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

LaunchBar 6.9 - Powerful file/URL/email...
LaunchBar is an award-winning productivity utility that offers an amazingly intuitive and efficient way to search and access any kind of information stored on your computer or on the Web. It provides... Read more
MacFamilyTree 8.2.1 - Create and explore...
MacFamilyTree gives genealogy a facelift: modern, interactive, convenient and fast. Explore your family tree and your family history in a way generations of chroniclers before you would have loved.... Read more
Spotify 1.0.57.474. - Stream music, crea...
Spotify is a streaming music service that gives you on-demand access to millions of songs. Whether you like driving rock, silky R&B, or grandiose classical music, Spotify's massive catalogue puts... Read more
VOX 2.8.26 - Music player that supports...
VOX just sounds better! The beauty is in its simplicity, yet behind the minimal exterior lies a powerful music player with a ton of features and support for all audio formats you should ever need.... Read more
beaTunes 5.0.1 - Organize your music col...
beaTunes is a full-featured music player and organizational tool for music collections. How well organized is your music library? Are your artists always spelled the same way? Any R.E.M. vs REM?... Read more
LibreOffice 5.3.4.2 - Free, open-source...
LibreOffice is an office suite (word processor, spreadsheet, presentations, drawing tool) compatible with other major office suites. The Document Foundation is coordinating development and... Read more
Sparkle Pro 2.2 - $79.99
Sparkle Pro will change your mind if you thought building websites wasn't for you. Sparkle is the intuitive site builder that lets you create sites for your online portfolio, team or band pages, or... Read more
calibre 3.1.1 - Complete e-book library...
Calibre is a complete e-book library manager. Organize your collection, convert your books to multiple formats, and sync with all of your devices. Let Calibre be your multi-tasking digital librarian... Read more
calibre 3.1.1 - Complete e-book library...
Calibre is a complete e-book library manager. Organize your collection, convert your books to multiple formats, and sync with all of your devices. Let Calibre be your multi-tasking digital librarian... Read more
Sparkle Pro 2.2 - $79.99
Sparkle Pro will change your mind if you thought building websites wasn't for you. Sparkle is the intuitive site builder that lets you create sites for your online portfolio, team or band pages, or... Read more

Latest Forum Discussions

See All

This War of Mine gets a new ending and m...
This War of Mine just got a big new update, featuring free DLC that adds a new ending to the game, among other exciting changes. The update is celebrating the game's two-year release anniversary. Apart from the new ending, which will be quite... | Read more »
Summon eight new heroes in Fire Emblem H...
Nintendo keeps coming at us with Fire Emblem Heroes updates, and it doesn't look like that trend is stopping anytime soon. The folks behind the game have just announced the new War of the Clerics Voting Gauntlet, expected to start next Tuesday. [... | Read more »
The best deals on the App Store this wee...
iOS publishers are pulling out all the stops this week -- there's a huge number of seriously great games at discounted prices this week. Let's not waste any time and get right down to business. [Read more] | Read more »
The House of da Vinci (Games)
The House of da Vinci 1.0.0 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0.0 (iTunes) Description: Enter The House of Da Vinci, a new must-try 3D puzzle adventure game. Solve mechanical puzzles, discover hidden... | Read more »
Solve the disappearance of history’s gre...
Blue Brain Games invites you to indulge in an immersive hands-on 3D puzzle adventure in similar vein to The Room series, with its debut release The House of Da Vinci. Set during the historic period of the Italian Renaissance (when Leonardo himself... | Read more »
Age of Rivals (Games)
Age of Rivals 3.3 Device: iOS Universal Category: Games Price: $.99, Version: 3.3 (iTunes) Description: Deep civilization-building strategy in a fast-paced card game! | Read more »
Panthera Frontier (Games)
Panthera Frontier 1.0 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0 (iTunes) Description: | Read more »
Angry Birds Evolution beginner's gu...
Angry Birds changes things up a fair bit in its latest iteration, Angry Birds Evolution. The familiar sling-shot physics mechanics are still there, but the game now features team-based gameplay, RPG elements, and a new top-down view. With all of... | Read more »
Sega Forever is for the retro game fans
Sega is launching a new retro games service titled Sega Forever, in a move that's sure to delight games enthusiasts with a bit of nostalgia. Sega's releasing five classic games for free. The titles include Sonic the Hedgehog, Phantasy Star II,... | Read more »
The Little Acre (Games)
The Little Acre 1.0 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0 (iTunes) Description: | Read more »

