• Deals with wire, cables, fiberoptics, radio waves, etc.

• Deals with network connectors and network topologies.
• This layer has to do with all that's physical (that you can see) and any issue with this layer means you have hardware problems in your network.
• All data is transferred across the network as a binary code (series of 0 and 1), i.e. bits.

• A wave of voltages sent across a network (radio wave, etc.).
• Every time the voltage is positive it's equal to 1, while it's equal to 0 every time it gets negative.
• Analog signal is subject to interference and loss of data.

2 SUBLAYERS:

• MAC (MEDIA ACCESS CONTROL)
• LLC (LOGICAL LINK CONTROL)

MAC (Media Access Control)

• Defines how packets are placed on the media (first come, first served, so everyone shares the same bandwidth).
• Defines physical addressing (the pshysical location of a device in a network defining what the device is; physical topologies are: bus, ring, star, mesh, hybrid, etc.)
• Defines logical topologies (the signal path through a physical topology: LAN,WAN, etc.)

• Tells the Data Link Layer what to do with a packet once a frame is received and where it's destined. 
• Flow control and sequencing of control bits

• Controls the passage of packets along the routes in the network.
• Reads protocol and address information and helps trasmit the data.
• Gathers information from different networks and different part of your network and gathers them (discovery) to find the quickest path 

• Makes sure that data is sent and received in the same order.
• Identifies each route in a network with a unique identification value (port)
• Determines the quickest path and assigns it a port value
• Error checking to make sure data arrives safely and without problems

Each message is singular and there's no agreement between sender and receiver about the amount of information that will be sent in both directions (UDP,SPX). It's like a postcard (we send it but we don't know when and whether it'll get to destination).

• Ensures data integrity preventing the sender from trasmitting more data than the receiver can handle (connection-oriented communication).
• Any received segment is acknowledged to the sender
• Any not acknowledged segments are retransmitted
• Segments are rearranged back (sequencing) in their proper order after arrival
• When too much data is received too fast, the receiver stores the exceeding data in a memory (buffer) only for short data bursts. If data keeps flooding the receiving machine, its buffer will be filled and the receiving machine will discard all the additional datagrams trasmitted (buffer overflow).

• Establishes, maintains and terminates communication.
• Keeps different applications data separate
• 3 modes: simplex, half duplex, full duplex

• Presents data to the Application Layer.
• Handles data processing (encryption, compression and translation, e.g.: ASCII to EBCDIC, BCD to binary, etc.)

• File, print, message, database and application services
• Provides the user an interface to read data (Internet Explorer, Mail, etc.)

TOP 3 LAYERS

(5-SESSION, 6-PRESENTATION, 7-APPLICATION)

Define how applications communicate with each other.

BOTTOM 4 LAYERS

(1-PHYSICAL, 2-DATA LINK, 3-NETWORK, 4-TRANSPORT)

Define how data is transmitted end-to-end.