

Acknowldeged connection-oriented services

1-consists of having the source machine sent independant frames to the destination machine without having the destination

machine acknowledged them, no logical connection is established beforehand or released afterward. 2- when this service is offered

there is still no logical connection used but its frame send is individually acknowledged ,in this way , the sender knows weather a

frame has arrived correctly if it had not arrived within a specific time interval .it can be sent again .3- with this service the source and

destination machines establish a connection before any data is transfered .each frame sent is numbered , and the DLL guarantees

that each frame sent is indeed received and exactly once ,provides the network layer process with the equivalent of a reliable bit

stream . Transfers go through 3 distinct phases :

1-connection is established 2-frames are transmitted 3- connection is released

16 Framing

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To provide service to the network layer, the data layer must use the service provided to it buy the physical layer: accept a

raw bit stream and attempt to deliver it to the destination. This bit stream is not free of errors. It is up to the data link layer to

detect an if necessary to correct errors. The usual approach is for the data link layer to brake the bit stream up in to discrete frame

and compute the check sum of each frame. At the destination the check sum is computed. If the newly computed check sum is

different from the one contained in the frame, the data link error knows that an error has occurred.

-Character count- use a field in the header to specify the number of characters in the frame.

-The data link layer at the destination sees the character count, it knows how many characters follow and hence where the

end of the frame is.

Note: The count can be alterate by transmission error. The character count is rarely used.

17 Sliding window flow controlEfficiency can be greatly improved by allowing multiple frames to be in transit at the same time

EX: A and B connected via full-duplex link

A is allowed to send n frames without waiting for any acknowledgment. Each frame is labeld with a sequence number.

B allocates buffer space for n frames, thus B can accept n frames. B acknowledges a frame by sending an acknowledgment

that includes the sequence number of the next frame expected. This ack also announces that is prepared to receive the next n

frames.

A maintains a list of sequence numbers that is allowed to send, and B maintains a list of sequence number that is prepared to

receive. Each of these list can be considered as window of frames.

The sequence number to be used occupies a filed in the frame and is of bounded size.

18 Stop and wait ARQ technique

Based on stop and wait technique. The service station transmits a single frame and than must wait on ACK. No other data

frames can be send until the destinations reply arrives at the SD (service destination).

2 Roots of errors could occur:

- the frame that arrives at the destination could be damaged, the receiver detects this by using an error detection technique,referred to earlier and simply discards the frame. After a frame is transmitted the source station waits for an ACK. If no ACK

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is received by the time the timer expires, than the same frame is sent again. This method requires that the transmitter

maintains a copy of the transmitted frame until an ACK is received for that frame.

The second sort of error is a damaged ACK

Ex: A sends a frame. The frame is received correctly by B, which respond by an ACK. The ACK is damaged in transit and is not

recognizable by A, which will therefore time out and resend the same frame. To avoid this problem frames are alternately labeled

with 0 and 1 and positive ACK are of the form ACK0 and ACK1.

In keeping with the sliding window connection an ACK0 acknowledges receipt of a frame numbered 1 and indicates that the receiver

is ready frame number 0. The principal advantage of stop and wait ARQ is its suppliant. The principal disadvantage is an inefficient

mechanism.

19 HDLC

The most important data link protocol is HDLC. Not only is HDLC widely used but it is the basis for other important data link

control protocols which use the same or similar formats and the same mechanism as employed in HDLC.

To satisfy a variety of applications HDLC defines 3 types of stations, 2 link configurations and 3 data transfer modes of

operation.

-Station types: -primary station, has the responsibility for controlling the operation of the link. Frames issued by the

primary are called commands.

-second station, operates under the control of the primary station. Frames issued by the second station

are called responses. The primary maintains a separate logical link with each secondary station on the line.

-combined station, combines the features of primary and secondly. A combined station may issue both

commands and responses.

The 2 link configurations are: -unbalanced conf. Consists of one primary and one or more secondly stations, and supports

full duplex and half-duplex transmission.

-balanced configuration; consists of 2 combined stations and supports both full and half-duplex transmissions.

