4G and 5G RAN
Disclaimer
I should begin this article with a disclaimer: I know very little about radio networks. I have never worked with 4G or 5G. My knowledge simply comes from a few afternoons spent researching this topic.
For me, this was one of the hardest topics to research because material about 4G and 5G is so vast, and yet it feels like so little is really applicable to what is needed to be understood for the Cisco SP exams.
I have included some of the better materials I found at the bottom of this article. Additionally, if you are looking for textbook reading material, I would suggest From GSM To LTE-Advanced Pro and 5G: An Introductino to Mobile Networks and Mobile Broadband. Admittedly, I only read the beginning of the book, but what I read was well-written. It just felt overkill for learning 4G/5G for the exam.
If you feel lost at first with RAN, don’t be too concerned. Revisit the material a few times and it will start to make sense.
What that stated, let’s get on to the material!
RAN
RAN stands for Radio Access Network. 4G and 5G networks are split into two general parts: the RAN and CN (core network). I think of the RAN as the wireless last mile, which connects mobile devices to the wireless network. In 4G, the core network is also referred to as the EPC (evolved packet core). The core contains devices that control call signaling and user traffic. Packet core refers to the fact that traffic is packet-switched. In the previous wireless generations, the core was circuit switched.
Basic 3G/4G RAN “Block”
The RAN is made up of the atennas, radio components, and signal modulators. These connect to the core network via the “backhaul.”
A basic setup is pictured below. The antennas broadcast the radio waves, and connect to the radio unit via an RF cable. The radio unit connects via fiber to a baseband unit on the ground at the foot of the radio tower. The BBU and RU communicate via CPRI (Common Public Radio Interface) over this fiber, as opposed to Ethernet. CPRI requires a constant stream of traffic at a certain timing, which will become important later when we look at centralized RAN. (It necessitates the use of DWDM). The BBU is what controls the radio.
Distributed RAN (D-RAN)
When each cell tower has its own BBU at the foot of the tower, we call it D-RAN. The BBUs are all distributed, one per physical cell tower location.
Centralized RAN (C-RAN)
To save on costs, you can centralize the BBUs in a central office, also called a “BBU Hotel.” Because the BBU connects to the RU via fiber, you simply run a longer fiber cable to the BBU Hotel. The RU and BBU have a strict latency requirement due to the timing of the CPRI protocol they speak, so you can only have a max of 10-15km distance between them.
The problem above is that you are running a 10km fiber strand for every single cell tower. To solve this you can use DWDM. Each RU-BBU pair needs its own p2p circuit because of CPRI, so DWDM/CWDM is really the only option to save on fiber costs.
5G Cloud-RAN
CPRI is not suited to the higher bandwidth and low latency requirement of 5G. To address this, the functions of the BBU have been split into the RU, and two new parts: the DU (distrbuted unit) and CU (centralized unit). The DU handles real time functions, and the CU handles non-real time functions. The RU was of course already present in 3G/4G, but now it has the ability to preform some functions that were previously preformed by the BBU.
The RU is still placed directly at the attenas as before. The DU must be close to the RU, no more than 15km away, because it handles real time functions. The CU can be even further away from the DU, allowing you to centralize the CUs at an even more central point. The DU latency requirement is less than 75us (microseconds), but the CU latency requirement allows for up to 25ms. A single CU can manage several DUs in a 1-to-many fashion, whereas a single DU is needed for each cell tower.
You’ll notice in the diagram above that the DU and CU functions are virtualized. This is another benefit of the BBU split: the DU and CU functions can take advantage of traditional virtualization features. (Scale up/down as needed, run on commerical off-the-shelf hardware, etc).
Also notice that the DWDM mux/demuxs are no longer required. Instead of CPRI, 5G uses eCPRI (Enhanced CPRI) as the protocol between the DU and RU, which uses Ethernet. This means the fronthaul has now been “packetized” and no longer needs a p2p circuit per tower. The fronthaul can now simply use a router instead of DWDM equipment. This gives you the benefit of fast failover technologies such as TI-LFA if you have redundant fiber paths.
Acronyms
Acronym
Meaning
CU
Centralized Unit, handles non-real time functions, further from the radio
DU
Distributed Unit, handles real-time functions, close to the radio
BBU
Baseband Unit
CSR
Cell Site Router
D-RAN
Distributed RAN (BBU at the tower)
C-RAN
Centralized RAN (BBU “hotel”)
Cloud-RAN
BBU split into DU and CU and virtualized
Further Reading/Watching
This is a very good, consise video
5G Packet Based Fronthaul - BRKSPG-2065 from the 2021 Digital Cisco Live
Tech Field Day panel on 5G RAN Transport. I like this video because of the interaction between participants trying to understand the material in real time.
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