cc notes lecture 02

7/29/2019 CC Notes Lecture 02

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Computer Communication &

Networks

Lecture # 02

Course Instructor:

Engr. Sana Ziafat


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Layering & Protocol Stacks


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Reference Models

OSI reference model

TCP/IP


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OSI Reference model

Open System Interconnection

7 layers

1. Crate a layer when different abstraction is needed

2. Each layer performs a well define function

3. Functions of the layers chosen taking internationally

standardized protocols

4. Number of layers large enough to avoid

complexity


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Seven layers of the OSI model


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Exchange using OSI Model


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Peer-to-peer Processes Layer x on one machine communicates with layer x on

another machine - called Peer-to-Peer Processes. Interfaces between Layers

Each interface defines what information and services alayer must provide for the layer above it.

Well defined interfaces and layer functions providemodularity to a network

Organizations of the layers

Network support layers : Layers 1, 2, 3

User support layer : Layer 5, 6, 7

It allows interoperability among unrelated softwaresystems

Transport layer (Layer 4) : links the two subgroups


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The interaction between layers in the OSI model


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OSI Layers


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Physical layer

Transporting bits from one end node to the next- type of the transmission media (twisted-pair, coax, optical fiber, air)

- bit representation (voltage levels of logical values)- data rate (speed)

- synchronization of bits (time synchronization)

- deals with the optical, mechanical and electrical features

- transmission modes ( half duplex, full duplex)

physical

connection


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The physical layer is responsible for movements ofindividual bits from one hop (node) to the next.

Note


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Protocols:

- Modems

- Optical Cables, Connectors

Network Devices:

- Hubs, Repeaters, and Amplifier.


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Data Link layer

Transporting frames from one end node to the next one

logical

connection

-framing - physical addressing

- flow control - error control

- access control


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Data Link layer- hop-to-hop delivery-


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Data Link layer- example-


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The data link layer is responsible for movingframes from one hop (node) to the next.

Note


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Network Devices:

-Bridge, Switch, ISDN Router, Intelligent

Hub, NIC, Advanced Cable Tester


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Protocols: Logical Link Control error correction and flow control manages link control and defines SAPs

Media Access Control controls the type of media being used:802.3 CSMA/CD (Ethernet)

802.4 Token Bus (ARCnet)802.5 Token Ring802.12 Demand Priority


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Network Layer The network layer is responsible for

the delivery of individual packets fromthe source host to the destination host.


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Network layer

End-to-End packet delivery From the original source to a destination

Needed when 2 devices are attached to

different networks What is the network definition here?

Main duties:1. Logical addressing

2. Routing

3. Switching

4. Congestion control and QoS

Not a message


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Source to destination delivery

Data Link

Network

layer


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Network layer- example -

Network layer addresses

Data Link layer addresses


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The network layer is responsible for thedelivery of individual packets from

the source host to the destination host.

Note


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Router works as the post office and network layer

stamps the letters (data) for the specific

destinations.

Protocols: These protocols work on the network

layer IP, ICMP, ARP, RIP, OSI, IPX and OSPF.

Network Devices: Network devices including Router,Frame Relay device and ATM switch devices work

on the network layer.


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Transport layer

Process-to-Process delivery of the entire message From the original source to a destination

Needed when several processes (running programs)

active at the same time

Main tasks: Port addressing

Segmentation and reassembly Congestion control

Flow control

Error control


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Transport LayerThe transport layer is responsible for the delivery

of a message from one process to another.


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Transport Layer


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Transport layer-an example of a reliable delivery -


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The transport layer is responsible for the deliveryof a message from one process to another.

Note


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Protocols: These protocols work on the

transport layerTCP, UDP, SPX, NETBIOS,

ATP and NWLINK.

Network Devices: The Brouter, Gateway andCable tester work on the transport layer.


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The session layer is responsible for dialogcontrol and synchronization.

Note


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Session Layer The session layer is responsible for

dialog control and synchronization.


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Presentation Layer


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Presentation layer is a best layer for

cryptography.


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The presentation layer is responsible for translation,compression, and encryption.

Note


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Application Layer The application layer is responsible

for providing services to the user.


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Services provided by Application layer:

- File transfer, Access

- Mail services


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Application layer

Enables user to access the network Provides services to a user

E-mail

Remote file access and transfer (Telnet, FTP) Access to WWW (HTTP)


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The application layer is responsible forproviding services to the user.

Note


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Protocols: FTP, DNS, SNMP, SMTP,

FINGER, TELNET, TFTP, BOOTP and SMB

protocol are operated on the application

layer.

Network Devices: Gateway network device is

operated on the application layer.


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OSI Reference Model

A convenient aid for remembering the OSIlayer names is to use the first letter ofeach word in the phrase:

All People Seem To Need Data Processing


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Summary of layers and protocols

Low-level protocols define the electrical and physical standards

to be observed, bit- and byte-ordering and the transmission and

error detection and correction of the bit stream

High-level protocols deal with the data formatting, including the

syntax of messages, the terminal to computer dialogue,

character sets, sequencing of messages


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TCP/IP Protocol


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TCP/IP Vs OSI Model


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Four Level of Addresses


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Relationship of Layers & Addresses in TCP/IP


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The physical addresses will change from hop to hop,but the logical addresses usually remain the same.

Note

E l 2 1


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In Figure 2.19 a node with physical address 10 sends aframe to a node with physical address 87. The two nodes

are connected by a link (bus topology LAN). As the

figure shows, the computer with physical address 10 is

the sender, and the computer with physical address 87is

the receiver.

Example 2.1


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Figure 2.19 Physical addresses

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As we will see in later lectures, most local-area networksuse a 48-bit (6-byte) physical address written as 12

hexadecimal digits; every byte (2 hexadecimal digits) is

separated by a colon, as shown below:

Example 2.2

07:01:02:01:2C:4B

A 6-byte (12 hexadecimal digits) physical address.

E l 2 3


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Figure 2.20 shows a part of an internet with two routersconnecting three LANs. Each device (computer or

router) has a pair of addresses (logical and physical) for

each connection. In this case, each computer is

connected to only one link and therefore has only one

pair of addresses. Each router, however, is connected to

three networks (only two are shown in the figure). So

each router has three pairs of addresses, one for eachconnection.

Example 2.3


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Figure 2.20 IP addresses

Example 2 4


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Figure 2.21 shows two computers communicating via theInternet. The sending computer is running three

processes at this time with port addresses a, b, and c. The

receiving computer is running two processes at this time

with port addresses j and k. Process a in the sendingcomputer needs to communicate with process j in the

receiving computer. Note that although physical

addresses change from hop to hop, logical and port

addresses remain the same from the source to

destination.

Example 2.4


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Figure 2.21 Port addresses

Example 2 5


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Example 2.5

As we will see in later chapters, a port address is a 16-bitaddress represented by one decimal number as shown.

753

A 16-bit port address represented

as one single number.


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Readings

Chapter 2 (B. A Forouzan)

Section 2.2,2.3, 2.4, 2.5

A


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Q & A

cc notes lecture 02

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