The data transfer modes: -Normal response mode(NRM): used with an unbalanced configuration, the primary may initiate

a data transfer to a secondary, but a secondary may only transmit data in the response to a command from the primary.-Asynchronous balanced mode(ABM), use with a balanced configuration. The secondary may initiate transmission without

explicit permission of the primary. The primary still retains responsibility for the line, including initialization, error recovery and

logical disconnection.

NRM-used as multi drop lines in which a number of terminals are connected to a last computer.

-used on point to point link, particularly of the link connects a terminal or other peripheral to a computer.

ABM-is the most widely used of the 3 modes, it makes more efficient of a full duplex point to point link as there is no poling

overhead.

ARM-is rarely used. It is applicable to some special situations in which a secondary may need to initiate transmission

20 HDLC frame structure

HDLC-uses synchronous transmissions. All transmissions are in the form of frames.

The flag address and control fields that proceed the information field are known as a header.

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The FCS and flag field following the data field are referred to as a trailer.

Flag field: -delimit the frame at both ends, with the unique pattern 01111110.

-single flags may be used as the closing flags for one frame and the opening flag at the next.

-on both side of the user network interface, servers are continuously hunting for the flag sequence to synchronize on the

start of a frame

-while receiving a frame, a station continuous to hunt for that sequence to determine the end of the frame

-to avoid frame- level sync because of possible appearance of pattern 01111110 somewhere inside the frame, a procedureknown as the bit stuffing is used.

-between transmission of the starting and ending flags, the transmitter will always insert an extra 0 bit after each

succession of five 1's in the frame

-the receiver after detecting a starting flag it monitors the last stream, while a pattern of five 1's appears, the six bit is

examined if this is a 0 it is deleted

-if the sixth bit is a 1 and the seventh bit is a 0 the combination is accepted as a flag

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-if the seventh and the sixth are both 1 the sender is indicating an abort condition

With the use of bit stuffing arbitrary bit patterns can be inserted into the data field of the frame, this prop is know as data

transparency.

21. Logical Link Control (LLC)

- LLC is part of IEEE 802 family of standards for controlling operators over a local area network (LAN). LLC is locking some features

found in HDLC and also has some features not found in HDLC

- The most obvious difference between LLC and HDLC is the difference in frame format

- Link control functions in the case of LLC are actually divided between 2 layers:

- a medium access control (MAC) layers and

- the LLC which operates on top of the MAC layer.

-the shaded position corresponds to the fields produced at the LLC layer

-The unshaded positions are the header are the header and trailer of the MAC frame

-The HAC layer includes source and destination addresses for devices attached to the LAN

-2 addresses are needed as there is no concept of primary and secondary in the LAN environment; therefore, both the

sender and receiver must be identified

-there are same control functions peculiar to medium access control that may be included in a HAC control field

-At the LLC Layer, these are 4 fields:

- the destination and source service access point (DSAP and SSAP), identify the logical user of LLC at the source and

destination system)

-the LLC control field has the same format as that of HDLC, limited to 7 bit sequence number

-Operationally, LLC offers 3 forms of services:

-the connection-made service is the same as the ABM of HDLC

-the other 2 services are: unacknowledged connectionless and acknowledged connectionless

22. Frame Relay

Is a data link control facility designed to provided a streamlined capability for use over high-speed switched networks.

It is used in places of X.25, which consists of both a data link control protocol (LAPB) and a network-layer protocol (called

X.25 packet layer)

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The data link control protocol defined for frame relay is LAPF (Link Access Procedures frameMade Bearer Services).

There are actually 2 protocols: a control protocol, which has similar features to HDLC and a core protocol which is a subset

of the control protocol

-There are several key differences between the LAPF control protocol and HDLC.

-link LAPB, LAPF control is restricted to ABM

-LAPF control protoc uses 7-bit sequence numbers; 3-bit sequence are not allowed

-the FCS for LAPF control is always a 16 bit CRC.

-the address field for LAPF control is 2,3, or 4 octets long containing a 10-bit, 16-bit or 23-bit DLCI (data link

connection identifier). The DLCI identifies a logical connection between a source and destination system. In addition the

address field contains some control lists that are useful for flow control purposes.

-The LAPF core consists of same flag, address, information and FCS fields as LAPF control. The difference is that there is nocontrol

field for LAPF core.