Price Scanner via MacPrices.net

ABBYY TextGrabber 6 for iOS Implements Instan...
ABBYY has announced the release of TextGrabber 6.0.0, an important feature update to the company’s productivity app developed for iOS and Android devices. TextGrabber 6.0 now offers Real-Time... Read more
vPhone, First Smartphone That Can’t Be Lost,...
Austin, Texas based Hypori has introduced the vPhone, a virtual smartphone that affords every business user the benefits of separate work and personal phones, conveniently delivered on a single... Read more
Save this weekend with 2016 refurbished MacBo...
Apple has dropped prices on Certified Refurbished 2016 15″ and 13″ MacBook Pros by as much as $590 off original MSRP. An Apple one-year warranty is included with each model, and shipping is free: -... Read more
New 27-inch 3.4GHz iMac on sale for $1699, sa...
MacMall has the new 2017 27″ 3.4GHz iMac (MNE92LL/A) in stock and on sale for $1699 including free shipping. Their price is $100 off MSRP. Read more
Clearance 2016 MacBook Pros available for up...
B&H Photo has clearance 2016 13″ and 15″ MacBook Pros in stock today and on sale for up to $400 off original MSRP. Shipping is free, and B&H charges NY & NJ sales tax only: - 15″ 2.7GHz... Read more
Apple Ranks 9th In comScore Top 50 U.S. Digit...
comScore, Inc. has released its monthly ranking of U.S. online activity at the top digital media properties for May 2017 based on data from comScore Media Metrix Multi-Platform. * Entity has... Read more
10.5-inch iPad Pros available for up to $20 o...
B&H Photo has the new 2017 10.5″ iPad Pros available for up to $20 off MSRP including free shipping plus NY & NJ sales tax only: - 64GB iPad Pro WiFi: $649 - 256GB iPad Pro WiFi: $749 - 512GB... Read more
Three Off-The-Beaten-Track iOS Apps That Dese...
One of the great things about using iPads and iPhones is the vast selection of apps available for most anything you want or need to do. The three outlined in this article have been in my core app... Read more
Apple No. 1 Spot In Gartner Top 100 Vendors i...
Gartner, Inc. has unveiled the top global 100 vendors in IT in 2016 based on their revenue across IT (excluding communication services) and component market segments. In the Gartner Global Top 100:... Read more
Clearance iMacs available for up to $300 off...
B&H Photo has clearance 21″ and 27″ Apple iMacs available starting at $949, each including free shipping plus NY & NJ sales tax only: - 27″ 3.3GHz iMac 5K: $1999 $300 off original MSRP - 27″... Read more

Jobs Board

*Apple* News Product Marketing Mgr., Publish...
…organizational consensus on strategy and vision for publisher tools, authoring, and Apple News Format.Carries this strategy and vision across the organization to Read more
*Apple* Retail - Multiple Positions - Apple,...
Job Description: Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
Security Data Analyst - *Apple* Information...
…data sources need to be collected to allow Information Security to better protect Apple employees and customers from a wide range of threats.Act as the subject matter Read more
Lead *Apple* Solutions Consultant - Apple I...
…integrity, and trust.Success Metrics/Key Performance Indicators:Quantitative* Year over Year growth in Apple Product and Beyond the Box sales in the assigned Point of Read more
*Apple* Solutions Consultant till v%u00E5r...
…ethics, integrity, and trust.Success Metrics/Key Performance Indicators:QuantitativeYear over Year growth in Apple Product and Beyond the Box sales in the assigned Point Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.