TweetFollow Us on Twitter

Jul 00 Getting Started

Volume Number: 16 (2000)
Issue Number: 7
Column Tag: Getting Started

Networks 201 pt. 3

by John C. Welch

Layer 1: The Physical Layer

Refresh

Last time around, we covered the sub-OSI parts of a network model, AKA Layer 0. We went over the physical wiring, and wiring types that networks use. We also covered connections between stations on a network, the pinouts of Ethernet cabling, and some basic information on fiber optic connections. Finally, we went over the various network topologies. This month we move up into the actual OSI model, starting with Layer 1, the Physical Layer.

Layer 1

The Physical Layer is the foundation for all the other layers in the OSI stack. It contains all of the specifications for any functions that deal with the transmission and reception of signals. There are four basic functions of the Physical Layer: Mechanical, Electrical, Functional, and Procedural. These cover all the functions that are needed for the transfer of electrical and / or optical signals and data. Examples of these are signaling methods, current requirements, components on the Network Interface Card, (NIC), and the characteristics of the connector on the NIC.

One of the common problems with talking about the Physical Layer is remembering its scope. In general, if it's outside of the host, it's outside of the Physical Layer, and the OSI model in general. This includes cabling, switches, routers and hubs. The usual boundary for the Physical Layer is the connector on the NIC. Part of the reason for the confusion over the scope of the layer is that it defines a lot of what the media and other Layer 0 components have to do, but doesn't actually include those components.

Since the Physical Layer transmits and receives data, let's look at what it has to do for those functions. For transmission, the Physical Layer has to:

  • Convert the frames it receives from the Data Link Layer into a stream of binary data.
  • Use the Media Access Method, as determined by the Data Link Layer 's media access method.
  • Transmit the binary data stream serially.
  • To receive data, the Physical Layer must:
  • Listen for inbound transmissions that are addressed to a device attached to its host.
  • Accept correctly addressed streams
  • Pass the stream data to the Data Link Layer for conversion to frames.

One important fact to note is that the Physical Layer does no error correction. It has no ability to detect errors in the data stream or frames it deals with. All it does is transfer data.

Signal Encoding

The Physical Layer primarily deals with getting signals in and out of the host, and encoding and decoding them. Regardless of the type of media used, these signals are all electromagnetic waves of some sort. Every electromagnetic wave has certain characteristics, the most obvious being its period or frequency. The frequency of a wave is the number of oscillations it makes every second. No wave is a straight line, but rather is a regular, reasonably symmetric curve that most resembles a sine wave. A diagram is shown in Figure 1.


Figure 1.

The symbol for wavelength is lambda, or l. The wavelength is the physical size of the signal through one complete oscillation, or cycle. The wavelength is also the inverse of the frequency. The frequency of a signal is the number of cycles it has in a given amount of time, usually one second. The measurement used to identify frequency is hertz, or Hz. If a signal has one cycle every second, its frequency is one Hz. If it has a thousand cycles per second, then it has a frequency of one KHz, and so on. Since frequency and wavelength are inverses of each other, the value of one can give you the value of the other. For example, the frequency of many Radar guns used by the police is 34GHz, or 34 billion cycles per second. Taking the inverse of this, using the formula 1/frequency, we get 1/34 x 109, or 2.9411 x 10-11, or a wavelength of .0294118 nanometers. Since nanometers are billionths of a meter, this is a very small wavelength indeed.

One of the more useful side effects of frequency has to do with the ability of a signal to transmit data. Since most encoding methods use the number of changes in a wave, the higher the frequency, the more data that can be transmitted by that wave. The disadvantage of higher frequencies is that the signal has less range and persistence. This is why faster LAN technologies tend to have less range than slower ones. (Note: I say 'tend to' here because when you go from copper - based LANs to fiber - optic based LANs, this falls apart somewhat, as a optical LAN has more range than a copper LAN, yet operates at much higher frequencies.) If we look at the signal in figure one above, you can see that the signal is either above or below a central point. The amount of distance between the top or bottom most point in the signal, and the central point is the amplitude, or strength of the signal. Normally, amplitude is measured in positive or negative volts, so our central point is zero volts. The positive and negative sections of the signal are states, and the transition points at zero volts are the state changes. As we will see, state changes are critical to encoding data in a signal. There are a few more parts of a wave that we need to understand, so we'll take a look at another wave form, the square wave, in Figure 2.


Figure 2.

