Ethernet is such a commonly used term these days, but few of us know the complete picture of this technology. I’m no expert on Ethernet, but having read multiple documents and links recently, I thought it would be good to write down the details of this technology.

Ethernet is a technological standard which originated out of PARC labs at Xerox. Robert Metcalfe is credited with the invention of this technology. But it is also a standard which is quite confusing since the IEEE specification is not generally used. To top it all, there are around four types of frame structures. Confused? Let’s get started from the basics.

What is a Network Topology?

A network topology is a connection layout of the elements in a network. Common topologies are Point-to-point, Bus, Star and Ring.(


Data terminal equipment (DTE) represents any node at which a frame terminates. It is usually the end user node. Data communication equipment (DCE) represents any node which forwards/switches frames towards its destination node. These are usually switch/bridge in an L2 network.

Types of Delay

Propagation delay/time is the time taken for a frame to travel on the wire between two nodes. Transmission delay/time is the time taken for a node to transmit the frame (including the headers) from its buffer onto the wire. Queuing delay is the time any frame waits in the queue of a node. A node may drop frames is its queue gets full. Processing delay is the time taken by a node to process the frame it receives.


The primary protocol which (was) used in Ethernet was Carrier Sense Multiple Access with Collision Detection (CSMA/CD). In this protocol, multiple nodes can access the medium of transmission at the same time. Hence, the name Multiple Access. Carrier Sense implies that the node first senses the medium (detects electrical signals) before transmitting. The node will transmit only if it finds that the medium is free, else it backs off its transmission for a random time and retries again. But well, things could go wrong. If two or more nodes do transmit at the same time (electric signals do take time to travel you see!) then a collision occurs, meaning the signals from the nodes overlap each other and the original signal is lost. This garbled signal will reach either of the transmitting nodes, and this node sends a signal on the medium. This is called the jamming signal. Hence other nodes wanting to transmit back off due to this and hence further collisions are saved. Hence the term Collision Detection. Now this phenomenon can occur if there is a contention for the medium. If the topology is Bus, then there is a scope for a collision. If two nodes are connected to a central node in a point-to-point method, then there can be no collision between the central node and any node.

When the same band is used for transmitting and receiving, once two nodes sends out a signal each, the signals overlap midway and reach each node. So the maximum time required for a node to detect a collision is approximately twice the signal propagation time. Also as the transmission rate increases at a node, the time taken to send out a frame decreases. But the signal propagation time is still the same irrespective of the transmission rate. Hence the way forward for better efficiency was to reduce collisions and decrease propagation time. Decreasing propagation time meant decreasing the distance between two node, which is not practical always.

Alternate to CSMA/CD ?

The method developed to overcome the above shortcomings was to use Full Duplex transmission. CSMA/CD is needed when Half Duplex transmission is used, i.e. the same channel/wire is used to send and receive frames. In Full Duplex mechanism, there is a separate channel/wire for sending and receiving frames. But this would give raise to a new problem. The transmission wire of the sender and the receive wire of the receiver have to be connected. Else Full duplex communication would not work right? Yes, this would be true when you are connecting two DTE’s point-to-point. There links are called Cross-over links or Cross cable. In usual cases, when there is DCE(s) present between two nodes (DTE), the cross over mechanism is implemented in the DEC i.e. the switch or bridge performs the swapping. Hence when connecting user nodes to a switch or bridge, a straight cable is used.

OSI Model

The Open Systems Interconnect model is a reference protocol stack model proposed by ISO. Layer two of this model is the Data Link layer. The function of the data link layer is to propagate frames between two adjacent nodes, perform flow control between two adjacent nodes, provide error detection/correction and interact with the physical layer.


Media Access Control (MAC) sub-layer is the lower part of Layer 2. The Logical Link Control (LLC) sub-layer is the upper part of Layer 2. The purpose of the MAC layer is to perform medium access, implement CSMA/CD if required, and interact with the physical layer. The LLC sub-layer performs multiplexing/demultiplexing to the network layer, provides flow control and automatic repeat request (ARQ) if needed.

