The world’s cellular networks join up exactly the way the world’s Internet service providers do, and 5G is a bet that three separate gains multiply.
Words you will meet
-
Internet Protocol Packet eXchange (IPX) — a private network joining cellular carriers.
-
5G New Radio — the 3GPP 5G standard. Most of what people mean by “5G”.
-
Millimeter wave millimeter wave Simple The very high radio frequencies 5G uses to go fast, which do not travel far. Precise The FR2 frequency group, 24 GHz to 52 GHz. It allows much faster data speeds than FR1, with two drawbacks: a much shorter range from base station to receiver, and high susceptibility to atmospheric interference — foliage and rain both cause problems outdoors. introduced in ch. 7 — open in glossary — the very high frequencies, fast and short-ranged.
-
Beam forming beam forming Simple Aiming a radio signal at one user instead of shouting in every direction. Precise What each antenna of a MIMO base station does: rather than broadcasting signals in all directions, it directs the signal at the user. It is how 5G raises spectral efficiency without the 17-fold increase in power that information theory would otherwise demand. introduced in ch. 7 — open in glossary — aiming a signal at one user instead of broadcasting.
-
Small cell station small cell station Simple A little extra base station filling a hole in coverage. Precise A station needed to fill coverage gaps between base stations and users, because millimeter wave signals are easily blocked by buildings and trees. In a highly populous region the distance between two small cells could vary from 10 to 100 meters. introduced in ch. 7 — open in glossary — a little station filling a coverage hole.
-
Network slicing network slicing Simple Cutting one physical network into several virtual ones with different promises. Precise A 5G core capability, managed alongside network function virtualization, that lets one physical network support different applications and services with different performance requirements. introduced in ch. 7 — open in glossary — one physical network, several virtual ones.
Why this matters
Two things happen here, and they are unrelated except that both close the chapter’s cellular story.
The first is a recognition. The global cellular network turns out to be a network of networks network of networks Simple The Internet is not one network but many separate networks joined together. Precise The structure of today's Internet: hundreds of thousands of access ISPs interconnected through regional ISPs and about a dozen tier-1 ISPs, together with PoPs, multi-homing, peering, IXPs and content-provider networks. introduced in ch. 1 — open in glossary , in the same sense chapter 1 meant it and for the same reasons. Nothing new has to be learned.
The second is an argument. 5G’s headline is a 100× capacity gain, and the book shows where that number comes from. It is one equation with three terms, and 5G raises all three.
A network of networks, again
Drag any device to rearrange the picture. Hover a link to see its rate, delay and length.
Two carriers, joined the same way two ISPs are joined. Play the second journey: when you roam, the visited carrier has to reach a database inside somebody else’s network, and this picture is why that is possible at all.
Read this diagram as text
- your phone — wireless link — base station
- base station — wired link — gateway
- base station — wired link — gateway
- base station — wired link — gateway
- HSS — the subscriber database — wired link — gateway
- gateway — wired link — public Internet and IPX
- public Internet and IPX — wired link — gateway
- gateway — wired link — base station
- gateway — wired link — base station
- gateway — wired link — base station
Your phone connects through a base station into its home network home network Simple The network a mobile device permanently belongs to. Precise A mobile device's permanent home, whose address the device keeps regardless of where it currently is. All of chapter 7's mobility discussion is about getting data to a device that is not currently on it. introduced in ch. 7 — open in glossary . That network belongs to a cellular carrier — the book names Verizon, AT&T, T-Mobile and Sprint in the United States, Orange in France, and SK Telecom in Korea.
The home network reaches other carriers, and the global Internet, through one or more gateway routers. Two carriers can join in either of two ways:
- over the public Internet, or
- over an Internet Protocol Packet eXchange (IPX) network — a managed network built specifically for interconnecting carriers [GSMA 2018a].
Two networks, one letter apart, and the same idea
The book makes the comparison itself. An IXP (Internet Exchange Point) Internet Exchange Point A building where many ISPs peer with each other; there are over 600 today. introduced in ch. 1 is where ISP (Internet Service Provider) Internet Service Provider Connects hosts, and other ISPs, to the rest of the Internet. introduced in ch. 1 s peer with each other, from section 1.3.3. An IPX is the cellular version: a managed network where carriers interconnect.
The two acronyms are nearly anagrams, which is unhelpful. The useful difference is who joins: Xchange Points join ISPs, and Packet eXchange networks join carriers.
So the global cellular network is a network of networks, exactly like the Internet. 4G networks can also peer with 3G cellular voice and data networks, and with the earlier voice-only networks.
This picture is what makes roaming work
Section 7.4.1 said a roaming device is authenticated by the HSS (Home Subscriber Service) Home Subscriber Service The 4G database holding what the home network knows about each subscriber. introduced in ch. 7 in its home network, reached by an MME (Mobility Management Entity) Mobility Management Entity The 4G element that authenticates a device, tracks where it is, and sets up its data path. introduced in ch. 7 in the visited one. That crosses a boundary between two companies.
