Networking 4 – Transmission Media

How do we Connect Computers to Make a Network?

Revision & recap: from KS3 you should know about the difference between LAN and WAN and remember the pros and cons of networking https://www.bbc.co.uk/bitesize/guides/zc6rcdm/revision/1 Note the opening paragraph: “A network is two or more computers (or other electronic devices) that are connected together, usually by cables or Wi-Fi.”, because it’s the end of that sentence that this lesson is about.
You should also have looked at the hardware used to build networks in the previous lesson (NICs, switches, routers, etc.) – there’s a recap here (https://www.bbc.co.uk/bitesize/guides/zh4whyc/revision/5).

Starter Activity

Test yourself knowledge from previous lessons here: https://www.bbc.co.uk/bitesize/guides/zc6rcdm/test

Video to Watch

NB: To be slightly pedantic, in that video it says that Bluetooth uses microwave frequencies, and Wi-Fi uses radio waves. Both right, but they are actually the same thing: first of all, microwaves are radio waves (check out your Physics EM Spectrum module). Secondly, the range of radio (microwave) frequencies used by Bluetooth is the same as those used by Wi-Fi – namely the ISM band at 2.4GHz. It’s also the same band that your microwave oven uses, which is why, if you’re warming up your dinner and stood by the microwave oven keeping an eye on it (as you should), the music over your Bluetooth headphones might start dropping out, or the picture quality of the Mandalorian as you stream it over Wi-Fi might suffer until the microwave goes “ding”. Incidentally, Wi-Fi, as it’s so popular, has also now pushed up into other frequency bands such as the 5GHz U-NII band to get more space and higher data rates.
Some of this content was also covered in last lesson’s video if you want an alternative point of view:

As far as we’re concerned, there are two ways of connecting computers in a network, wired and wireless (ignoring “copying data onto a USB stick, pile of disks or box of tapes and carrying them over to the other computer“, which doesn’t really count, but can, on occasion, actually be the fastest way to transfer large amounts of data, as memorably noted by Prof. Andrew S. Tanenbaum, and colloquially referred to as “sneakernet“. Don’t assume that hi-tech is always best…).

You can also split the connections into “guided” and “unguided“. To guide something means to point it in the right direction and make sure it doesn’t wander off anywhere (like the guided roadways for the Bristol Metrobus). So you can see that wired connections are guided (they go where you want them to go), and wireless connections are unguided – when you transmit a wireless signal, it doesn’t just go where you want it to, it will spread out all over the place (even if it’s a focused, directed beam) and other people will be able to receive it (and possibly snoop on your messages).

Wired Transmission Media

The main types you’ll come across nowadays and need to know about are:

Ethernet Cable (IEEE802.3)

The ubiquitous (common!) Ethernet cables are copper cables that transfer data as electric signals, mostly in LANs. If you dissected one (don’t! At least, not one of the school’s ones!), you’d find multiple pairs, each pair wrapped around itself (see the video above) – they’re twisted together like that to reduce interference (interference, or noise, is the enemy of telecommunications – the more noise there is, the harder it is to get your data through in one piece, or at all). Ethernet was invented waaaay back in the early seventies by Bob Metcalfe, the idea coming from his PhD work. (I was fortunate enough to meet him (briefly – shook hands and that was it!) because he was giving a keynote talk and participating in a panel session at the Ethernet’s 40th Anniversary Celebrations in Dallas, Texas, which were part of a wider IEEE 802 meeting in November 2013 that I was attending for the Wi-Fi/802.11 sessions. Met Vint Cerf there too.)

To further reduce interference and improve shielding, you can also wrap an outer layer of metal (like cooking foil) around all the twisted pairs in the cable. If you do that, it’s called Shielded Twisted Pair (STP) – if you don’t, then it’s called Un-shielded Twisted Pair (UTP).

STP will give you a higher data rate (less noise) than UTP… but it costs more.