-Thus there is no means of doing flow and error control, which results in a more streamlined operation.

Asynchronous Transfer Mode (ATM) Like frame relay, ATM is designed to provide a streamlined data-transfer capability across high-

speed networks, Unlike frame relay, ATM is not based on HDLC.

It is based on a completely new frame format, know as a cell, that provides a minimum processing overhead.

The cell has a fixed length of 53 octets, or 424 bits.

23. PPP The Point-to-Point Protocol

The PPP protocol is defined in RFC 1661 and further elaborated on in several other RFCs(1662,1663). If handles errordetection,

-supports multiple protocols

-allows IP addresses to be negotiated at connection time, permits authentification PPP provides 3 features:

1) A framing method that unambiguously delimitates the end of one frame and the start of the next one. The frame format

also handles error detection.

2)A link control protocol for bringing lines up, testing them, negotiating options and bringing them down again gracefully

when they are no longer needed.

This is called LCP (Link Control Protocol). It supports synchronous and asynchronous circuits and byte-oriented and bit-

oriented encodings.

3. A way to negotiate network-layer options in a way that is independent of the network layer protocol to be used. The

method chosen is to have different NCP (Network Control Protocol) for each network layer supported.

Typical scenario of a home user celling up on Internet service provider to make a home PC -> a temporary Internet host.

Once the parameters have bean agreed upon a service of NCP packets are sent to configure the network layer.

Typically, the PC wants to run a TCP/IP protocol stock, so it need an IP addres.

Then are not enough IP addresses to go around, so normally each Internet Protocol gets a block of them and then

dynamically assigns one to each newly attached PC for the duration of its login session.

If a provider owns n IP addresses, it can have up to n machines logged in simultaneously, but its total customer base may

be many times that. The NCP for IP assigns the IP address. At this point the PC is now an Internet host and can send and receive IP

packets.

When the user is finished, NCP tears down the network layer connection and frees up the IP address.

The LCP shuts down the data link layer connection. Finally the computer tells the modem to long up the phone, releasing

the physical layer conncection.

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24.IEEE 802 Reference Model

Fig.4.1 relates the LAN protocols to the OSI architecture. This architecture was developed by the IEEE 802 comity and has

been adopted by all orghanizations working on the the specifications of LAND standards. It si generally refered as IEEE 802 reference

model

The lowest layer of IEEE 802 ref model corresponds to the physical layer of OSI model and includes such functions as:

- Encoding/decoding signals- Bit transmission / receptionIn addition:

- -a specification of the transmission medium and topology.- Above the physical layer are the functions associated with providing service to LAN users. These include:- -An transmissionassemble data into a frame with address and errordetection fields- -An reception: disassemble frame, perform address recognition and error detection- -Govern access to the LAN transmission medium- -Provide an interface to high layer and perform error control

These functions are grouped into a logical Link control layer (LLC)The functions in the first three bullet intens are treated by a separate layer, called medium access control (MAC).

The separation is done for the following reasons:

-The logic required to manage access to a shared access medium is not formed in traditional layer 2 data link

control.-For some LLC several MAC options may be provided.

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Fig 4.2 illustrates the relationship between the levels of the architecture. User data are passed sown to LLC, which appends

control information as a header, creating an LLC protocol data unit (PDN). This control is used in the operation of the LLC

protocol. The active LLC PDN is then passed down to the MAC layer, which appends control information at the front and

back of the packet, forming a MAC-frame. The control information in the frame is needed for the operation of the MAC

protocol. For context, the fig also shows the use of TCP/IP and an application layer above the LAN protocols

25.MAC technique

MAC=Medium access control

All MANs and LANs consist of collections of devices that must share the networks transmission capacity. Some means of

controlling access to the transmission medium is needed to provide for an ordered and efficient use of that capacity. This is the

function of MAC protocol.

The key parameters in any MAC technique are:

-where and how;

Whererefers to whether control is exercised in a centralized or distributed fashion.

Centralized. A controller is designated that has the authority to grant access to network.

Decentralized. The stations collectivity performs a MAC function to dynamically determine the order in witch stations transmit.The second parameter (HOW ) is constrained by the topology and is a trade-off between among competing factors

including ast, performance and complexity.

