Physcial layer: About Cellular
In the previous section, Internet access was discussed in the context of Wi-Fi hotspots, where a device can connect only when it is within the limited range of an 802.11 access point. However, Wi-Fi coverage is relatively small, so another solution is needed when users want wireless Internet access outside a hotspot. The natural answer is to extend cellular networks so that they support not only voice communication but also Internet connectivity.
The goal of cellular Internet access is to provide reasonably high-speed wireless communication while maintaining seamless mobility. In other words, users should be able to stay connected while traveling in a car, bus, or train, and continue activities such as web browsing, messaging, or even video conferencing. This section introduces the overall evolution of cellular Internet technologies, focusing on the wireless first hop and the network infrastructure that connects mobile users to the larger telephone network and the Internet.
7.4.1 Overview of Cellular Network Architecture
To explain cellular architecture, the section adopts terminology from GSM (Global System for Mobile Communications), one of the most influential cellular standards. GSM emerged in Europe as a digital replacement for incompatible analog cellular systems and later became a dominant global standard.
The term cellular comes from the fact that the coverage area of a network is divided into many small geographic regions called cells. Each cell contains a Base Transceiver Station (BTS) that communicates wirelessly with mobile devices inside the cell. In practice, the size and quality of a cell depend on many factors, including transmission power, physical obstacles such as buildings, and antenna height.
Several BTSs are typically managed by a Base Station Controller (BSC). The BSC allocates radio channels, performs paging, and supports handoff when a user moves between cells. Together, the BTSs and the BSC form the Base Station System (BSS). At a higher level, the Mobile Switching Center (MSC) handles key control functions such as user authorization, call setup and teardown, and mobility support. Gateway MSCs then connect the provider’s cellular network to the public telephone network.
GSM-based 2G systems use a combination of frequency division multiplexing (FDM) and time division multiplexing (TDM). In this design, frequency bands are divided into smaller sub-bands, and time is divided into slots within each band, allowing multiple calls to share the same channel resources efficiently.
Cellular Generations: From 1G to 4G
Cellular technologies are commonly grouped into generations.
- 1G systems were analog and designed only for voice communication.
- 2G systems introduced digital voice communication.
- 2.5G added early support for data services.
- 3G supported both voice and data, with a stronger emphasis on Internet connectivity.
- 4G moved toward a fully IP-based architecture with much higher data performance.
This progression shows how cellular networks evolved from voice-centric systems into mobile Internet platforms.
Wi-Fi Versus 4G
An interesting question raised in the section is whether wireless Internet access will be dominated by Wi-Fi or by cellular systems such as 4G. Wi-Fi has the advantage of widespread deployment, low cost, and built-in support across many devices. At the same time, 4G offers broad-area mobility and Internet access even while the user is moving.
Rather than replacing one another, Wi-Fi and 4G are more likely to coexist. In practice, users often have access to both technologies, and devices can choose whichever provides better service at a given location.
7.4.2 3G Cellular Data Networks
While 2G focused mainly on connecting voice users to the public telephone network, 3G expanded cellular systems to support true Internet access for mobile subscribers. With 3G, users could browse the web, send email, use location-based services, and watch streaming content through the cellular data network.
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A 3G architecture consists of two major parts:
- the radio access network, which provides the wireless link between users and the network
- the core network, which connects cellular users to the public Internet
In the 3G core network, two important components appear:
- SGSN (Serving GPRS Support Node): responsible for delivering datagrams to and from mobile devices in the radio access network, while also supporting user authorization, mobility management, and forwarding
- GGSN (Gateway GPRS Support Node): acts as a gateway between the cellular network and the wider Internet
On the wireless edge, the Radio Network Controller (RNC) manages multiple base stations, similar to the role of the BSC in 2G. One major technical difference is that 3G UMTS uses W-CDMA rather than GSM’s FDM/TDM scheme. This allows more advanced and higher-capacity wireless communication. 3G data services such as HSPA significantly improved download speeds compared with earlier generations.
7.4.3 4G: LTE
The transition to 4G LTE brought a major architectural change. The most important idea behind 4G is that the network becomes all-IP. Unlike 3G, where voice and data followed partly separate paths, 4G treats both as IP traffic. This reflects the shift from traditional telephone-oriented design to a modern Internet-oriented architecture.
Two key characteristics define 4G:
- A unified all-IP core network
Voice and data are both carried in IP datagrams, making the system more consistent and efficient. - A clear separation of the data plane and the control plane
User data forwarding and network control functions are handled by different components.
The main 4G components include:
- eNodeB: the LTE base station, responsible for forwarding user traffic between the wireless edge and the core network
- P-GW (Packet Data Network Gateway): allocates IP addresses and enforces QoS policies
- S-GW (Serving Gateway): acts as a data-plane mobility anchor through which user traffic passes
- MME (Mobility Management Entity): handles control-plane functions such as connection and mobility management
- HSS (Home Subscriber Server): stores subscriber information, authentication data, roaming information, and QoS profiles
LTE also uses more advanced radio resource allocation. It combines time and frequency multiplexing with OFDM (Orthogonal Frequency Division Multiplexing), allowing the network to assign small time slots across frequency channels to users in a flexible way. This makes it possible to improve transmission efficiency, adapt to channel conditions, and support higher data rates. LTE-Advanced further extends this capability and can provide downlink bandwidths of hundreds of Mbps.
WiMAX
The section briefly mentions WiMAX, another 4G wireless technology based on IEEE 802.16 standards. Although technically important, it has not achieved the same level of widespread deployment as LTE.
Final Summary
This section shows how cellular systems evolved from traditional voice networks into high-speed mobile Internet platforms.
- 2G established the basic cellular structure for digital voice communication.
- 3G extended this structure to support Internet data services.
- 4G LTE transformed the network into a fully IP-based system with higher speeds, better mobility support, and a cleaner architectural separation between control and data functions.
In short, the history of cellular networking is the story of a transition from voice-centered communication infrastructure to mobile Internet infrastructure.