The square wave is named for its shape, and is a common wave form used in the transmission of data. The square wave is also useful for identifying certain parts of a wave that are used in signaling and encoding. The leftmost edge of the wave is the leading, or rising edge. The upper and lowermost parts of the wave are the peaks, and the middle line, where the wave is transitioning from a high to a low is the trailing, or falling edge. All parts of the wave can be used in signaling and data encoding, depending on which method is used.

Regardless of the method chosen, there are three basic ways to modulate a wave so as to allow it to carry data. (In truth, these are the basis for many, many more modulation techniques, and other trucks to get as many bits per cycle as possible onto a wave, but for our purposes, these three will do.) The first method is amplitude modulation, also known as AM or ASK, for Amplitude Shift keying. Since the amplitude of a signal is its strength, the way this works is suggested by the name. If the data being carried by the signal is a binary 1, then the pulse amplitude is increased by a given amount. If the data is a binary 0 then the pulse amplitude is decreased by the same amount. A middle amplitude indicates no data. Amplitude Modulation is simple to implement, and widely used for voice transmission, as in AM radio. Due to limitations in the modulation technique itself, ASK is only used for transmissions up to 1200 baud.

The next method used is Frequency Modulation, a.k.a. FM or FSK. This functions by introducing a second signal into the first signal. If a binary 1 needs to be transmitted, the frequency of the first signal is upshifted, or increased. If a binary 0 is being transmitted, the signal frequency is downshifted, or decreased. If no data is being transmitted, the frequency is left alone. FSK is more efficient and reliable than ASK, and is used for data rates up to 1200 baud.

The final method is known as Phase Modulation, or PSK. In this method, the phase of the signal is altered to indicate a binary 0 or 1. If a binary 0 is sent, the phase is not altered. If a binary 1 is sent, the phase is switched to its opposite. This allows one bit per shift(s) to be sent. If more than one phase shift is used, then more bits can be sent in a set of shifts, or burst. PSK is the most efficient, and reliable of the three modulation techniques, allowing data rates of up to 9600bps. These may not seem like fast data rates, but in combination with other encoding methods, (most of which we will cover in other articles, as they are protocol - dependant, and therefore beyond the scope of the Physical Layer.), we get the data rates we are accustomed to on today's LANs.

Whichever form of modulation and encoding is chosen, that is then modified by other methods to allow the signal to pack more bits per cycle, thereby increasing efficiency of the signal as a data carrier. Depending on the methods used, and the frequency of the signal, it is possible to encode anywhere from two to forty bits per cycle in a signal. So a cable that has a maximum bandwidth of 500MHz is capable of transmitting several gigabits of data per second. Since fiber optic cables can handle much higher frequencies than copper, it is obvious why fiber is becoming more popular as a way to move the staggering amounts of data that a modern network is required to move.

One thing to remember is that any form of encoding, no matter how fast or efficient, imposes an overhead that subtracts from the bits a signal can carry. As an example, Gigabit Ethernet uses an encoding scheme known as 4B/5B. 4B/5B states that for every 4 - bit string of data transmitted, a 5 - bit pattern is used to encode it. This means that 20% of the data transmitted is overhead. If Gigabit Ethernet were to be strictly defined as 1000, or even 1024Mbps, then the best speed you could actually get would be in the 800Mbps range. To get around this, Gigabit Ethernet is actually defined as 1250Mbps, so when the encoding and other overhead is accounted for, the usable speed is still very close to 1Gbps.

Conclusion

It may seem that we have glossed over a lot of the Physical Layer. That is partially true. Much of the Physical Layer deals with extremely low level electrical issues that would be of only academic use to the network administrators. Since we covered physical media in the previous article, there is no need to re-hash that here. Also, anything that is protocol - specific is the property of the higher layers. The important thing to remember is that the Physical Layer is only concerned with ones and zeros, and the transfer of those things. The next article will deal with the Data Link Layer , and the Medium Access Control sublayer, and that is where we will get into much of the protocol specific issues of a network.

Bibliography and References

  • Tannenbaum, Andrew S. Computer Networks. Third Edition Prentice Hall, 1996.
  • Sportack, Mark. Networking Essentials Unleashed. SAMS Publishing, 1998.
  • Stallings, William. Local & Metropolitan Area Networks. Fifth Edition Prentice Hall, 1997.

John Welch <jwelch@aer.com> is the Mac and PC Administrator for AER Inc., a weather and atmospheric science company in Cambridge, Mass. He has over fifteen years of experience at making computers work. His specialties are figuring out ways to make the Mac do what nobody thinks it can, and showing that the Mac is the superior administrative platform.