NB: the cable is just the physical medium, you also need hardware (the “physical layer” or “PHY” – chips, circuits and firmware on the NIC that control the electrical signals on the cable) and software (the “medium access control” layer, or “MAC” – firmware and software that coordinates how all devices share fairly and play nicely on the medium) just to connect a single device to the shared medium. Protocols like Ethernet (IEEE 802.3) and Wi-Fi (IEEE 802.11) are defined from the MAC layer downwards – and offer a consistent interface to the stack above – so you can write your software without having to care about whether the computer it eventually runs on uses Ethernet or Wi-Fi.
The higher layers of protocol stack (network layer, TCP/IP, HTTP, FTP, SMTP, applications etc.) are then needed to do anything useful on over the network (like talk to other computers, send emails, read webpages).

Coaxial Cable

Coaxial cables (so called because everything is wrapped around a single, central axis in layers like an onion) are even better shielded than STP, and so can support even higher data speeds – but it’s more expensive to make and therefore buy. Coax (pronounced co-ax) also typically has longer range (i.e. you can have a longer cable).

Some of the earliest version of Ethernet (IEEE802.3) used coaxial cables (we’re talking late 1970’s/early 1980’s!) before switching to the cheaper twisted-pair cabling described above (purely for cost / economic reasons).

Nowadays you’ll mostly see it used to connect your router out to your cable box (e.g. with Virgin Media) or used to connect TVs to TV aerials (e.g. your Freeview box).

Fibre Optic Cable

A quick detour over to the Science department is needed for this… think about glass/plastic prisms and beams of light – refraction to be specific. You can bend light as it goes from air to glass and vice versa. If you hit just the right angle, you can make it reflect too (called Total Internal Reflection) – you sometimes see it in nature when the surface of a body of water looks like a mirror (e.g. swim under the surface of a pool and look up). If you make a long length of glass, like a long hair or filament, and carefully aim a beam of light down it at just the right angle, you can make that beam of light bounce down it for a really long distance without any losses. In fact, if you use a laser as the light source, and start modulating the beam (i.e. flashing it on and off really fast – think Morse code with a torch but at Superman speed), you can send signals over massive distances and at really high data rates.
And, best of all, no interference from any outside electrical signals.
Congratulations, you’ve just discovered fibre optics and optical fibres 🙂

But, it’s expensive – you should be able to see a pattern here. The faster the connection, the more you have to pay for it.

Because of the cost, it’s mainly used for commercial WAN networks, e.g. the Virgin Media backbone (the bit in the street, rarely do you get fibre all the way to your home) or even between countries (e.g. London to the rest of Europe).

There’s a huge ongoing project in Bristol (partners including the University of Bristol, Bristol City Council, etc.) that is making use of spare optical fibre that has been laid over the city in the past, called Bristol Is Open, which allows researchers to try out new “Smart City” ideas – for more info, see https://www.bristolisopen.com/overview/.

Wireless Transmission Media

Big reminder/dose of common sense (like the one about sneakernet above) here, namely “Dr Haines’ First Law of Telecommunications” – always use a wire if you can. Don’t go wireless unless you have to (e.g. you need to be able to move around) – wireless is always noisier, lower bandwidth, more hackable, less private, more power-hungry and more expensive. But, it’s more convenient, so guess what we use most of the time nowadays 🙂
The main types you’ll come across nowadays and need to know about are:

Wi-Fi (WLAN, IEEE 802.11)

The IEEE802.11 dates right back to the 1990s. The very first version had three PHY layers to choose from (spread spectrum that we’d recognise today, frequency hopping that is like Bluetooth nowadays, and even a short-lived light-based version). The first versions offered a whopping 1-2 Mbps data rate…

Wi-Fi has spent most of its life operating in the same frequency band as Bluetooth, microwave ovens and wireless baby monitors (the unlicensed 2.4GHz ISM band), but has since expanded into the 5GHz band, the 60GHz millimetre wave band, and even (ironically) back to light (with the IEEE802.11bb amendment that’s due to be standardised in 2021).

The thing to remember with Wi-Fi is that it’s just a replacement for an Ethernet cable: it connects you to a LAN. The only reason it allows you to get to the Internet is because of all the other equipment in the network (routers etc.), just as with an Ethernet connection.

Wi-Fi Logo

Bluetooth (WPAN)

Bluetooth is named after King Harald Bluetooth, a Danish king that united all of Denmark’s warring kingdoms into one country – the analogy being that Bluetooth unites all devices through the magic of radio communications (this also explains the “Nordic rune” logo, see below).