In general we can categorize access control techniques as being either:

1. synchronous2. asynchronous

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1. not optimal in LANs and MANs because the needs of stations are unpredictable.2. it is preferable to allocate capacity in an asynchronous(dynamic) fashion more or less in response to immediate demand.

-round robin

-contention

-reservation

26.standardized medium access techniques

Round robin

With round robin each station in turn is given the opportunity to transmit. During that opportunity the station may decline

to transmitter may transmit subject to specified upper band.

When it is finished the station relinquishes its turn and the right to transmit passes to the next station in logical sequence.

Control of sequence may be centralized or distributed.

RR=efficient when many stations have to transmit over an extended period of time.

When only a few stations have data to transmit over an extended period of time, other techniqueslargely depending on

whether the data traffic has a stream or bursty characteristic may be preferable.

-stream traffic is characterized by length and fairly continuous transmission. (Voice transmission etc..)

-bursty traffic is char by: short sporadic transmissions (interactive terminal-host traffic)

Reservation

For stream traffic: well suited

-time on the medium is divided into slots

-a station reserves future slots for an extended or even an indefinite period.

Reservation can be centralized or decentralized.

Contention

For bursty traffic- appropriate.

-all stations contend for time in a way that can be rather rough than

Tumble

-these techniques are distributed by nature

RR and contention are the most common techniques

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27.LLC protocol

1. makes use of the asynchronous balanced mode of HDLC in order to support connection mode LLC service. This is referredto as type 2 operation

2. supports a connectionless service using the numbered information: PDU; this is known as type 1 operation3. supports an ack connectionless service by using 2 new numbered PDUs- type 3 operations4. allows multiplexing by the use of LLC service access points (LSAPs)

TYPE 1 operation: the unnumbered information (UI) PDU is used to transfer user data: no ACK however there is a error detecti on

and discord at the MAC level.

TYPE 2 operation: a data link connection is established between 2 LLCSAPs prior to data exchange.

TYPE 3 operation: each transmitted PDU is acknowledged. A new unnumbered PDU the Acknowledged Connectionless (AC)

information PDU is defined

The buss/tree LANs

Topology: is a multipoint configuration:=>requirement.

1. the need for a medium access control technique2. the other design issue has to do with signal balancing:

-the signal strength of the transmitter must be adjusted to be within

certain limits imposed by: -the receivers requirements

-to maintain adequate SIN ratio

-must not be so strong that it overlaps the

dranitry of the transmitter, as the signal

become distorted

signal balancing is now easy for multipoint line.

It must be performed for all permutations of stations taken 2 a time: nx(n-1) permutations

For a 2 stations network=> 39800 signal strength constrains must be satisfied simultaneously.

A common solution is to divide the medium into smaller segments within which pair wise balancing is possible using amplifiers or

repeaters between segments.

28.Ring repeaters function and states

Repeaters in addition to serving as an achievement on the ring serves attachment point.

Data insertion is achieved by the repeater(as a packet circulates past a repeater, the address field is copied. If the attachedstation recognizes the address the remainder of the packet is copied.

Perform the data insertion and reception function in a manner not unlike that of tops. The removal however is more difficult on a ring. For a bus or tree signals propagate to the endpoints and are absorbed by

terminators , hence shortly after the transmission ceases, the bus or tree is clean of data. However because the ring is a

closed loop, packet will circulate indefinitely unless is removed.

A packet may be removed by:

The address repeater The transmitting repeater after it has made one trip around the loop. This approval is more desirable because:

-it allows automatic acknowledgement

-it allows multicast addressing: one packet is simultaneously to

multiple stations

Data insertion: a variety of strategies can be used for determining how and when packets are inserted onto the ring. These are its

effect, medium access control protocols (MAC protocols)

The repeater can be seen to have 2 main purposes:

1. to contribute to proper functioning of the ring2. to provide an access point for attached stations to send and receive data

Corresponding to these 2 purposes are 2 states:

1. listen state2. the transmit state

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A. Each received hit is retransmitted with a small delay required to allow the separator to perform required functions-scan passing hit stream pertinent patterns(address of attached stations permission

to transmit)

-copy each incoming bit and sent it to the attached station

-modify a bit as it passes by (inserting strategies, bits may be modified, for example to indicate that the packet ha been

copied; this would serve as an DCK)

B. in this state, the separator receives bits from the station and retransmits them on its outgoing links. During the period of

transmission, bits may appear on the incoming ring link. This are 2 possibilities and they are treated differently:

-the hosts would be from the packet that the repeaters still in the process of sending (if bit length of the ring is shorter that the

packet. In that case the repeater passes the bits back to the station, which can check as a form of DCK.)

