Foreword
========

When I was a graduate student studying parallel computing in the early
1990s, the World Wide Web exploded onto the scene, and I wanted to
understand how the Internet made such rapid advances possible. I
picked up a copy of the first edition of *Computer Networks: A Systems
Approach*, and I started reading. I was delighted to learn not only
the Internet’s history and main concepts, but also to see examples of
real code that illustrated how the protocols actually worked and how
they could be changed. I was hooked. I loved the idea of working in
the computer networking field, where the technologies we build can
help bring people together and lower the barriers to innovation so
that anyone who can write software can contribute.

As time went on, I saw that, while the Internet enabled rapid advances
in end-host applications, the inside of the Internet was harder to
change. Network devices were closed and proprietary, and protocol
standards evolved slowly and painfully. In response, enabling
innovation inside the network became a passion of mine, and of many
other technologists eager to make the Internet better–more secure,
performant, reliable, cost-effective, and easier to manage.
Gradually, the computer networking field changed, with the advent of
software-defined networking, giving network owners–such as the
hyperscalers running large data centers–much more control over the
software inside their networks. (See *Software-Defined Networks: A
Systems Approach* for more of this story!) Nowadays, computer
networking really is about software. It’s a welcome change.

In recent years, the excitement has moved to the network edge with the
emergence of 5G cellular access networks. No longer are wireless
communication, computer networking, and cloud computing siloed parts
of some larger ecosystem. They are brought together, allowing wireless
devices to connect to nearby computing resources. Far beyond providing
mobile Internet access, these networks enable exciting real-time
applications like augmented and virtual reality (AR/VR), Internet of
Things (IoT), self-driving cars, drones, robotic control, and more.
Plus, these networks are not only the purview of large carriers that
must invest significant resources in wireless spectrum, cell towers,
and more. Rather, individual enterprises, campuses, and communities
are deploying private 5G within their own organizations to support
their own applications, using lightly regulated spectrum like CBRS
(Citizens Broadband Radio Service). Opportunities for innovation
abound!

However, now there is so much more to learn, to understand the many
parts of 5G access networks as a single coherent system. The learning
curve can be steep. Even fairly technical people usually know one or
at most two parts of the system, and do not know how the parts relate
to the larger whole. For example, I knew a good amount about computer
networking, a little about cloud computing, and not much about
wireless communication. *Private 5G: A Systems Approach*–this
book–changed that. I learned what I needed to know about the radio
access network, and how the pieces come together to be more than the
sum of their parts. More than that, the book shows that, despite the
complexity of the 3GPP cellular standards and the long and frustrating
history of closed network equipment in the cellular networking space,
open source software is gaining a foothold. Open source platforms like
Aether and Magma–both discussed in this book–are seeing practical
deployment in a wide variety of settings. The book even has a guide
for readers to bring up the Aether platform, including a 5G small
cell.


For me, then, the story comes full circle. Private 5G networks are
something you can touch, code, and deploy yourself. Armed with the
knowledge of how 5G access networks work, and with hands-on experience
with open source software, just imagine the places you’ll go!

| Jennifer Rexford
| Princeton, New Jersey