Saint's Log

Introduction to Networks – Part IV

Cellular networks are a ubiquitous part of modern daily life. The history of cellular technology is definitely worth knowing, even if only at the high level presented in this video on the Evolution of Mobile Standards (1G, 2G, 3G, 4G, and 5G).

2G introduced digital modulation and came in variants like TDMA, CDMA, and GSM. 3G uses spread spectrum in the radio portion of the network whereas 4G uses Orthogonal Frequency Division Mutiplexing (OFDM). 4G also separates the user and control planes whereas they were on the same hardware in 3G (and therefore couldn’t scale independently). The following videos from Sunny Classroom are brief but helpful explanations of these communications concepts.

5G offers lower end-to-end latency and higher uplink and downlink throughput than 4G because it has more bands (low, mid, and high) vs just low and high with 4G. It is also a programmable network, which lets developers access network stats via APIs.

Mobility

The discussion in the class proceeded to mobility, introducing the concept of cellular handoff, which can broadly be classified into mobile assisted and mobile controlled handover. See Handoff in Wireless Mobile Networks for more details. Another classification of types of handover is based on when the UE disconnects from one cell: hard handoff vs soft handoff. Soft handoff ensures that calls are not dropped. The professor was drawing hexagonal cells when illustrating these and I realized I had no idea why they are hexagonal. Here’s why:

antennas in a coverage area are in a hexagonal pattern… because it requires fewer cells to represent a hexagon compared to triangle or square – meaning network carriers can cover a wider area with less base stations. – The Fundamentals of Cellular System Design

The transmissions to and from the base stations can be done via frequency division duplexing or time division duplexing. See Frequency Division vs. Time Division Duplexing in Wireless Communications or the video below for more details.

LTE

Next, we took a closer look at the architectural details of 4G. The key components in the LTE packet core are the Serving Gateway (SGW), PDN Gateway (PGW), Mobility Management Entity (MME), Policy and Charging Rules Function (PCRF), and Home Subscriber Server (HSS). Their relationship is explained on this LTE (4G) Network Architecture page. See this 4G Architecture: LTE Network Elements and Interfaces page as well. These videos also cover the basics of LTE:

An interesting aspect of the LTE packet core is that a 5G base station can be attached to it. Contrast this mode, known as 5G non-standalone to 5G standalone mode, where a 5G radio is connected to a 5G packet core. See this post on Non-standalone and Standalone: two standards-based paths to 5G for a detailed review of these modes. One advantage of the 5G packet core is that it allows for cloud-based implementations. The Boost Mobile Network, for example, is 100% implemented in the cloud. Is AWS set to flex cloud on telecom? has a discussion of such a transition (to the cloud) in the telecom space.

The antennas used on the base stations can be of multiple types, e.g. SISO and MIMO. MIMO Antennas Explained: An In-Depth Guide provides more details on the differences between these designs. A key benefit of MIMO is that it eliminates performance degradation caused by multipath wave propagation.

We also dug into the LTE and 5G network evolution, from network deployment, to network growth, then finally coverage and capacity optimization. Deployment may involve dual-radio in UEs and EPC capabilities to support interoperability with earlier generations like 2G/3G. 5G is currently in the deployment phase since 5G SA has not yet been fully rolled out. Network growth may involve cell splitting in the RAN for capacity as well as expansion of the core network. Coverage and capacity optimization may involve spectrum aggregation, advanced network topologies, and advanced antenna techniques.

The continued growth in application and device diversity, RAN complexity, and QoS variance is making networks more complex and thus harder to optimize under the current network management paradigm. Self-organizing networks (SON) were designed to address this problem. Here is an overview of SON.

SON is used to set many required configuration parameters when introducing a new eNB or gNB to a network e.g. IP addresses from DHCP, transmit power, beam width, supported connections, connecting to neighboring base stations via the X2 (4G) or XN (5G) connection, etc. SON can also be used for driving energy savings by shutting down carriers when less capacity is required e.g. in the middle of the night (without dropping emergency calls). Another application is coverage and capacity optimization, which involves adjusting transmission power and continuously adjusting antenna tilt to increase capacity (thus decreasing coverage) or to increase coverage (thus decreasing capacity). Mobility handover optimization is also required to avoid too early/too late handover or a ping pong between base stations. The SON architecture can be centralized, distributed, or hybrid.

5G

Finally, we took a look at 5G technology, which has much lower latencies, much higher throughput, and high capacity. Some of the key technologies I learn about include Fixed Wireless Access, in which the UE does not move, and mmWave. More info on the latter is available at What is mmWave and how does it relate to 5G? 5G also supports modified air interfaces (modified OFDM), massive MIMO, device-to-device communication, separated user and control planes, and network virtualization.

An important capability that 5G introduced is positioning, which has many potential use cases e.g. industrial, automotive, and AR/VR. See 5G positioning: What you need to know for more details. In the industrial setting, for example, 5g all in one boxes are deployed in the 5G private networks. They have a base station and a packet core in a single piece of hardware, e.g. RAK All-in-One 5G box (the first one in the search results).

The 5G core network architecture is significantly different from the LTE packet core (eNB, SGW, PGW, MME, HSS, and PCRF). It moved to a service based architecture where microservices expose functionality via APIs. This makes the 5G network programmable and extensible. This 5G System Overview covers the overall 5G architecture. These are a few of the 5G components:

• Access and Mobility Management Function (AMF)
• Session Management Function (SMF)
• User Plane Function (UPF)
• Network Repository Function (NRF)
• Network Slice Selection Function (NSSF), which allocates slices to users
• Authentication Server Function (AUSF)
• Policy Control Function (PCF)
• Unified Data Management (UDM), which is functionally similar to 3G and 4G’s HSS
• Application Function (AF), which can let applications retrieve data like latencies

Summary

This is the final post in the Introduction to Networks series of posts. It has been an extremely enlightening course. I have appreciated how much more extensive it was than I expected from an introductory course as I try to stay on top of the fast moving tech space.