-for some strategies, more than one package would be on the ring at the same tine. If the repeater, while transmitting receives

bits from a packet it did not originate it must buffer them to be transmitted later.

The 2 states: listen and transmit are sufficient for proper ring operation.

A third state: the bypass state is also useful. In this state a bypass relay can be activated so that signals propagate pass the

repeater with no delay other than from medium propagation.

2 benefits: -provides partial solution to the reliability problem

-improves performance by eliminating repeater delay for those stations

that are not achieved on the network

30. IEEE 802.5 Token ring operation

Token ring is the most commonly used protocol for ring-topology LANs. In this section we examine 2 standards

LANs that use taken ring: IEEE 802.5 and FDDT.

IEEE 802.5 MAC

MAC Protocol

The token ring technique is based on the use of a small frame, called a token , that circulates when all stations areidle.

A station wishing to transmit must wait until it detects a token passing by. It then seizes the token by changing one

bit in the token, which transforms it from a token into a start-of-frame sequence for a data-frame. The station then

appends and transmits the reminder of the fields need to an at a frame.

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When a station seize a token and begins to transmit a data frame, there is no token on the ring, so other stations

wishing to transmit must wait. The frame on ring will make a round trip and be aborted by the transmitting station. The

transmitting station will insert a new token on that ring when both the following conditions have been met:

- the station has completed transmission of its frame.- The heading edge of the transmitted frame has returned to the stationOBS: if the bit length of the ring is less than the frame length, the first condition implies the second; if not, a

station could release a free token after it has finished transmitting but before it begins to receive its own transmission. It

causes that only one data frame at a time may be transmitting, thereby simplifying error-recovery procedures.

Once the new token has been inserted to the ring, the next station downstream with data to send will be able toseize the token and transmit.

A sends a packet to C, which receives it and then sends its own packets to A and D.

OBS: - under highly loaded additions there is some inefficiency with token ring because a station must wait for the

token to come around before transmitting.

- under heavy loads, which is when it matters, the ring functions in a round-robin fashion, which is bothefficient and fair.

31.Internetworking architecture approaches.

In describing the internetworking function,2 dimensions are important:

-the mode of operation(connection-mode,connectionless)

-the protocol architecture

The mode of operation determines the protocol architecture

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32. Arhitecure for connectionless internet protocol.

Where connection-mode operation corresponds to the virtual circuit mechanism of a packet switching network connectionless-

mode operation corresponds to the datagram network

All DTE`s and all routers share a common network layer protocol known as an IP.

Below this IP,a protocol is needed to access a portion of the subnetwork.

Thus,there are 2 protocols operating in each DTE and router at the network layer.

-an upper sublayer that provides the internetworking function.

-a lower sublayer that provides subnet access.

IP was developed as a part of DARPA internet projects

Somewhat later,when the international standards community recognized the need for a connectionless approach to

internetworking,the ISO connectionless network protocol was standardized.

IP provides a connectionless or datagram service,between 2 end systems.There are a number of advantages to this approach.

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-flexibility;it can deal with a variety of networks

-can be made highly robust.

-is just for connection less transport protocols

33.INTERNET PROTOCOL OPERATION

The figure depicts the operation of the internet protocol for data exchange between Host A on one LAN And Host B on another

department LAN through the WAN.The figure shows the format of data unit at each shape.

The end systems must all share a common internet protocol. In addition the end systems must share a common internet

protocol ,must share the same protocols above IP.

The IP at A receives blocks of data to be sent to B from the higher layers of software in A.

The IP attaches a header specifying the global internet address of B.

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That address is logically in 2 parts:

-network identifier

-end system identifier

The result is called an internet-protocol data unit or simply a datagram.The datagram is then encapsulated with that protocol

and sent to the transmitter which ships off the LAN fields to read the IP header

The router then encapsulates the datagram with the X25 protocol fields and transmits is across the WAN to another router.