Raw Ethernet

This standard was developed by Novell specifically to carry IPX packets in a local area network. It is no longer in use today.

Ethernet, Ethernet-II

Ethernet is a layer 2 protocol and standard initially developed by Xerox. Later, Digital Equipment Corporation (DEC), Intel and Xerox together refined the specification and Ethernet-II (also called DIX Ethernet) was born. Ethernet used half duplex links along with CSMA/CD protocol in a bus topology to connect the nodes. Ethernet-II uses full duplex links in a star topology to connect nodes. Ethernet-II is now popularly called just Ethernet.

The Ethernet frame encapsulates the packet received from the network layer and transmits it to the next hop. Each host implementing the Ethernet standard is connected using connectors and wires to the next hop which may be a DCE or DTE (more on wires and connectors below). Each host is thus represented by a 48 bit address called MAC address. When a host is transmitting a frame, it should know the MAC address of the destination node, else transmission would not be possible.

The first field of the frame is the preamble which denotes to the receiving node that a frame is about to arrive. Next is the start of frame byte which denotes end of preamble and beginning of the frame. The next two fields are the destination and source MAC address, which are 6 bytes each. The next field is EtherType which is used for demultiplexing. The value in EtherType identifies the network layer protocol at layer 3. Next, the payload forms the majority of the frame. The frame ends with a frame check sequence for error detection. Consecutive frames are transmitted separated by and Inter-frame gap. So far, so good. But where does the LLC sub-layer fit in this standard. The answer is DIX Ethernet doesn’t use any LLC sublayer.  The EtherType field is used to demultiplexing and flow control is left to the upper layers. The popular values for EtherType are available here:

IEEE Ethernet (802.2 and 802.3)

DIX Ethernet was in use widely but was not a standard blessed by anybody! Hence, IEEE comes into the picture. Since DIX Ethernet didn’t have an LLC sub-layer, it didn’t strictly conform to the OSI model. Hence IEEE replaced the EtherType field with the Length field. Hence additional mechanism was needed to identify the network protocol, which was the job of LLC header. IEEE 802.2 standard defined the LLC sub-layer and its frame format. This LLC frame was inserted in the Ethernet Type to make the model consistent with OSI model and the result was the 802.3 standard. There are many variations of the 802.3 depending on the media used for the links.

The LLC header consisted of three fields, S-SAP, D-SAP and Control field. It defined each host (here, the network layer) as a Service Access Point (SAP). S-SAP represents the source and D-SAP represents the destination. If the first bit of DSAP is 0, it represents an address of a single host. If it is 1, it means the DSAP is a group of hosts (i.e. multicast group). The different DSAP values can be found here:

The control field represents whether LLC is Type 1, 2 or 3. Type 1 is unacknowledged connection-less session, meaning there is no flow control. Type 2 means it is an acknowledged connection-oriented session, which means flow control is implemented in layer 2. Type 3 is Acknowledged connection-less session, which means flow control is present, but no concept of session between two nodes.

Now since the DSAP field is used to identify the network layer protocol, and 7 bits are useful in that field, 128 possible protocols can be identified. Also it is a requirement that these values are standards, thus providing no room for proprietary or experimental protocols. And what I call as glaring oversight, ARP is not a standard DSAP, which means IPv4 protocol will not work on IEEE 802.2 Ethernet frame!


After the above problem was identified, it was necessary for another fix. Sub-network Access Protocol (SNAP)! Though it is called a protocol, it is really a 5 byte header extension to the LLC header. The first three octets is an IEEE OUI (See References). The next two bytes represents the Protocol ID which specifies the network layer protocol. This means, IEEE-SNAP Ethernet can support 64k network protocols. But this is exactly the support provided by DIX Ethernet without all these complications right? Precisely!