 

Community Search:
MacTech Search:

Software Updates via MacUpdate

Macs Fan Control 1.5.14 - Monitor and co...
Macs Fan Control allows you to monitor and control almost any aspect of your computer's fans, with support for controlling fan speed, temperature sensors pane, menu-bar icon, and autostart with... Read more
VueScan 9.7.96 - Scanner software with a...
VueScan is a scanning program that works with most high-quality flatbed and film scanners to produce scans that have excellent color fidelity and color balance. VueScan is easy to use, and has... Read more
FileMaker Pro 19.6.1 - Quickly build cus...
FileMaker Pro is the tool you use to create a custom app. You also use FileMaker Pro to access your app on a computer. Start by importing data from a spreadsheet or using a built-in Starter app to... Read more
Duet 3.1.0.0 - Use your iPad as an exter...
Duet is the first app that allows you to use your iDevice as an extra display for your Mac using the Lightning or 30-pin cable. Note: This app requires a iOS companion app. Release notes were... Read more
Firefox 107.0.1 - Fast, safe Web browser...
Firefox offers a fast, safe Web browsing experience. Browse quickly, securely, and effortlessly. With its industry-leading features, Firefox is the choice of Web development professionals and casual... Read more
War Thunder 2.21.1.91 - Multiplayer war...
In War Thunder, aircraft, attack helicopters, ground forces and naval ships collaborate in realistic competitive battles. You can choose from over 1,500 vehicles and an extensive variety of combat... Read more
Numbers 12.2.1 - Apple's spreadshee...
With Apple Numbers, sophisticated spreadsheets are just the start. The whole sheet is your canvas. Just add dramatic interactive charts, tables, and images that paint a revealing picture of your data... Read more
DEVONthink Pro 3.8.7 - Knowledge base, i...
DEVONthink is DEVONtechnologies' document and information management solution. It supports a large variety of file formats and stores them in a database enhanced by artificial intelligence (AI). Many... Read more
Drive Genius 6.2.3 - $79.00
Drive Genius features a comprehensive Malware Scan. Automate your malware protection. Protect your investment from any threat. The Malware Scan is part of the automated DrivePulse utility. DrivePulse... Read more
VLC Media Player 3.0.18 - Popular multim...
VLC Media Player is a highly portable multimedia player for various audio and video formats (MPEG-1, MPEG-2, MPEG-4, DivX, MP3, OGG, ...) as well as DVDs, VCDs, and various streaming protocols. It... Read more

Latest Forum Discussions

See All

‘Genshin Impact’ Version 3.3 Pre-Install...
Following the reveal of the release date and more for Genshin Impact (Free) version 3.3 ‘All Senses Clear, All Existence Void’, HoYoverse showcased the Genius Invokation TCG that arrives this week in the update. | Read more »
TouchArcade Game of the Week: ‘Sling Min...
The world of PC games has always blown my mind because there’s just SO MUCH stuff out there that it’s not uncommon at all for there to be a game that’s well-liked and well-reviewed, and seemingly quite popular with a solid fanbase, and have it be... | Read more »
SwitchArcade Round-Up: Reviews Featuring...
Hello gentle readers, and welcome to the SwitchArcade Round-Up for December 2nd, 2022. So, today turned out a little quieter than the usual Friday. It was so quiet, in fact, that I decided to pen a few reviews. The Knight Witch, Railbound, and Donut... | Read more »
Blue Archive reveals its latest event st...
Nexon has announced the new update for Blue Archive, under the name of An Unconcealed Heart. Featuring a battle between two academies, the story will follow a group struggling to gain recognition, and will bring three new students to recruit. [... | Read more »
Dead Cells+ Is Out Now on Apple Arcade a...
Following the major update for Dead Cells on iOS and Android a few days ago, Playdigious has brought Dead Cells+ () to Apple Arcade. As an App Store Great, Dead Cells+ includes all prior paid DLC and content updates. It also has exclusive mobile... | Read more »
SwitchArcade Round-Up: ‘Romancing SaGa’,...
Hello gentle readers, and welcome to the SwitchArcade Round-Up for December 1st, 2022. Wow, December. We’re already at the last month of the year? Phew. I have a lot of work to finish in the next few weeks. As for today, we’ve got a little news, a... | Read more »
‘Railbound’ Update Now Available Adding...
One of our favorite puzzlers released this year is Railbound from Afterburn Games, which hit in early September and earned our Game of the Week recognition for being an absolutely ace logic puzzler. The goal is to place rail pieces down in order to... | Read more »
The Seven Deadly Sins: Grand Cross celeb...
Netmarble Corporation has pulled out all the stops to celebrate the 3 and a half year anniversary of The Seven Deadly Sins: Grand Cross. The Grand Cross 3.5th Year Anniversary the Ultimate One, a rather wordy title, brings with it a brand new... | Read more »
‘Skullgirls Mobile’ Major Update 5.2 Out...
Developer Hidden Variable pushed out a major update for Skullgirls Mobile (Free) a few hours ago. The version 5.2 update brings in Black Dahlia (before the console and PC game), Retakes, XP Treats, free gifts, and more. Since launch, Skullgirls... | Read more »
Out Now: ‘Disgaea 4’, ‘Romancing SaGa: M...
Each and every day new mobile games are hitting the App Store, and so each week we put together a big old list of all the best new releases of the past seven days. Back in the day the App Store would showcase the same games for a week, and then... | Read more »

