bitmindframes

Physical Layer Protocols and Services:

L1 controls how data is placed on the media.
It has to encode bits into signals and Tx/Rx these signals across the physical media.
The delivery of frames across media requires.

Physical media and connectors.
A representation of bits – code.
Encoding of data and control information.
Tx and Rx circuitry on devices.

The purpose of L1 is to create the electrical, optical, or radio signal that reqresents the bits in each frame.
It also must retrieve signals from media, restore them to bit representations, and pass them to L2 as a complete frame.
The type of signal – depends on the type of media.

Copper = electrical pulses.
Fiber = patterns of light.
Wireless = patterns of radio transmissions.

Identifying where a frame starts and stops.

Is the job of L2, but L1 may add its own signals.

Standards:

Similar to L2, L1 technologies are defined by:

ISO.
IEEE.
ANSI.
ITU-T.
EIA/TIA.
FCC and CRTC.

The technologies defined by these organizations include four areas of L1-2 standards:

1. Physical and electrical properties of the media.
2. Mechanical properties (materials, dimensions, pin outs) of the connectors.
3. Bit representation (encoding).
4. Definition of control information signals.

Physical Layer Fundamental Principles:

Encoding:

A method of converting bits into a predefined “code”.
Codes are groupings of bits in a predictable pattern.

Signalling:

The electrical, optical, or wireless signals that = 1 or 0.

Physical Layer Signaling and Encoding:

Each signal placed onto media has a specific amount of time to occupy the media. This is refered to as its bit time.
Requires synchronization between transmitter and receiver.
Synchro is accomplished with a clock.
In LANs, each end maintains its own clock.
Signaling Methods
Bits are represented by changing one or more of the following:

Amplitude.
Phase.
Frequency.

Some methods use one voltage level during the bit time to = 0 and a different voltage = 1.
Other methods use transitions or their absence to = 1 and 0.
Signalling methods can be complex. 2 Examples:

NRZ signalling:

Low voltage = 0.
High voltage = 1.
Only for slow speed data links.
Inefficient and susceptible to EMI.
Boundaries between bits lost in long strings of 1s or 0s.

Manchester Encoding:

Bit values are represented as voltage transitions (moving up or down).
Low ? high = 1.
High ? low = 0.
Must occur in the middle of each bit time (setup transition).
Transitions ensure bit times are synchronized (always changing)
Not efficient, but used in legacy Ethernet (10mBps).

Encoding Patterns:

Can be symbolic grouping of bits prior to being sent.
At higher speeds > possibility of data corruption.
Coding groups help detect errors more efficiently.
L1 needs to recognize the beginning and end of frames.
One way to provide frame detection is to begin and end each frame with a pattern of bits.

Code Goups:

Are often used as an intermediary step at higher speeds.
Although they add extra bits, they improve robustness.
Designed to force an ample number of bit transitions to synchronize this timing.
Also ensure the number of 1s and 0s in a string are balanced.
This is called DC balancing (every 5 bits co group???).
Code groups have 3 types of symbols:

Data symbols – Symbols that represent the data of the frame as it is passed down to the Physical layer.
Control symbols – Special codes injected by the Physical layer used to control transmission. These include end-of-frame and idle media symbols.
Invalid symbols – Symbols that have patterns not allowed on the media. The receipt of an invalid symbol indicates a frame error.

4B/5B:

4 bits (nibble) of data are turned into 5-bit code (symbols).
Each nibble encoded as 5-bit symbols.
Ensures at least 1 level change per code.
Used by Fast Ethernet.

Data Carrying Capacity:

3 ways to measure capacity (all in kbps or Mbps).
Bandwidth – raw or theoretical capacity.
Throughput – Measured transfer rate.

Usually less than bandwidth.
Affected by amount and type of traffic, the number of users, etc..
Cannot be faster then the slowest link in a path.

Goodput – measures the transfer of usable data.

Measures the effective transfer of user data between L7 entities.
Through put minus traffic overhead.

Types of Physical Media:

Copper Media.

The most commonly used media (eg UTP).
Coax cable is also copper.
Generally make use of modular jacks/plugs.
RJ-45s are used in LANs and in some WANs but are used in many other areas.