This router ships off the x25 fields and recovers the datagram which is then wraped in lan fields appropriate to LAN2 and sends

it to B

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34.IP SERVICES

IP provides 2 service primitives at the interface to the next-higher layer.

The SEND primitive is used to request transmission of a data unit .The DELIVER primitive is used by IP to notify the user of the

arrival of a data unit.

The parameters associated with the 2 primitives are:

-Source address:Internetwork address of sending IP entity

-Destination address: address of destination IP entity

-Protocol:Recipient protocol entity

-Type of service indicators:Used to specify the treatment of the data unit in its transmission throught component networks.

-Identifier: used in combination with the source and destination addresses and user protocol to identify the data unit

uniquely.It is needed for reassembly and error reporting

-Don`t fragment identifier:indicates wether an IP can segment data to accomplish delivery.

-Time-to-live:measured in network hops.

-Data length:length of data to be transmitted

-Option Data:options requested by IP user

-Data : user data to be transmitted

Identifier,don`t fragment and time to live are present in SEND primitive list but not in the deliver primitive.

These 3 parameters provide instructuins to IP protocos that are not of concern to the recipient IP user

The sending IP user includes the type-of service parameter to request a particular quality of service.

The user may specify one or more of the services listed in tabe 6-1.

The options parameter, allows for future extensibility and for conclusions that are usually not invoked,.

The currently defined options are:-Security:allows a security label to be attached to a datagram.

-Source routing:a sequence list of addresses that specify the route to be followed.

Routing is strict or loose

-Route recording: A field is allocated to record the sequence of routers limited by the datagram.

-Stream identification:names reserved resources used for stream service.This service provides special handling for volatile

periodic traffic

-Time stamping:The source IP entity and some are all intermediate routers add a timestamp(precision to milliseconds)to the

data unit as it goes by.

35.ICMP

Internet control message protocolThe IP standard specifies that a compliant implementation must also implement ICMP

ICMP provides a means of transferring messages from routers and other hosts to a host .ICMP provides a mean for transferring

messages from routers and other hosts to a host..ICMP provides free feedback problems in the communication environment.

Examples of its use:

-when a datagram cant reach its destionation

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-when the router does not have buffering capability to forward a datagram

-when the router can direct the station to send traffic on a shorter route

In most cases an ICMP message is sent in responde to a datagram,either by a router along the datagrams path,or by the

intended destination host

Although ICMP is a user of IP an ICMP message is constructed and then passed down to IP,which encapsulates the message with

an IP header and then transmits the resulting datagram in the usual fashion.Because ICMP messages are transmitted in IP

datagrams,their delivery is not guaranteed and their use cannot be considered reliable.

All ICMP messages start with a 64-bit header consisting of the following:

-Type:specifies the type of ICMP message

-Code: Used to specify parameters of the message that can be encoded-Checksum:for the entire ICMP message

-Parameters: used to specify more lengthy parameters.

ICMP includes:

-Destionation unreachable

-Time exceeded

-Parameter Problem

-Redirect

-Echo

-Echo reply

-Time stamp

-Time stamp reply

-|Adress mask request

-Adress mask reply

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36. BORDER GATEWAY PROTOCOL (ERP)

The BGP was developed for use in conjunction with internets that empty TCP/IP protocol suite, although the concepts are applicable

to any internet. BGP has become the standardized exterior router protocol for the internet.

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Functions: BGP was designed to allow routers, called gateways in the standard, in different autonomous systems (ASs) to cooperate

in the exchange of routing in f.

The protocol operates in terms of messages, which are sent over TCP connections. The repertoire of messages is summarized in

table 6.2.

Table 6.2 BGP 4 messages

Open Use to open a neighbor relationship with another router.

