OSI Model

Physical Layer 1

Where transmission of bits across the network occurs and includes physical and electrical network characteristics

• Cables are also part of the physical layer

• Hubs, Access points, Media converters, Bluetooth, Wi-FI, (NFC)

• are essentially repeaters, passing along whatever is received

Transition Modulation

If it changes during the clock cycle, then a 1 is represented (otherwise, a 0 is represented)

Asynchronous

Uses start and stop bits to indicate when transmissions occur from the sender to the receiver

Synchronous

Uses a reference clock to coordinate the transmissions by both sender and receiver

Broadband

Divides bandwidth into separate channels

Baseband

Uses all available frequencies on a medium (cable) to transmit data

• uses a reference clock

Time-Division Multiplexing (TDM)

Each session takes a turn using time slots, to share the medium between all users

Statistical Time-Division Multiplexing (StatTDM

Dynamically allocates the time slots on an as-needed basis

Frequency-Division Multiplexing (FDM)

Divides the medium into channels based on frequencies and each session is transmitted over a different channel

Data Link layer 2

Packages data into frames and transmits those frames on the network

• Network Interface Cards

• Bridges

• Switches

Media Access Control (MAC)

Physical addressing system of device which operates ob logical topology

• unique 48-bit physical addressing system is assigned to every network interface card (NIC) produced

Vender code

is first part of MAC address [D2:51:F1]:3A:34:65

Unique value

is last part of MAC address D2:51:51[3A:34:65]

Logical Link Control (LLC)

Provides connection services and allows acknowledgement of receipt of messages

• Logical Link Control provides basic error control funtions

Isochronous

Network devices use a common reference clock source and create time slots for transmission

Synchronous

Network devices agree on clocking method to indicate beginning and end of frames and can use control character

Asynchronous

Network devices reference internal clocks and us start and stop bits for synchronization

Packet Switching

Data is divided into packets and then forwarded

• Most networks use packet switching

Circuit Switching

Dedicated communication link is established between two devices

• a dedicated and constant communication path is maintained for the duration of a conversation

Message Switching

Data is Divided into messages which may be stored and then forwarded

Route Discovery and Selection (Layer 3)

Manually configured as a static route or dynamically through a routing protocol

• RIP

• OSPF

• EIGRP

Connection Services

Augment Layer 2 connection services to improve reliability

• Flow control

• Packet reordering

Internet Control Message Protocol

Sends error message and operational information to an IP destination

Transmission Control Protocol (TCP) (Layer 4 Protocol)

A connection-oriented protocol that is a reliable way to transport segments across the network

• Three-way handshake

• Reliable

• Connection-oriented

• Segment retransmission and flow control

• through windowing

• Segment sequencing

• Acknowledges segments

Three way handshake (TCP) (Layer 4 Protocol)

• Step 1 (SYN): In the first step, the client wants to establish a connection with a server, so it sends a segment with SYN(Synchronize Sequence Number) which informs the server that the client is likely to start communication and with what sequence number it starts segments with

• Step 2 (SYN + ACK): Server responds to the client request with SYN-ACK signal bits set. Acknowledgement(ACK) signifies the response of the segment it received and SYN signifies with what sequence number it is likely to start the segments with

• Step 3 (ACK): In the final part client acknowledges the response of the server and they both establish a reliable connection with which they will start the actual data transfer.

User Data-gram Protocol (UDP) (Layer 4 Protocol)

Connection-less protocol is an unreliable way to transport segments across the network. If dropped, the sender is unaware. Use when audio or video streaming does not require obtaining every single piece of the file

• Datagram

• Unreliable

• Connectionless

• No windowing or retransmission

• No Sequencing

• No acknowledgment

Windowing

Allows the clients to adjust the amount of data in each segment, Sends information while minimizing retransmissions and maximizing throughput

Buffering

Occurs when devices allocate memory to store segments if bandwidth is not readily available

Session Layer 5

Keeps conversations separate to prevent intermingling of data

• Set up

• Maintain

• Tear down

• Transfer data

• Reestablish connection

• Acknowledge receipt

Example

• H.323

• NetBiois

Set up Session (Layer 5 Protocol)

Checking of user credentials and assigning numbers to sessions to help identify them

Maintain Session (Layer 5 Protocol)

Were data Transfers back and forth across the network

Tear Down Session (Layer 5 Protocol)

Ending of session after the transfer is done or when the other party disconnects. Tearing down a session can be done mutually or after the communication has finished

H.323 (Layer 5 Protocol)

Used to set up, maintain, and tear down vice and video connections

Real-time Transport Protocol (RTP) (Layer 5 Protocol)

Streaming audio/video (phone call or Facetime session}

NetBIOS (Layer 5 Protocol)

Used to share file over a network

Presentation Layer 6

Formats the data to be exchanged and secures that data with proper encryption

Ensure data is readable by the receiving system

Provides proper data Structures

Negotiates data transfer syntax for the Application Layer 7

• Data Formatting or Encryption

• American Standard Code for Information Interchange (ASCll)

• GIF

• JPG

• PNG

• Movies files

• scripting languages

• Standard text

• Picture

Data Formatting

Data is formatted by the computer to have compatibility between different devices

Encryption (Layer 6 Protocol)

Used to scramble the data in transit to keep it secure from prying eyes and provide data confidentiality

Application Layer

Provides application-level services where users communicate with the computer

Application Services

Unites communicating components from more that one network application

• POP3

• IMAP

• SMTP

Service Advertisement

Sending out ofannouncements to other devices on the network to state the services they offer

Encapsulation

The process of putting headers (and trailers around some data

Protocol Data Unit (PDU)

A single unit of information transmitted in a computer network

SYN (Synchronization)

Used to synchronize connection during the three-way handshake

ACK(Acknowledgment)

Used during the three-way handshake, but also used to acknowledge the successful receipt packets

FIN(Finished)

Used to tear down the virtual connections created using the three-way handshake and the SYN flag

RST (Reset)

Used when a client or server receives a packet that it was not expecting during the current connection

PSH (Push)

Used to ensure data is given priority and is processed at the sending or receiving ends

URG(Urgent)

Similar to PSH and identifies incoming data as urgent

MAC Address

Physical address that is used to identify a network card on a local area network

Ether Type

Used to indicate which protocol is encapsulated in the payload of a frame

Default frame

1500 MTU

Jombo Frames

9000 MTU