Whilst Wi-Fi is a WLAN (wireless local area network), Bluetooth is a WPAN (wireless personal area network) – shorter range, fewer devices. Rather than connecting you to a LAN, Bluetooth is just meant to connect your personal devices together over a fairly short range (e.g. wireless headphones, smart watch, mobile phone, diabetes monitor, heart-rate belt, smart electric toothbrush, car stereo, etc.)

What Bluetooth does have in common with Wi-Fi is that it also uses that (crowded, noisy) 2.4GHz ISM band, so has to compete for bandwidth with everyone else.

To do that, to avoid the noise and the interference, it uses something called Frequency Hopping, first invented by the Hollywood actress Hedy Lamarr during WWII to remote control Navy torpedoes. Frequency Hopping, as the name suggests, avoids noise and interference by changing frequency and “hopping” around the frequency band according to a predefined pattern (specifically, it’s a 1MHz wide carrier bouncing around a 79MHz wide band, so it has 79 channels to choose from). If a frequency that it lands in happens to be noisy, that’s OK, it’ll move on and be in a clearer one momentarily – and it’s jumping 1600 times a second, so it really doesn’t stay on any one channel for long. There’s even a technology called “adaptive frequency hopping” which lets the system identify really noisy channels and not hop back there again in the near future (some people even have patents on specific ways of doing that identification – note the first name on the list of inventors at the bottom 😉 ).

Bluetooth logo

Cellular (Mobile)

Cellular – what your smartphone is connecting to when you’re not on Wi-Fi, i.e. 3G / 4G / 5G, is a horse of an entirely different colour. Where Wi-Fi and Bluetooth came from the computing industries, cellular has come from a different direction – telephony (landlines, switchboards and big national organisations such as BT and AT&T). So, even though it ends up doing a similar thing as far as you as an end-user are concerned, in the background it gets there in a very different way.

Thinking of Wi-Fi and Bluetooth, they’re duking it out, fighting for bandwidth, in the noisy, unlicensed, Wild West ISM band. Cellular, however, operates in entirely different bands (900MHz, 1800MHz, …) that have been auctioned off by all the governments around the world, paid for (costing millions if not billions in each country) by the network operators (Vodafone, EE, TIM, Sprint, etc.) and are now privately owned, for the sole use of each network.

So, no fighting in the band with other systems, much less interference, and the operators are in control: they tell your device what frequency to go on, how to connect, what data rate it’s going to get, whether it even can connect, and they can fine tune it all to work like clockwork. They’ve also paid for and built masts, base-stations and back-haul cables across most of the country. This is why you have to pay for your cellular data (and it’s rationed) but your Wi-Fi is free and never-ending: cellular is much more expensive to build, manage and run.

Originally cellular just did voice calls (0G and 1G analogue), then we started to be able to send data over the links and even text messages (that was during 2G “GSM”, the arrival of digital… I remember it well as I worked on it*), then data rates increased (3G) and now (4G, 5G) we’re looking at even greater data rates, and new applications (such as supporting the Internet-of-Things and dealing better with big crowded events – ever tried making a call at a football match or concert?).

* Your humble author, when he was working for NEC in the mid- to late-nineties, actually sent the first text message ever sent on any NEC mobile phone – other engineers had sent text messages on other companies’ phones already (and, indeed, the world’s first ever text message was sent on Vodafone by a British engineer called Neil Papworth in 1992), but having got the first NEC 2G hardware up and running, debugged the device driver software and installed the protocol stack, I sent the first text message any NEC phone had ever sent. Bit of a claim to fame at least.

Satellite

Satellite radio communications, as the name implies, involves satellites – man-made objects orbiting the Earth that we can bounce radio signals off of (some satellites literally just pass the signal on to another part of the Earth, like a bent-pipe, others do some processing such as boosting the signal, or error checking and forwarding).

The big positive is that satellite footprints (the area of the earth that they can send a signal too) can reach anywhere on the Earth, depending on the orbit of the satellite(s) – that’s why you see explorers and people in remote places (e.g. watch the pilot episode of Batwoman…) using satellite phones – you can spot a satellite phone because it has a massive antenna the size of a small baguette.

