Tutorial: How do IP packets and MAC Frames go together?
It is important to understand how packets and frames are related, and in particular, IP packets vs. Ethernet or MAC frames.
Simple network example. Routers move packets from one circuit to another.
Packets are for networks. A packet is a block of user data, such as a piece of an e-mail message, with a network address on the front. The network address is the final destination. The standard for network addresses is IP.
Network equipment like routers receive an IP packet on an incoming circuit, examine the indicated destination IP address, use it to make a route decision, then implement the decision by forwarding the packet to the next router, on a different circuit.
A frame is a lower-level idea. Frames are used to communicate between stations on the same circuit. The circuit may have multiple stations physically connected onto it, like a wireless LAN, a few stations connected by a LAN switch, or only two stations like a point-to-point LAN cable. Each station has a Media Access Control (MAC) address, sometimes called a hardware address, link address or Layer 2 address.
A frame has framing to mark the beginning and end, sender and receiver MAC addresses to indicate the stations on the circuit, control information, a payload and an error detection mechanism.
The frame is transmitted on the circuit, and all stations on the circuit receive it. If an error is detected at a receiving station, the frame is discarded and might have to be retransmitted somehow.
If no errors are detected, the receiver compares the destination MAC address on the received frame to its own MAC address, and if they are the same, processes the frame, extracting the data payload and passing it to higher level software on the receiver.
If the MAC addresses are not the same, the receiver ignores it and waits for the next one.
The end result is that the payload in the frame is communicated to the correct station on the same circuit, with no errors.
Packet with its IP address vs. frame and its MAC address
The main purpose of packets is to append an IP address to your data. The IP address is used by network equipment to make route decisions: to relay the packet from one circuit to a different circuit. This is accomplished by receiving the packet then transmitting it to a different machine, usually the next router in the chain.
To actually transmit a packet to another router, the packet is inserted as the payload in a frame, then the frame is broadcast on the circuit that connects to the next router.
Notice that there are two addresses: the IP network address and the MAC address.
The IP address on the packet is the final destination, and so does not change. The MAC address on the frame indicates the destination on the current circuit, and so is changed as the data is forwarded from one circuit to another.
This and related topics are covered in:
• Instructor-Led Course 101
Telecom, Datacom and Networking for Non-Engineering Professionals
• DVD-Video V3 Fundamentals of Datacom and Networking
Telecom 101 – Fourth Edition, 2016 released and on sale!
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Telecom 101 covers the core knowledge set required in the telecom business today: the technologies, the players, the products and services, jargon and buzzwords, and most importantly, the underlying ideas… and how it all fits together.
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Last month, the days were getting shorter and shorter, and the sun was reaching a lower and lower height in the sky every day.Tutorial: Packetized Voice
This is Section 2.9.1 of the new Telecom 101, 4th edition
print and ebook available January 2016
.
Brought to you by BOOT CAMP
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– – –
This diagram provides a very high-level block diagram view of the processes involved in communicating speech in IP packets from one person to another:
Voice in IP Packets End-to-End
Starting on the left, commands from the speaker’s brain cause combinations of lungs, diaphragm, vocal cords, tongue, jaw and lips to form sounds.
A microphone is positioned in front of the mouth and acts as a transducer, creating a fluctuating voltage which is an analog or representation of the sound pressure waves coming out of the speaker’s throat.
This is fed into a codec, which digitizes the voltage analog by taking samples of it 8,000 times per second and coding the value of sample into binary 1s and 0s. Typically, the value of each sample is represented with a byte, meaning 64 kb/s to be transmitted.
Approximately 20 ms worth of coded speech is taken as a segment and placed or encapsulated in an IP packet.
The IP packet begins with a header, which is control information, the most interesting part being the IP address of the source telephone and IP address of the destination telephone.
IP packets are moved from the source to the destination over a sequence of links.
The links are connected with routers, which relay the packets from one link to the next.
Lower level functions such as framing and link addressing are usually performed following the IEEE Ethernet and MAC standards.
At the lowest level, the links are physcially implemented with Category 6 LAN cables, DSL modems, Cable modems, fiber optics and radio systems.
At the destination, the bits are extracted from the IP packet and fed into a codec, which re-creates the analog voltage.
This voltage drives a speaker, which re-creates the sound pressure waves, which travel down the ear canal to the inner ear, causing hairs on the cochlea to vibrate, triggering neural impulses to the brain, making the listener think they are hearing something.