Play the second journey above and watch the path. It is only possible because the carriers are interconnected — and it is why this short section sits where it does.
5G: what it is aiming at
The book sets a target and then measures 5G against it. The ultimate wide-area data service would offer gigabit connection speeds everywhere, extremely low delay, and no limit on how many users and devices a region can hold.
Such a service opens the door to applications that do not work today. The book names pervasive augmented and virtual reality, and control of both autonomous vehicles and factory robots over wireless links. It also names fixed wireless, which replaces DSL (Digital Subscriber Line) Digital Subscriber Line Home Internet access over the existing telephone wire. introduced in ch. 1 and cable with a radio link from a base station to a modem in a house.
5G is expected to make a big step towards that target. The prediction the book quotes [Qualcomm 2019] is a 10× increase in peak bitrate, a 10× decrease in delay, and a 100× increase in traffic capacity over 4G.
”5G” is not one thing
Principally it means 5G New Radio, the standard adopted by 3GPP. But other 5G technologies exist. The book’s example is Verizon’s proprietary 5G TF network, which operates on 28 and 39 GHz and is used only for fixed wireless Internet service, never in smartphones.
| eMBBEnhanced Mobile Broadband | URLLCUltra Reliable Low-Latency Communications | mMTCMassive Machine Type Communications | |
|---|---|---|---|
| What it buys you | |||
| Aimed at | |||
| State of play as the book was written |
Cells marked ⓘ have a reason behind them — click to read it.
The book is explicit that 5G is not one cohesive standard [Dahlman 2018]. Click a cell for the reason.
The frequencies, and the price of them
| Group | Frequencies | Where it was deployed first | What it costs you |
|---|---|---|---|
Cells marked ⓘ have an explanation — click to read it.
Everything interesting about 5G follows from this table. Click a cell for the reason.
5G is not backward compatible, and this is expensive
The physical layer of 5G is not backward-compatible with 4G systems such as LTE (Long-Term Evolution) Long-Term Evolution The dominant 4G cellular standard. Its architecture is a set of gateways plus a subscriber database. introduced in ch. 7 . In particular, it cannot be delivered to existing smartphones by upgrading base stations or pushing software updates.
So the transition to 5G requires carriers to make substantial investments in physical infrastructure. Compare this with every software change in chapters 4 and 5, which shipped as a new version of a routing daemon.
Where the 100× comes from
Capacity is the product of three terms [Björnson 2017]:
The units are the argument. Cell density is in cells/km², available spectrum is in hertz, and spectral efficiency measures how well each base station uses each hertz, in bps/Hz/cell. Multiply the units out and the cells and the hertz both cancel, leaving bps/km².
What each symbol means
- cell density — how many cells you fit into a square kilometre (cells/km²)
- available spectrum — how wide the frequency band is (Hz)
- spectral efficiency — how well one base station uses each hertz (bps/Hz/cell)
Read aloud: How many bits per second a square kilometre can carry is how many cells you pack in, times how much radio band each has, times how many bits per second each hertz is worth.
capacity40.0 Gbps/km^2
capacity = cell density x available spectrum x spectral efficiency
units = cells/km^2 x Hz x bps/Hz/cell
= 4 x 2.00e+9 x 5
cells and Hz both cancel, leaving bps/km^2
capacity = 40.0 Gbps per square kilometre
4G LTE works up to about 2 GHz; 5G FR2 runs 24-52 GHz, a band 28 GHz wideChange any number above and the arithmetic re-runs, carrying the units through.
Move one slider at a time. The book’s claim is that 5G raises all three at once — and the answer is a product, so three modest gains multiply into a large one.
| Term | Unit | Why 5G’s is larger |
|---|---|---|
Cells marked ⓘ have an explanation — click to read it.
Multiply three larger numbers together and you reach the book’s 100× claim. Click a cell for the argument.
Raise all three and the products compound. The book’s conclusion is a 100× increase in capacity in urban areas. And because the frequency band is so much wider, it expects peak download rates of 1 Gbps or higher.
The catch, in one paragraph
Millimeter wave signals are easily blocked by buildings and trees. Small cell stations small cell station Simple A little extra base station filling a hole in coverage. Precise A station needed to fill coverage gaps between base stations and users, because millimeter wave signals are easily blocked by buildings and trees. In a highly populous region the distance between two small cells could vary from 10 to 100 meters. introduced in ch. 7 — open in glossary are needed to fill the coverage gaps between base stations and users. In a highly populous region the distance between two small cells could vary from 10 to 100 meters [Dahlman 2018].
That is a lot of hardware for one city block. It is the honest price of the first term in the equation above.