External Signal Interference:

EMI and RFI can distort and corrupt the data signals on copper.
Fluorescent lights, electric motors are sources of noise.
Cable shielding or pair twisting are designed to minimize signal degradation due to electronic noise.
Susceptibility can be limited by:

Selecting the cable type suited to the environment.
Designing infrastructure to avoid sources of interference.
Properly handling and terminating cables.
UTP:

4 twisted pairs of wires encased in a flexible plastic sheath.
Colour codes identifies wires and aids in cable termination.
Twisting helps cancel unwanted signals.
EMI the same on both wires, receiver cancels 1 from the other.
Also helps internal crosstalk – EMI from adjacent pairs.
TIA/EIA-568A stipulates standards for LAN installations.
Cables in higher categories support higher data rates:

Cat5 – used commonly in 100BASE-TX FastEthernet.
Cat5e – now the minimum acceptable cable type.
Cat6 – recommended type for new building installations.

Coax Cable:

Two other types of copper cable are used.

1. Coaxial.

Conductior + insulation + metallic cover + jacket.
Often used for TV (may be hybrid HFC) and antennas.
Used in early Ethernet.

2. Shielded Twisted-Pair (STP):

Uses 2(?) pairs of wires wrapped in metallic braid or foil.
Provides better noise protection then UTP ? costs more.
Was specified for use in TR (token ring).
10 Gb Ethernet has a provision for STP, so possibly renewed interest.

Electrical Hazards:

Copper wires can conduct electricity in undesirable ways.
A defective device can conduct current to other devices.
Cabling could present undesirable voltage when connecting devices.
Copper may conduct lightning strikes to network devices.
Insulation and sheaths may be flammable or produce toxic fumes.

Fiber Media:

Uses either glass or plastic fibers.
The bits are encoded as light impulses.
Immune to EMI and electrical issues.
At present primarily backbone cable.
PVC jacket, strengthening materials, fiber + cladding.
Cladding prevents light loss.
Light travels in one direction only ? 2 fibers for full duplex.
Currently 40 gbps in use, 10 Tbps in development!.

Generating and Detecting the Optical Signal:

Either lasers or LEDs generate light pulses.
Photodiodes detect light ? volts ? bits ? frames.
Single-mode, - laser, single ray – small core – long distance.
Multimode – LED – light enters at different angles – in long runs may become blurred (modal dispersion) – limits length. (thinker fiber).
Thicker core.
MM fiber (LEDs) is cheaper then SM.

Wireless:

Carries electromagnetic signals at radio and microwave freq.
Not restricted to conductors or pathways.
Certain materials and terrain limits effective coverage.
Is susceptible to environmental interference.
Network security is a major component.

Types of Wireless Networks:

802.11 – Wi-Fi , is a WLAN (Wireless LAN) technology that uses contention or non-deterministic system with CSMA/CA < 100 meters.
802.15 – WPAN (Wireless Personal Area Network) – “Bluetooth” < 10 meters.
802.16 – WiMAX (Worldwide Interoperability for Microwave Access) – broadband ~ < 50 Km.
GSM (Global System for Mobile Communications) – data transfer over cellular networks GPRS (General Packet Radio Service) protocol.
Other technologies include satellite, microwave and infrared.

The Wireless LAN:

Requires a WAP and Wireless NICs.
802.11a – 5 GHz – 54 Mbps – 35 meters.
Smaller coverage less building penetration.
802.11b – 2.4 GHz – 11 Mbps – 38 meters.
Langer range and are better penetration.
802.11g – 2.4 GHz – 54 Mbps – 38 meters.
Interoperable with 11b.
802.11n – 2.4 or 5 GHz – 100 to 210 Mbps – 70 meters.
Currently still in draft form.

Connector Termination:

Improper terminations are a potential source of performance degradation.
It is essential that terminations are high quality.

Fiber Connectiors.

ST (straight tip) – widely used with Multi-mode.
SC (subscriber connector) – widely used with Single-mode.
LC (lucent connector) – becoming popular for SM and MM.
MT-RJ (not mentioned here)
3 common termination and splicing errors:

Misalignment – fibers not precisely aligned.
End gap – fibers do not completely touch.
End finish – ends not well polished or dirty.

Optical Time Domain Reflectometer (OTDR) to test fiber optic cables.
Field test – bring flashlight – quick way to find broken fiber.

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