CSC414ComputerSystemFundamentals

THINK BIG WE DO

U R Ihttp://www.forensics.cs.uri.edu

Digital Forensics CenterDepartment of Computer Science and Statics

Data Communications

and theOSI Model

Data Communications

and theOSI Model

Data CommunicationData

- Messages to be shared between sender and receiver

Packets

- Envelopes used to address messages

Protocols

- Establish accurate and appropriate meaning to the messages

- Understood by both senders and receivers

- Standard interaction between entities

From

Greeting

Body

Closing

Signature

Recipient

Street

CityState Code Zip Code

Stamp

Return Address

Data CommunicationCommunications channel

- Logical Connection

- Used to capably and reliably transport messages

Physical connection

- Independent of the messaging- Includes signaling that represents the

messages being transported

Communication StacksProtocols for Communication

Physical Layer

Data Link Layer

Transport Layer

Application Layer

Network Layer

100BASE-TX

MAC Address

TCP; UDP

HTTP;SMTP;SSH;FTP

IP; DHCP

Physical Layer

Data Link Layer

Transport Layer

Application Layer

Network Layer

Presentation Layer

Session Layer

MIME Encoding; SSL

Sockets

OSI Model TCP/IP Model ExamplesTransmission Control Protocol / Internet Protocol

OSI Model Application Layer

- Layer where message is created

- Includes any application that provides software that can communicate with the network layer

- Protocols are specific to purpose of application- Web Browsing - HTTP

- File Transfer - FTP

- Mail - SMTP

- Requested data and files use URLs- Universal Resource Locators

- URL includes protocol information

Application Layer

OSI Model

http://www.uri.edu:80/courses/cs/index.html

protocol computeraddress

directory file

port

OSI ModelPresentation Layer

- Presents data to application in a form it can use

- Makes application independent of data representation- Translate ASCII characters to Unicode Characters

- Encryption and decryption of messages

- Example protocols- SSL - Secure Sockets Layer

- MIME - Multi-purpose Internet Mail Extensions

- Used to encode mail attachments

Application Layer

Presentation Layer

OSI Model

OSI ModelSession Layer

- Establishes a dialogue between two cooperating applications or processes at the ends of the communication link

- Responsible for- Establishing the session between the applications

- Controlling the dialogue

- Terminating the session

- Example- Sockets or ports

- Used as an end point or connection point on the computer

Application Layer

Presentation Layer

Session Layer

OSI Model

Protocol Port UseFTP 20 file transferSSH 22 secure loginSMTP 25 mail

finger 79 Info about systemHTTP 80 web browsingPOP3 110 mailMySQL 3030 database requests

http://www.uri.edu:8080/courses/cs/index.html

Alternative Port

OSI ModelTransport Layer

- Provides services that support reliable end-to-end communications

- Generates the final address of the destination

- Responsible for all end-to-end communication facilities

- Packetization of a message, - breaking up of the message into packets of reasonable size

- Example protocols- TCP - Transmission Control Protocol

- Socket to socket communications

- UDP - User Datagram Protocol (streaming video)

Transport Layer

Application Layer

Presentation Layer

Session Layer

OSI Model

OSI ModelNetwork Layer

- Addressing and routing of packets to proper destination

- Unreliable, connectionless, packet switching service

- Does not guarantee delivery nor check for errors

- Communications within a local network:

- No routing is required because nodes are directly addressable

- Physical addresses for corresponding IP addresses are looked up in a table

- IP appends a header with the physical address and passes the datagram to the data link layer (layer 2)

- Communications sent outside of the local network

- At each intermediate node, the network layer removes the current node address and determines the next node address

- New address is added to the packet and passed to the data link layer (layer 2)

Transport Layer

Application Layer

Network Layer

Presentation Layer

Session Layer

OSI Model

Routers and gateways function at this layer

OSI ModelData Link Layer

- Reliable transmission and delivery of packets between two adjacent nodes- Packets at this layer are called frames

- Software logical link control sublayer- Error correction, flow control, retransmission, packet

reconstruction and IP datagram/frame conversions

- Numbers frames and reorders received frames to recreate the original message

- Hardware medium-access control sublayer- Defines procedures for access the channel and detecting errors

- Responsible for services such as data encoding, collision handling, synchronization, and multiplexing

Data Link Layer

Transport Layer

Application Layer

Network Layer

Presentation Layer

Session Layer

OSI Model

Medium Access Control (MAC) Address used at this layer

OSI ModelPhysical Layer

- Layer at which communication actually takes place consisting of a bare stream of bits

- Primarily implemented in hardware by a network interface controller (NIC)

- Physical access protocol includes- Definition of the medium

- Signaling method, signal parameters, carrier frequencies, lengths of pulses, synchronization and timing issues

- Method used to physically connect the computer to the medium

Physical Layer

Data Link Layer

Transport Layer

Application Layer

Network Layer

Presentation Layer

Session Layer

OSI Model

Data CommunicationMessage Transmission

Physical Layer

Data Link Layer

Transport Layer

Application Layer

Network Layer

Presentation Layer

Session Layer

OSI Model

My Message

My Message

My Message

My Message

My Message

My Message

My Message

My Message

Sender's Computer

Intermediate NodeRouter

Data CommunicationMessage Transmission

My Message

My Message

Physical Layer

Data Link Layer

Transport Layer

Application Layer

Network Layer

Presentation Layer

Session Layer

OSI Model

My Message

My MessageMy MessageMy MessageMy Message

Sender's Computer

Data CommunicationMessage Transmission

Physical Layer

Data Link Layer

Transport Layer

Application Layer

Network Layer

Presentation Layer

Session Layer

OSI Model

My Message

Sender's Computer

Receiver's Computer

Data CommunicationMessage Transmission

Physical Layer

Data Link Layer

Transport Layer

Application Layer

Network Layer

Presentation Layer

Session Layer

OSI Model

Sender's Computer

Receiver's Computer

"The Network" or "Internet"

THINK BIG WE DO

U R Ihttp://www.forensics.cs.uri.edu

Digital Forensics CenterDepartment of Computer Science and Statics

Data Communicationsand the

OSI Model

Data Communicationsand the

OSI Model