– – –
It’s important to note that the voice packets are communicated directly from one telephone to the other over the IP network.
The packets do not pass through a CO telephone switch, for example.
– – –
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Development #4: A Worldwide Standard for Mobile Wireless
A closer look at the fourth item in our list of eight major recent developments and trends in telecom.
Telecommunications technology is constantly changing and improving – seemingly faster and faster every year – and at Teracom, we keep our training courses up to date to reflect these changes. In a previous post, we identified eight major developments and trends in telecommunications incorporated in our training.
In this post, we take a closer look at the fourth development:
a worldwide standard for mobile wireless has finally been achieved with 4G LTE.
Mobility means it is possible to start communicating with a particular radio base station, then when moving physically away, be handed off to another base station down the road to continue communications uninterrupted. In a non-mobile system (like WiFi), communication ceases if you move too far away.
The first generation (1G) of mobile radio was characterized by analog FM on frequency channels. Numerous incompatible systems were deployed: AMPS in North America, TACS in the UK, NMT in Finland and others.
The second generation (2G) was digital, which means modems communicating 1s and 0s between the handset and base station. Again, several incompatible systems were deployed, and two warring factions emerged, which could be called the “GSM/TDMA faction”, and the “CDMA faction”.
By far, the most popular 2G system was GSM, a European technology where a number of users time-share a single radio channel. Another system was IS-136, called “TDMA” in North America, deployed by the company currently known as AT&T Wireless in the US and Rogers in Canada.
A less popular 2G system employed CDMA, using technology patented by American company Qualcomm, and deployed by Verizon, Sprint and Canadian telephone companies.
These 2G systems were totally incompatible. A basic phone from a carrier could not work on another carrier unless they both used exactly the same system.
To try to avoid a repeat of the incompatibility for the third generation, the International Telecommunications Union (ITU) struck a standards committee in year 2000 called IMT-2000, its mission to define a world standard for 3G.
They failed. IMT-2000 instead published a 3G “standard” with five incompatible variations. The two serious variations were both CDMA – but differed on the width of the radio bands, the control infrastructure and synchronization method among other things.
The GSM/TDMA faction favored the deployment of CDMA in a 5 MHz wide band. This was called IMT-DS, Direct Spread, Wideband CDMA and Universal Mobile Telephone Service (UMTS). Its data-optimized version was called HSPA.
The CDMA faction favored a strategy that was a basically a software upgrade from 2G, employing existing 1.25 MHz radio carriers. This is called IMT-MC, CDMA multi-carrier, CDMA2000 and 1X. Its data-optimized version was called 1XEV-DO.
Again, these 3G systems were completely incompatible. A basic UMTS phone could not work on a 1X network.
Market forces finally pushed the two camps together.
The fact that there were far more users in the GSM/TDMA faction meant that their phones were less expensive, had better features and appeared on the market first. This put the carriers in the CDMA/1X faction at a disadvantage. This trend was continuing into 3G, where UMTS phones would have the same advantage over 1X phones.
Then, Steve Jobs invented the world’s most popular consumer electronic product, the iPhone – but only permitted carriers in the GSM/TDMA/UMTS faction to have it. This severely tilted the playing field.
In the face of this, the CDMA/1X faction threw in the towel, and decided to go with the GSM/TDMA/UMTS faction’s proposal for the fourth generation (4G), called LTE, to level the playing field.
And once this was agreed, Steve Jobs allowed the iPhone on all networks. One of the legacies of Steve Jobs will not just be the iPhone, but ending the standards wars by pushing the industry to agree on LTE as a single worldwide standard for mobile communications as of the fourth generation, using the leverage of his iPhone.
I hope you’ve found this article useful!
If you like, you can watch a video segment of our instructor explaining why LTE became a worldwide standard on youTube
Additional explanation of cellular concepts, TDMA, CDMA and LTE is available in:
– Teracom online tutorials
– Course 101: Telecom, Datacom and Networking
for Non-Engineering Professionals
– Textbook T4210 Telecom, Datacom and Networking
for Non-Engineers
– Online Course 2206 Wireless Telecommunications
(part of the CTNS certification coursework)
– Online Course 2232 Mobile Communications
(part of the CWA certification coursework), and
– DVD Course V6 “Wireless”