But that massive antenna hints to the disadvantage: it needs that massive antenna because satellites are a long, long way up. A mobile phone basestation is, at most (i.e. you’re in the middle of nowhere) 35 km away from you. The lowest of Low Earth Orbit satellites are 160 km away – geostationary orbit satellites can be 35,000 km up. So it requires a lot of power (and it can be susceptible to bad weather) … and there’s a lot of delay (think of live news reports when the reporter is talking via satellite link).

And, if you thought building and maintaining a cellular network is expensive, wait until you get a load of the bill for putting something into orbit and keeping it there. So it’s really, really expensive.

But it can connect computers that are a very, very long distance apart.

Satellite

Typical Speeds

Before we start looking at speeds (data transfer rates), for comparison, Virgin Media (for example) are currently offering packages ranging from 50-350Mbps, where Mbps is mega bits per second, i.e. 1,000,000 bits (binary digits) transferred every second. (1Gbps, 1 gigabit per second, is 1,000,000,000 bits per second; 1Tbps, 1 terabit per second, is 1,000,000,000,000 bits per second.)

And remember that’s the transfer rate at your router: if you start sharing it with your family (especially younger siblings streaming videos all the time, and parents doing video conferencing), you’re all sharing that peak rate. Even worse, if you connect to your router over a Wi-Fi connection (or some Heath Robinson contraption of Ethernet-over-power-line adaptors, mesh Wi-Fi notes and Wi-Fi relays just to get coverage everywhere in your Faraday-Cage-like 1930’s home…) then you’ll only get the data rate of the slowest link in that chain – if you’re connecting your laptop with an IEEE802.11-1997 connection running at 1Mbps, then even if you’ve optical fibre direct from your ISP coming in at the front of your house, the best you’re going to get is 1Mbps. So the data rate (speed) of your network is very important… hence this list:

  • Wired: Ethernet cable (Category 5, “Cat-5”): 100 Mbps
  • Wired: Ethernet cable (Category 5e, “Cat5e”): 1000 Mbps
  • Wired: Ethernet cable (Category 6, “Cat 6”): 10 Gbps
  • Wired: Fibre Optic (glass) – 100 Tbps
  • Wireless: Wi-Fi (landmark IEEE802.11 releases) – these are peak rates, the most you can get:
    • 802.11 (1997): 1-2Mbps
    • 802.11b (1999): 11Mbps
    • 802.11a (1999): 54Mbps
    • 802.11n* (2009): 600Mbps
    • 802.11ac (2013): 3Gbps+
    • 802.11ax (2019): 10.5Gbps+
  • Wireless: Bluetooth WPAN: 1-2Mbps (peak)
  • Wireless: Cellular – these are the rates you will get if the network grants you it (they own the channels, remember)
    • 2G (1990’s): 9.6kbps, up to 20kbps with GPRS multi-slot
    • 3G (2000’s): 144kbps – 384kbps, 2Mbps if stationary
    • 4G (2010’s): 10Mbps – 100Mbps (300Mbps theoretically)
    • 5G (2020’s): 50Mbps – 2Gbps (near Shannon limit, so just using more bandwidth to get the rate up)

* RE: IEEE802.11n – One of the earliest pre-802.11n systems (using a MIMO antenna array) was demonstrated in Geneva at the ITU-T World Fair in 2003 by an intrepid band of brilliant research engineers based here in Bristol…

Task

Which is the best networking solution for…

  1. Connecting my smart watch to my smart phone?
  2. Connecting my smart phone to the Internet while I’m sat in a cafe?
  3. Connecting my smart phone to the Internet while I’m walking down the street?
  4. Connecting my laptop to the Internet when I’m going to be working in different rooms during the day?
  5. Connecting my PC to the printer?
  6. Connecting my PC to the file server and to the Internet?
  7. Connecting my home to my Internet Service Provider?
  8. Connecting my network to another network on the other side of town?
  9. Connecting my network to another network on the other side of Europe?

Extended Learning

Draw the network topology (i.e. what transmission media are making each connection, what units and black boxes – e.g. wireless access point, router, file server, printer, what computers and devices are attached) of a network of your choice. It could be your home, or, if that’s not a very exciting network (or is too exciting and confusing because it’s evolved over two decades and is now held together with magic dust and electric tape), then feel free to imagine what the network might be like at school, or anywhere else you can think of.

Quizizz

To wrap up your learning on this, try this Quizizz quiz, “Year 9 – Computing – Networking – Transmission Media“.