The 5G core
The 5G core does the same three things the 4G core does: it relays data traffic from end devices, authenticates devices, and manages device mobility. It contains all the same elements — mobile devices, cells, base stations, the MME, the HSS, and the Serving and PDN gateways.
Chapter 5’s separation of planes, arriving in the cellular world a second time — and this time it is the reason the core can be pure software.
Read all steps as text
- 1 · The 4G core, for reference — The 5G core does the same three things the 4G core does: it relays data traffic from end devices, authenticates devices, and manages device mobility. It contains all the same elements — mobile devices, cells, base stations, the MME, the HSS, and the Serving and PDN gateways.
- 2 · The MME splits into two — The 5G core decomposes the 4G Mobility Management Entity into two functional elements. The Access and Mobility Management Function receives all the connection and session information from end-user equipment, but only handles connection and mobility management. The Session Management Function handles session management, talks to the decoupled data plane, performs IP address management, and plays the role of DHCP.
- 3 · The user plane is pushed to the edge — Control- and user-plane separation lets the User-Plane Function be distributed and pushed to the network edge. The 5G core consists purely of virtualised software-based network functions, and it also holds distributed servers and caches across the network, which cuts latency.
The 5G Core network manages all 5G mobile voice, data and Internet connections. It has been redesigned to sit better alongside the Internet and cloud-based services, and it holds distributed servers and caches across the network, which cuts delay. Network function virtualization, from chapters 4 and 5, and network slicing network slicing Simple Cutting one physical network into several virtual ones with different promises. Precise A 5G core capability, managed alongside network function virtualization, that lets one physical network support different applications and services with different performance requirements. introduced in ch. 7 — open in glossary are both managed there.
The differences from the 4G core are two:
- it is designed for complete control- and user-plane separation, which is chapter 5’s idea; and
- it consists purely of virtualised software-based network functions — NFV (Network Function Virtualization) Network Function Virtualization Running middlebox functions as software on commodity hardware. introduced in ch. 4 as the whole architecture, not as an implementation detail.
Where this leaves 5G
The book is careful. As of its writing in 2020, 5G is in early deployment and many of its standards are not yet final. It lists four open questions and answers none of them. Does 5G become a pervasive broadband wireless service? Does it compete successfully with WiFi (IEEE 802.11 wireless LAN) IEEE 802.11 wireless LAN Wireless LAN access; a user must be within a few tens of metres of the access point. WiFi is a trade name, not an abbreviation. introduced in ch. 1 indoors? Does it become critical to factory automation and autonomous vehicles? And does it take us a big step towards the ultimate wide-area wireless service?
Five slips in these four pages
- A doubled phrase. Page 574 opens “let’s take a step back at take a look at how the global cellular network is organized”. The “at” should be “and”.
- A wrong word. Page 575: the home network “is connected to the networks of other cellular carriers and to the global Internet, though one or more gateway routers”. It means through.
- A contradiction. Page 575 describes the ultimate service as having “unrestricted limitations on the number of users and devices”. A limit that is unrestricted is not a limit. The sense wanted is no limitations.
- A wrong cross-reference. Page 577 says the 5G core contains all the network elements “that we encountered in Section 7.4.2”. Section 7.4.2 is the protocol stacks; the elements it then lists — the base stations, the MME, the HSS and the two gateways — are all introduced in section 7.4.1.
- A stray word. Page 577: “there are some major differences in that the new 5G core architecture”. Drop “that”.
Check yourself
Check yourself
0 of 8 answered1.Two cellular carriers need to exchange traffic. What are their options?
2.predictYou are roaming on a carrier that is not yours. Which network holds the database that says who you are, and how is it reached?
3.Why is the short range of millimeter wave frequencies not purely a drawback?
4.predictUsing the calculator: you double the available spectrum and leave the other two terms alone. What happens to capacity?
5.Information theory says doubling spectral efficiency needs a 17-fold increase in power. How does 5G get around this?
6.Which of the three 5G standards was still being standardised as the book was written?
7.The 5G core splits the 4G MME into two. Which half plays the role of DHCP?
8.Why can a carrier not deliver 5G by upgrading its base station software?
What to remember
- Cellular carriers interconnect over the public Internet or over an IPX, the cellular counterpart of an IXP. The global cellular network is therefore a network of networks, and that is what lets a visited carrier reach your home subscriber database.
capacity = cell density × available spectrum × spectral efficiency, and 5G raises all three — which is where 100× comes from. Doubling efficiency by raising power would cost 17× the power, so 5G uses MIMO (Multiple Input Multiple Output) Multiple Input Multiple Output Using several antennas at each end to raise the rate over one channel. introduced in ch. 7 with beam forming instead.- 5G’s physical layer is not backward compatible with 4G, so the transition costs physical infrastructure rather than software. The 5G core is purely virtualised software, splitting the 4G MME in two and pushing the user plane to the edge.