The physical layer, hubs, and ethernet

The main functions of the physical layer is to send bits and receive bits, as well as state transitions.

• Bits come in only values of 1 or 0 (morse code with numerical values).
• State transitions – changes in voltage from high to low and vice-versa.

Hubs at the physical layer

What is a hub?

• A hub is really a multiple port repeater.

Hubs in a network

• All devices in the same collision domain.
• All devices in the same broadcast domain.
• Devices shares some bandwidth.

What is a Physical Star Network?

• A physical star network is where the hub is a central device and cables are extended in all directions from it.

Note: Hubs and repeaters can be used to enlarge the area covered by a single LAN segment, however, this is NOT RECOMMENDED.

Ethernet at the physical layer

• Created by a group called DIX (Digital, Intel, Xerox)
• There are three types of ethernet
  • 802.3u – Fast Ethernet
  • 802.3ab – Gigabit Ethernet on Category 5
  • 802.3ae – 10Gbps over fiber and coax

Electronics Industries Association and The Newer Telecommunications Industry Alliance (EIA/TIA)

• The standard body that creates the physical layer/specifications for ethernet.
• EIA/TIA specifies that ethernet use a registered jack (RJ) connector with a 4 5 wiring sequence on unshielded twisted pair (UTP) cabling (RJ45).

Cables

• Cables are measured in decibals (dB)
• Cabling in corporate and home markets is measured in categories.
  • A higher quality cable will have a higher rated category and lower attenuation.
    • ex. category 5 > category 3

Key term:

• Crosstalk – the unwanted signal interference from adjacent pairs in the cable.

A closer look at the Data-Link layer

The Data-Link layer provides physical transmission of the data and handles error notifications, network topology, and flow control.

Lets take a closer look at it’s function

1. Ensures messages are delivered to the proper device on LAN using hardware addresses and will translate messages from the Network layer to transmit.
2. Transmit messages into pieces called a data frame and adds a customized header containing the hardware destination and source address.
3. Data-Link uses binary values and hexadecimal values.

Binary Values

Nibble Values	Byte Values
8  4  2  1	128  64  32  16  8  4  2  1

Binary to Decimal Chart

Binary Value	Decimal Value
10000000	128
11000000	192
11100000	224
11110000	240
11111000	248
11111100	252
11111110	254
11111111	255

Hexadecimals

Hex is short for hexadecimal, which is a numbering system that uses the first six letters of the alphabet (A through F) to extend beyond the available ten digits in the decimal system. Hexadecimal has a total of sixteen digits.

Hex to Binary to Decimal Chart

Hexadecimal	Binary	Decimal
0	0000	0
1	0001	1
2	0010	2
3	0011	3
4	0100	4
5	0101	5
6	0110	6
7	0111	7
8	1000	8
9	1001	9
A	1010	10
B	1011	11
C	1100	12
D	1101	13
E	1110	14
F	1111	15

Understanding how flow control operates at the Transport layer

A quick overview of the Transport layer and its function:

• Reliable networking is used at the transport layer
  • Reliable networking = using acknowledgements, sequencing, and flow control in the network.
• Segments and reassembles data into a data stream from the upper application layer and unites the information into the same data stream for further processing.

Flow Control

• Flow control prevents a sending host on one side of the connection from overflowing the buffers in the receiving host.
  • an event that can result in data loss
• Reliable data transports and employs a connection oriented communications sessions between systems and the protocols involved, to ensure that the following will be achieved:
  • Segments delivered are acknowledged back to the sender upon their reception.
  • Any segments not acknowledged are re-transmitted.
  • Segments are sequenced back into their proper order upon arrival at their destination.
  • A manageable data flow is maintained in order to:
    • avoid traffic congestion
    • overloading
    • data loss
• The purpose of flow control is to provide means for the receiver to be able to govern the out of data sent by the sender.

Important additional functions of the OSI reference model's upper layers

 

Application layer

• The application layer acts as an interface between the actual applications programs.
  • Example - Microsoft Word – actually does not reside at the application layer, but instead just interfaces at the application layer protocol, using data presented to it by the presentation layer.

Presentation layer

• The presentation layer presents data to the application layer up top.
• Responsible for data translation and code formatting.
• Acts as a translator that presents and provides coding and conversion functions.
  • Example – EBCDIC to ASCII
• Some presentation layer may also involves multimedia operations too.

Session Layer

• The session layer is responsible for setting up, managing, and then tearing down sessions between layer entities.
• Coordinates communication between systems and serves to organize their communication by offering THREE different modes:
  • Simplex
  • Half Duplex
  • Full Duplex
• Keeps different application data separate from other applications data.

A quick overview of the OSI layers and their functions

Applications

• file
• print
• message
• database
• application services

Presentations

• data encryption
• compression
• translation

Session

• dialog control

Transport

• end-to-end connection

Network

• routing

Data Link

• framing

Physical

• physical topology

Facts and notes on the OSI reference model bottom layers

The bottom layers of the OSI reference model include

1. Transport
2. Network
3. Data Link
4. Physical

Functions at each of the bottom layers

Transport
- provides reliable or unreliable delivery
- performs error correction before re-transmit

Network
- provides logical addressing, which routers use for path information

Data Link
- combines packets into bytes and bytes into frames
- provides access to media, using MAC address
- performs error detection, NOT correction

Physical
- moves bits between devices
- specifies voltage, wire speed, and pin-out of cables

The following operates at all seven layers of the OSI reference model

1. Network Management Stations (NMS)
2. Web and application servers
3. Gateways (not default gateways)
4. Network Hosts

Facts and notes on the OSI reference model

1. The OSI reference model has seven layers and is divided into two groups.
2. The top three layers define how the applications within the end stations will communicate with each other and with users.
3. The bottom four layers define how data is transmitted end to end.

The upper layers of the OSI reference model include

1. Application layer
2. Presentation layer
3. Session layer

*Note: Users interfaces with computer at the application layer.

*Note: The upper layers are responsible for applications communications between hosts.

Diagram

The OSI reference model and it's seven layers

The OSI reference model has seven layers:

Application layer (layer 7)
Presentation layer (layer 6)
Session layer (layer 5)
Transport layer (layer 4)
Network layer (layer 3)
Data Link layer (layer 2)
Physical layer (layer 1)