As a result, the processing required for IEEE-SNAP and IEEE-802.2 frame is more that that of DIX frame, not to mention the reduction in IP payload as well. Hence DIX Ethernet is the most commonly used Ethernet standard. However, IEEE-SNAP frame has been standardized in RFC 1042.

Where does VLAN’s fit in?

Virtual LAN (VLAN) provides a mechanism to separate the collision domain. This is achieved by tagging each frame and treating them as part of a different LAN. Hence when data has to travel between VLAN, it has to pass through a Layer 3 device much like the case between two LAN’s.

The IEEE 802.1q (also called dot1q) represents a VLAN tagging mechanism to achieve the separation of the collision domains. This is achieved by adding 32 bits in the Ethernet header between the EtherType or length (depending on the frame). In its simplest form, the two fields in 802.1q tag is the Tag Protocol Id (TPI) and Tag Control Information (TCI). TPI is set to 0x8100 for dot1q tagging. The TCI consists of the VLAN id and priority information for the frame. Do check this:

Confusion Receiving?

Well, since there are four types of Ethernet frames, a receiving node would be confused as to which type of frame it is receiving. Obviously!

Right after the Source MAC, if a TPI is seen, it means that the frame is VLAN tagged, else it should either be EtherType or Length field.

The maximum length of the payload for IEEE 802.3 frame is 1500 bytes. Hence if the field after Source MAC (and VLAN tag if present), is less than or equal to 1500, it is a IEEE-802.2 frame. Once the 802.2 frame is read, if the DSAP and SSAP are set to 0xAA, then SNAP header exists. Else it it just LLC header.

If the field after Source MAC (and VLAN tag is present) is greater than of equal to 1536, it is a DIX/Ethernet-II frame. Hence all EtherType have values greater than 1536. Example it is 0x0800 for IPv4.

Media types

Well since we talked about the Ethernet implementations at the nodes, how about moving to the actual wire. The transmission medium can be copper or optical fibers. The transmission mechanism is base-band. It means the entire bandwidth offered by the medium is utilized. Also it uses digital signals.

In broadband transmission, it is possible to have multiple transmissions simultaneously using frequency division multiplexing. It uses Analog signals and needs separate channels for transmission and reception. Broadband transmission is commonly used for cable television.

We do know that flow of electrons generates a magnetic field perpendicular to it. Hence when two such wires are twisted together, the interference caused by their respective magnetic fields cancel out each other. This medium is called Twisted pair. It maybe provided with a shielding resulting in Shielded Twisted pair (STP) or no shielding giving Unshielded Twisted Pair (UTP). These cables categorized in different categories like Cat 5, Cat 6, etc depending on their quality and length supported between two stations. Another medium used is the coaxial cable which has a single wire for transmission and a protection layer around it. It is also possible to convert the electrical signals to light and transmit light using optic fibers. These different medium are connected to the nodes using connectors. RJ-45 connector is the most common connector we come across. BNC is another type of connector, typically used with coaxial cables.

The IEEE 802.3 specifies various standards stating the required conditions for connecting various nodes. Let’s expand 10BaseT also called IEEE 802.3i standard. 10 represents 10Mbps transmission speed at the node. Base represents base-band transmission on the wire. T represents the wire is twisted pair type. 10BaseF represents 10Mbps transmission speed with base-band transmission over optic fiber cable.

I do hope at the end of this long article, you are still with me and haven’t closed your browser! I hope I have been able to provide some insight into the world of Ethernet which is as you know now is very complex. I’m not an expert, I’m constantly learning as you do, and will update this article as and when needed. So do remember, when you say that you are connecting an Ethernet cable next time, it probable you are connecting a Cat5 or above cable using 10BaseT or 100BaseT IEEE 802.3 standard with an RJ45 connector at each end to equipment/node which has support to transmit and receive IEEE802.2, IEEE802-SNAP and DIX Ethernet at the Data Link layer, apart from all the other things that happen magically!


Is the 930 Iconic?