Update Used to (1) transmit info about a single rout and/or (2) to list multiple routers to be withdrawn

Keepalive Used to (1) acknowledge an Open message and (2) periodically confirm the neighbor relationship

Notification Send when an error condition is detected

Free functional procedures are involved in BGPneighbor acquisition

- Neighbor recheability- Network recheability

Two routers are considered to be neighbors if they are attached to the same subnetwork. If the writers are in different autonomus

systems, they may wish to exchange writing info. For this purpose, it is necessary to first perform neighbor acquisition. In the

neighbor acquisition process, one router sends a request message to another, which may either accept or refuse the offer. To

perform neighbor acquisition, one router sends an open message to another. If the target router accepts the request, it returns

keepalive message in response. Next, the neighbor reachability procedure is used to maintain the relationship. The two routers

periodically issue keepalive messages to each-other. The find procedure specified by BGPs is network reachability.

-each router maintains a database of the subnetworks that it can reach and the proffered route for reading that

subnetwork.

-whenever a change is made to this database, the router issues an update message that is broadcast to all other

routers implementing BGP. By the broadcasting of this update message, all of the BGP routers can build up and maintain

writing info.

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37. BGP

MESSAGES

illustrates the formats of all of the BGP messages. Each message begins with a 19-octet header containing 3 fields, as indicated by

the shaded position of each message in the figure: -Marker. Reserved for authentication

-Length. Length of message in octets

-Type. Type message: Open, Update, Notification, Keepalive

To acquire a neighbor/a rout: Opens TCP connection to the neighbor of internet. -> Sends an open message: -identifies the AS

which the sender belongs;

-provides IP address of the router. The keepalive message consists simply of the header. Each router

issues these messages to each of its peers often enough to prevent the Hold Time from expiring.

38. BGP ROUTING INFORMATION EXCHANGE

The essence of BGP is the exchange of writing info among participating routs in multiple ASs. This process can be quite complex.

A router that implements BGP will also implement an internal routing protocol such as OSPF. Using OPFR1 can exchange routing infowith other routers within AS1 and build up a picture of the topology of the subnetworks and routers in AS1 and construct a routing

table. Next, R1 can issue an Update message to R5 in AS2. This could include: -AS_PATH: the identity of AS1

-NEXT_HOP: the IP address of R1

-NLR1: a list of all subnetworks in AS1

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Suppose that R5 also has a neighbor relationship with another router in another AS, say R9 in AS3. R5 will forward the information

just received from R1 to R9 in a new Update message. This message includes the following: -AS_PATH: the list of identifiers

{AS2, AS1} -NEXT_HOP: the IP address of R5

-NLRT: a list of all subnetworks in AS1

This message informs R9 that all of subnetworks listed in NLR1 are reachable via R5 and that the autonomus systems traversed are

AS2 and AS1.

R9 must now decide if this is its preferred rout to the subnetwork listed. If R9 decodes that the rout provided in R5s update

message is preferable, than R9 incorporates the routing info into its routing database and forwards this new routing info to other

neighbors. This new message will include an AS_PATH field of {AS1, AS2, AS3}. Routing update info is propagated through the

larger internet.Open Shortest Path First (OSPF) protocolinterior routings protocol.

OSPS uses what is known as a link state routing algorithm. Each router maintains descriptions of the state of its local links to

subnetworks, and from time to time transmits update state information to all of the routers of which it is aware. Every router

receiving an update packet must ack. it to the sender.

The OSPF protocol (RFC 2328) is now wildly used as the interior router protocol in TCP/IP networks.

OSPF computes a rout through the internet that incurs the least cost based on a user configurable metric of cost.

Each router maintains a database that reflects the known topology of the autonomus system of which it is apart. The topology is

expressed as a direct graph. The graph assists of:-vertices, or nodes of 2 types:

1. router 2.network, which is of 2 types in turn:

a. transit b. stub, if it is not a transit network

-edges of 2 types

1.graph edgesthat connect 2 router vertices when the corresponding routers are connected to each other by a

direct point to point link

2.graph edgesthat connect a router vertex to a network vertex when the router is directly connected to the

network

39. IPv6- types of addresses

Prefix, which identifies various categories of addresses.

IPv6 allows 3 types of addresses:

(1) Unicast. An identifier for a simple interfaces . A packet sent top a unicast address is delivered to the interface identified bythat address.

