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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