Price Scanner via MacPrices.net

Holiday Sale: Apple AirPods Pro for only $199...
Amazon has new 2022 AirPods Pro in stock and on sale for $199.99 shipped as part of their Holiday sale. Their price is $50 off Apple’s MSRP, equaling their Black Friday price, and it’s the lowest... Read more
New Holiday Sale: Apple retailers are offerin...
Several Apple retailers lowered prices on 10.9″ iPad Airs overnight to lows of $100 off MSRP starting at $499. Their prices are the lowest available for iPad Airs anywhere this Holiday season right... Read more
New Holiday sale at Amazon: Take $50 off Appl...
Amazon has Apple’s new 10th-generation iPads in stock and on sale, for the first time, for $50 off MSRP starting at only $399. Their discount applies to all models and all colors. With the discount,... Read more
Holiday Sale: Get an 8.3″ Apple iPad mini for...
Sams Club has 10.9″ 64GB iPad minis on Holiday sale for $80-$100 off MSRP through December 7, 2022. With their discount, prices start at $399 — the cheapest price for a new iPad mini from any of the... Read more
Sams Club Holiday December Event sale: Apple...
Apple AirPods Max headphones are on sale at Sams Club for $110 off MSRP ($439) as part of their December Event sale, ending on December 7, 2022, valid for all colors. Sale price for online orders... Read more
Apple’s 10.2″ 64GB 9th-generation iPads are o...
Sams Club has 9th-generation 64GB iPads on Holiday sale for $60 off MSRP through December 7, 2022. With their discount, prices start at $259 — the cheapest price for a new iPad from any of the Apple... Read more
11″ 128GB WiFi M2 iPad Pro on sale for $749,...
B&H Photo has the new 11″ 128GB WiFi M2-powered iPad Pro (in Space Gray or Silver) on Holiday sale for $749 including free 1-2 day shipping to most US addresses. Their price is $50 off MSRP and... Read more
Find the best Holiday sale price on an iPad u...
We’ve updated our iPad Price Trackers with the latest information on the new 10th-generation iPads, M2-powered iPad Pros, M1 iPad Airs, iPad minis, and 9th generation iPads from Apple’s authorized... Read more
Apple retailers are offering $100-$150 Holida...
Apple retailers have posted their most-recent Holiday sale prices on 13″ MacBook Airs. Take up to $150 off MSRP on M2-powered Airs with these sales with prices starting at only $1099. Free shipping... Read more
Holiday Sale: Apple’s 14″ MacBook Pros with M...
B&H Photo is offering $200-$300 discounts on Apple’s 14″ MacBook Pros with M1 Pro CPUs as part of their Holiday 2022 sale, with prices starting at $1799. Free 1-2 day shipping is available to... Read more

Jobs Board

Support Technician II - *Apple* Support - O...
…problems and acting as a liaison between customers and resolving groups. As an Apple Technical Specialist, you will be supporting many of our popular Apple Read more
*Apple* Electronic Repair Technician - PlanI...
…a highly motivated individual to join our Production Department as an Apple Electronic Repair Technician. The computer repair technician will diagnose, assemble, Read more
Lead Developer - *Apple* tvOS - Rumble (Uni...
…earnings, and positive sentiment About the role: We are looking for a Lead Apple tvOS Developer to join our application engineering team to expand our video centric Read more
Tier 1 Endpoint Engineer - *Apple* - Red Ri...
…Desk on site, at our Client's location, with a focus on support to Apple products. This position will handle technical support requests directly from customers and Read more
Product Manager II - *Apple* - DISH (United...
…you will be doing We seek an ambitious, data-driven thinker to assist the Apple Product Development team as our new Retail Wireless division continues to grow and Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.