What is the 930? Why does it have to be iconic? Well, I’m referring here to the Nokia Lumia 930, also sold as Nokia Lumia Icon in the USA. As a user of this new phone, I would like to pen my thoughts as to whether it does live up to its name in the States.

First things first, for some strange reason, this phone is not (yet) available in India. Even though it was listed on Nokia’s India website, it isn’t now. Flipkart and Infibeam have a “Coming Soon” tag from a few months. So if you live in India and want to lay your hands on this phone, head over online to or any other website who ship internationally. I had a good experience with clove, hence the mention here.

After being a staunch Symbian & Nokia user for more than 7 years, I decided it was time to move on to a phone with better hardware and what the world now calls as “modern OS”. Being a Symbian user, I had gotten used to a few things, and few more were taken for granted. As we all know, Symbian OS based phones are no longer sold nor are supported. When they did however, they did rule the smartphone world introducing many firsts. Also having owned five Nokia phones in the past 7 years, you kind of get used to the legendary build quality, the amazing camera and the works. But my last phone, Nokia 701 started showing signs of aging. Being a 2010 product, it wasn’t snappy or quick anymore, the web browsing sucked, no support in terms of os or app updates and the camera doesn’t hold a chance in today’s gadget world.

Having considered the HTC One (M8) – too expensive, Sony Xperia Z2 – too expensive, I kept debating between the Nokia Lumia 930 – first choice or HTC One (E8) – in case I wouldn’t be able to get my hands on 930. I did want to steer clear of Android or iOS for my own personal reasons (which is not up for debate here, just my choice!). Eventually I ordered the 930 from Clove and received the handset in exactly a week. There was no trouble at all from the ordering to shipping and the delivery process.

Coming to the phone, the first thing that strikes you is the build quality and the solid feel. The aluminium band on the side gives a premium feel and the phone feels good to hold in the hand. The screen is a gorgeous 5″ full HD AMOLED display (what ever AMOLED means!), it just looks gorgeous, videos and images are vivid and clear and sunlight legibility is excellent. There are volume, power and camera buttons on the right side and they feel solid and built to last. The screen surface feels buttery smooth and I resisted a lot to put on a scratch guard! The phone has built in wireless charging and the package I received had a wireless charger. I was skeptical whether it would work as well as a normal charger, but it does work very well. Make sure the bottom half of the phone is in contact with the charging pad, the pad will emit a small white light when it is charging the phone.

The phone runs Windows Phone 8.1 out of the box, with customizable backgrounds and themes and different sized live tiles. There is a pull down notification bar for your one touch toggles and notifications, though I feel it needs improvement. There is no toggle for Mobile Data, the Bluetooth button toggles between on/off but the Wi-Fi button takes you to the Wi-fi menu. Why Microsoft?

The Volume buttons allow you to adjust the Call/Notifications and Media/Apps volume and a there is a toggle on the screen for vibrate. When playing music, the volume buttons provide an option to pause and switch to next/previous song. Coming form the Symbian world, you are used to profiles, but that feature here is alien. An App called “Volume” did fill in this functionality but doesn’t work on version 8.1. Microsoft, please help them make it work!

The call quality is excellent. Even in noisy conditions, I can hear clearly and I’m told I’m heard clearly as well! Something which had started to fade in my old 701. But the dallier doesn’t have smart dialing! Why?

The contacts/people menu is good and phone book is very well integrated into LinkedIn, Facebook, Twitter and Outlook. You can check the latest social happenings from your contacts just by navigating to their name in People and you can assign a custom ringtone per contact. You can also organize your contacts in groups, but that’s about it. You have no way to set a group ringtone, or SMS tone or make any other customization’s. MS, are you listening?

Messaging is decent, the word flow keyboard is excellent. You can type anything in a jiffy. But no folders in Messaging! Another “taken for granted” feature from Symbian phones of the past. You can’t select each message in a thread too, though a fix for this is coming. Please bring back the folders…..

