Network Fundamentals - OSI Physical Layer

Physical Layer

The function of the OSI Physical Layer is to create signal (electrical, optical or microwave)  that represents the bits in the frame from Data Link layer.




Transmission of frames required :

• A physical media and its connectors
• Circuit on the network devices
• A method to represent the bits on the media
• A method for data encoding and control

Three Types of Signal

Standards

4 areas of standards:

• Physical and electrical properties of a media
• Mechanical properties of the connectors
• Encoding – bit representation of signals
• Definition of control signals

Physical layer process

A process involving physical components that carry encoded data sent out as a signal suitable for the media.

• Physical components
• Encoding – a method to convert a group of bits into a predefined code which can be understood by sender and receiver
• Signaling – a method of representing the bits in “0” and “1”

Signaling

Each signal placed onto the media has a specific amount of time to occupy the media, called bit time.

Bits are represented on the medium by changing the Amplitude, Frequency and/or Phase.

Encoding

Encoding is a method to convert a group of bits into a predefined code.

Data-Carrying Capacity

3 methods to measured data transfer speed:

• Bandwidth – Digital Bandwidth measures the amount of information flow from one place to another in a given period of time in a specific media.
• Throughput – measures the transfer of bits across the media over a given period of time. It does not equal to the specified bandwidth of a media
• Goodput  – measures of usable data transferred over a given period of time, the transfer rate of actual data

Units – kbps, Mbps, Gbps, Tbps

Throughput and Goodput

Ethernet Media

Notation: 1000BASE-T
               Bandwidth:1000
               Signaling:BASE
               Media or Segment Length: T 

Network media

Three common types of network media:

• Copper cable – use copper wires to transfer data in electrical signals between network devices
• Fiber optic – use glass or plastic fibers to transfer light signals
• Wireless – use electromagnetic signals at radio and microwave frequencies to transfer data bits

Copper media

Copper cable is most commonly used media.

3 types of copper media:

• UTP – Unshielded Twisted-pair
• STP – Shielded Twisted-pair
• Coax – Coaxial cable

Unshielded Twisted-pair (UTP)

UTP consists of 4 pairs of color-coded wires which twisted together and wrapped in a flexible plastic sheath

Commonly used in LAN connection because of lowest cost and easy installation; but it has a distance limitation (max 100m) (attenuation: loss of energy in a signal as it travels longer distance)

Noise & Crosstalk

In shielded cable, shielding material protects the data signal from external noise and also internal noise generated by electrical signals within the cable.

Crosstalk is the interference caused by magnetic field around the adjacent pairs of wires inside the cable or around the nearby cable. In twisted cable, the twisted wires cancel the noise from each other.

Unshielded Twisted-pair (UTP)

UTP cables are divided into categories according to their bandwidth, e.g. Category 5 (Cat5), Enhanced Category 5 (Cat5e), Category 6 (Cat6)

Three types of UTP cable using specific wiring conventions (T568A and T568B):

• Ethernet Straight-through
• Ethernet Crossover
• Rollover

Ethernet Straight-through and Cross-over cables

Pinout of the straight-through and cross-over cables

Rollover cables

UTP cable connection

• Group A devices: PCs, Servers, Routers
• Group B devices: Hubs, Switches
• Use straight-through cables for connecting devices in different groups.
• Use crossover cables for connecting devices in the same group.
• Rollover cables – used to connect a workstation serial port to a console port of a Cisco device, using an adapter

Network Cabling

Shielded Twisted-pair (STP)

STP use 4 pairs of wires which are wrapped in an metallic braid or foil

STP provides better noise protection than UTP, but STP is of higher price

Copper Media Connectors

Coaxial cable

In the past, coax was used to carry high-frequency radio and TV signals over wire.
It is still used in connecting cable TV
New coax technologies hybrid fiber-coax (HFC)
2 types of old coaxial cable:

• Thicknet (10Base5) - was operated at 10Mbps with a maximum length of 500 m
• Thinnet (10Base2) - was operated at 10Mbps with a maximum length of 185m

 Coaxial cable and connector

Coaxial cable connection

Disadvantage:
One device fails will cause whole network to fail.

External Interference

The electrical signals in a copper cable is easily affected by electromagnetic interference from outside, e.g. fluorescent lights, electric motor, radio waves

Fiber Media

Fiber-optic cabling uses either glass or plastic fibers to guide light impulses from source to destination.

Optical fiber media implementation issues include:

• More expensive (usually) than copper media over the same distance (but for a higher capacity)
• Different skills and equipment required to terminate and splice the cable infrastructure
• More careful handling than copper media

Advantages: Long segment length, high bandwidth, immunity to electromagnetic interference

Because light can only travel in one direction over optical fiber, two fibers are required to support full duplex operation.

 
Either lasers or light emitting diodes (LEDs) generate the light pulses that are used to represent the transmitted data as bits on the media.

Note:
The laser light transmitted over fiber-optic cabling can damage the human eye. Care must be taken to avoid looking into the end of an active optical fiber.

Fiber Media

Single-mode and Multimode Fiber

Wireless Media

 

Wireless media carry electromagnetic signals at radio and microwave frequencies that represent the binary digits of data communications.

Wireless connection

Radio wave uses air as transmission medium, users are free from connection of a physical cable

Open areas are best for wireless connections – within buildings, interference comes from walls, metal air ducts and office machinery

Disadvantages:

• Wireless media is open to anyone with a wireless receiver,  this increases the need of security in wireless network
• Wireless signal is more easily affected by signals from microwave ovens, fluorescent lights, home-used wireless devices like phones and Bluetooth devices
• A wireless connection is slower than a cable connection

Types of Wireless networks

Four common types of wireless networks:

• Standard IEEE 802.11 – known as “Wi-Fi”, is a Wireless LAN (WLAN) technology uses CSMA/CA media access process
• Standard IEEE 802.15 – Wireless Personal Area Network (WPAN), known as “Bluetooth”, can communicate between 1-100m
• Standard IEEE 802.16 – WiMAX, uses a point-to-multipoint topology to provide wireless broadband access
• Global System for Mobile Communications (GSM) – includes Physical layer specifications that enable the implementation of the Layer 2 General Packet Radio Service (GPRS) protocol to provide data transfer over mobile phone networks

Wireless LAN

A wireless LAN needs 2 types of network devices:

• Wireless Access Point (AP) – concentrates the wireless signals from users and connects to a copper-cabled network structure
• Wireless NIC adapters – provides wireless communication capability to each network host

802.11 wireless LAN standards:

• IEEE 802.11a – 54Mbps, 5 GHz, cover small area
• IEEE 802.11b – 11Mbps, 2.4 GHz, longer range and able to pass through walls
• IEEE 802.11g – 54Mbps, 2.4 GHz
• IEEE 802.11n – published in Oct 2009, 2.4 GHz or 5 GHz,  100-210Mbps with distance upto 70m (indoor) or 250m (outdoor)