(2) Anycast. An identifier for a set of interfaces (typically belonging to different nodes). A packet sent to an anycast address isdelivered to one of the interfaces identified by that address (the nearest one, according to the writing protocols measure

of distance)

(3) Multicast. An identifier for a set of interfaces (typically belonging to different nodes). A packet sent to a multicast address isdelivered to all interfaces identified by that address.

Unicast Addresses (represented in figure 6.17(a))

- ProviderBased Global; - Link-local;- Site-local;- Embedded IPv4;-Loop back

A provider based global unicast address provides for a global addressing across the entire universe of connected hosts.

The address has five fields after the Format Prefix.(fig 6(a))

- Registry IDidentifies the registration authority which ensures the provider position of the address

- Provider IDa specific internet service provider, which ensures the subscriber position of the address

- Subscriber IDdistinguished among multiple subscribers attached to the provider of the address

- Subnet IDtopologically connected group of nodes within the subscriber network

- Node IDidentifies a single node interface among the group of interfaces identified by the subnet prefix.

Anycast addresses

Enables a source to specify that it wants to contact any one node from a group of nodes via single address. A possible use of an

anycast address might be found within a routing header to specify an intermediate address along a route.Anycast addresses are allocated from the same address space as unicast addresses. Thus, members of an anycast group must be

configured. To recognize that addresses and routers must be configured to be able to map an anycast address to a group of unicast

interfaces addreses.

Multicast addresses:

IPv6 includes the capability of addressing a predefined group of interfaces with a single multicast address. A packet with a multicast

address is to be delivered to all members of the group.

Multicasting is a useful capability in a number of contexts:

For example: it allows lists and routers to send neighbor discovery messages only to those machines that have registered to receive

them.

40-Error messages :

ICMPV6 includes 4 error messages : -destination unreadable-Packet too big-time exceeded-parameter problem .Each of these messages refers to a prior ipv 6 packet and then sent to the originating source .the message body includes as much of

the original packet as possible , up to a limit on the size of the ip v6 packet carrying this message of 576 octets

(1)(2)messages provides a mechanism for listing that communication is possible between entities . the recipient of an echo request

message is obliged to return the message body in an echo reply message . an identifier and sequence number are associated with

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the echo request message to be matched in the echo reply message . the identifier might be used like a service acces point to

identify a particular session and the sequence number might be incremented on each echo request sent.

ICMPv6 : A new version , known as ICMPv6 has been specified (RFC/885) to work with ip v6 .the key features of ICMPv6 are :

-uses a new protocol number.

-both protocols use the same header format

-Some little-used ICMP messages have been omitted form ICMPv6

-the maximum size of ICMPv6 is longer ( 576 octets including ipv6 headers ) number so as to exploit the increased size of packets

that ipv6 guarantees .it will be transmitted without fragmentation .

The group management messages implements the procedures of the INTERNET GROUP MANAGEMENT PROTOCOL (IGMP)).IGMP is

an extension of ICMP ( that provides a mechanism for deciding wether a router shouldnt forward multicast IPv4 datagram .in ICMPv6 , there are actually 3 different messages with different type values :

-group membership query

-group membership report

-group membership termination

a host may join a multicast group by sending a group membership report on a subnet with multicast adress in the body of the

message .Routers on the subnet receive the report and store the info that at least 1 node on that sunbnet is a member of the group

. a host may terminate its membership by sending a group membership termination message .At regular intervals , routers send out

group membership query messages ,each host which still wishes to be a member of the group or groups replies for each appropriate

group with w group membership report .

29. CSMA/CD OPERATION

This leads to the following rules for CSMA/CD:

1. If the medium is idle transmit; otherwise go to step 2.2. If the medium is busy, continue to listen until the channel is idle, then transmit immediately.3. If a collision is detected during transmission, transmit a brief jamming signal, to ensure that all stations know that there has

been a collision and then cease the transmission.

4. After transmitting the jamming signal, wait a random amount of time, then attempt to transmit again.FIG 4-14 CSMD/CD OPERATION POZA POZA POZA

- A1 t0, station A begins transmitting a packet addressed to D.- A1 t1, both B and C are ready to transmit. B senses a transmission, and so defers. C , however is still unaware of As

transmission and begins its own transmission. When As transmission reaches C, at t2, C detects the collision back to A,

where it is detected sometime later, t3 at which time A ceases transmission.