Internet Explorer 11 is a pleasure to use. It just is. And I prefer it to Chrome from Android. Don’t believe me? Do check it out. Windows Store works well too. Though the quality of apps have improved drastically, it is still quite a catch up to Android.

The Call & SMS filter is decent. Though you can expect some non-spam numbers to be blocked. I couldn’t find any third party application for blocking phone numbers. Bad! Quiet Hours is a much awaited feature to come in the Denim update, so that I’m not woken up in the middle of the night to incoming mail, but can allow emergency calls. Bravo! This feature should be made mandatory on all phones!

The home screen is a disappointment for me personally. The 930 doesn’t support Glance, hence you will have to press the power button to know if you have missed any notifications. A pulsating windows logo would be very nice! Also the home screen can support only six notifications. Why? Why? Why?

But you do have an option to allow the background to change every now and then automatically. The Kid’s corner is a good concept. Now I do not have to worry about my phone or any apps when I hand it over to my little niece to play games. Though support for password protection of apps would be a big welcome. Are you listening MS?

The transparent Live tiles work very well. The tiles refresh automatically if the app supports it. Weather, Calendar, People, Photos and few others are excellent apps. All of them behave as expected. E-mail setup is a breeze and managing your e-mails is an absolute pleasure. Whats-app works very well too. I haven’t tried any other chat applications.

The live tiles can be grouped in a folder, with live folders expected to come in the Denim update. Alarms and Calculator does its job. Though I would prefer to use the app “ZDClock” to manage my reminders and alarms. MS, please allow customizable snooze times for the alarms! Files is a decent file manager, though you can do more with “Pocket File manager”. The Bing apps – Weather, Fitness, News, Travel, et al work very well. There is no default sound or call recorder, though fully functional third party apps are available. You can use “Transfer My Data” to transfer data from your old phone if it was a Nokia.

Shazam and Skype are pre-installed. Many popular apps are available on the Windows store. Squirrel is a good Pocket client, TOI , ET and NDTV take care of the news, Tripadvisor, MoneyControl, BookmyShow, Flipkart, Amazon, et al work very well.

OneDrive is amazing, many third party apps like myFinance (Expense manager) use it to sync files to the cloud. So you can access your files anywhere as long as you can access your Microsoft Account.

Video and Music playback is bliss. No faults here whatsoever. The speaker is loud and clear. Headphone output is amazing. Though surprisingly the package didn’t include a headset. Microsoft, come on, How much did you save here?

Nokia’s HERE maps and navigation is an absolute beast. It is extremely fast with places and City lens tightly integrated into it. Search works very well and auto saves the places. You can organize your favorites the way you want. Navigation works very well and Transit left me surprised. It does know the bus routes between ITPL and Koramangala and the transit points in between. I least expected this 🙂

Coming to the Camera, It’s a 20MP PureView CarlZeiss Camera which will snap a high resolution and an over-sampled picture every time you click. The camera button provides good feel for auto focus. The Camera app is great and allows you to set exposure, flash, ISO, white balance and the works. You even have a burst mode to click pictures in succession or a Panorama app to stitch a good panorama picture. The Storyteller app allows you to add captions to your photos and make a slide show kind of presentation while the Cinemagraph allows you to create an animated Photo. The photos I have snapped, in my experience, were in one word, AMAZING. It just nullifies the other negatives and niggles this phone has. Flash or no flash. The Camera just clicks great pictures very fast.

And that kind of seals the deal for me. The last “Nokia only made” phone, has served me well for the past few weeks and I haven’t gone back to my trusty old Symbian phone. It just works. No hanging, no sluggishness. Smooth quick transitions. I’m sure I will stick around to this phone for a long time to come. So is it Iconic? MS, fix the SMS folders, notifications, password protected apps and third party Volume control profiles, it truly will be iconic. Right now, Well It Almost Is!


Disclaimer: The above article is true to the best knowledge of the author and based on his first hand experience, opinion and preferences.