The term cellular refers to the fact that the region covered by a cellular network is partitioned into a number of geographic coverage areas, known as cells, shown as hexagons on the left side of Figure 7.18. As with the 802.11WiFi standard we ­studied in Section 7.3.1, GSM has its own particular nomenclature. Each cell contains a base transceiver station (BTS) that transmits signals to and receives signals from the mobile stations in its cell. The coverage area of a cell depends on many factors, including the transmitting power of the BTS, the transmitting power of the user devices, obstructing buildings in the cell, and the height of base station antennas. Although Figure 7.18 shows each cell containing one base transceiver station residing in the middle of the cell, many systems today place the BTS at corners where three cells intersect, so that a single BTS with directional antennas can service three cells.

Figure 7.18 Components of the GSM 2G cellular network architecture

The GSM standard for 2G cellular systems uses combined FDM/TDM (radio) for the air interface. Recall from Chapter 1 that, with pure FDM, the channel is partitioned into a number of frequency bands with each band devoted to a call. Also recall from Chapter 1 that, with pure TDM, time is partitioned into frames with each frame further partitioned into slots and each call being assigned the use of a particular slot in the revolving frame. In combined FDM/TDM systems, the channel is partitioned into a number of frequency sub-bands; within each sub-band, time is partitioned into frames and slots. Thus, for a combined FDM/TDM system, if the channel is partitioned into F sub-bands and time is partitioned into T slots, then the channel will be able to support F.T simultaneous calls. Recall that we saw in Section 6.3.4 that cable access networks also use a combined FDM/TDM approach. GSM systems consist of 200-kHz frequency bands with each band supporting eight TDM calls. GSM encodes speech at 13 kbps and 12.2 kbps.

A GSM network’s base station controller (BSC) will typically service several tens of base transceiver stations. The role of the BSC is to allocate BTS radio channels to mobile subscribers, perform paging (finding the cell in which a mobile user is resident), and perform handoff of mobile users—a topic we’ll cover shortly in Section 7.7.2. The base station controller and its controlled base transceiver stations collectively constitute a GSM base station subsystem (BSS).

As we’ll see in Section 7.7, the mobile switching center (MSC) plays the central role in user authorization and accounting (e.g., determining whether a mobile device is allowed to connect to the cellular network), call establishment and teardown, and handoff. A single MSC will typically contain up to five BSCs, resulting in approximately 200K subscribers per MSC. A cellular provider’s network will have a number of MSCs, with special MSCs known as gateway MSCs connecting the provider’s cellular network to the larger public telephone network.

The 3G core cellular data network connects radio access networks to the public Internet. The core network interoperates with components of the existing cellular voice network (in particular, the MSC) that we previously encountered in Figure 7.18. Given the considerable amount of existing infrastructure (and profitable services!) in the existing cellular voice network, the approach taken by the designers of 3G data services is clear: leave the existing core GSM cellular voice network untouched, adding additional cellular data functionality in parallel to the existing cellular voice network. The alternative— integrating new data services directly into the core of the existing cellular voice network—would have raised the same challenges encountered in Section 4.3, where we discussed integrating new (IPv6) and legacy (IPv4) technologies in the Internet.

There are two types of nodes in the 3G core network: Serving GPRS Support Nodes (SGSNs) and Gateway GPRS Support Nodes (GGSNs). (GPRS stands for Generalized Packet Radio Service, an early cellular data service in 2G networks; here we discuss the evolved version of GPRS in 3G networks). An SGSN is responsible for delivering datagrams to/from the mobile nodes in the radio access network to which the SGSN is attached. The SGSN interacts with the cellular voice network’s MSC for that area, providing user authorization and handoff, maintaining location (cell) information about active mobile nodes, and performing datagram forwarding between mobile nodes in the radio access network and a GGSN. The GGSN acts as a gateway, connecting multiple SGSNs into the larger Internet. A GGSN is thus the last piece of 3G infrastructure that a datagram originating at a mobile node encounters before entering the larger Internet. To the outside world, the GGSN looks like any other gateway router; the mobility of the 3G nodes within the GGSN’s network is hidden from the outside world behind the GGSN.

The 3G radio access network is the wireless first-hop network that we see as a 3G user. The Radio Network Controller (RNC) typically controls several cell base transceiver stations similar to the base stations that we encountered in 2G systems (but officially known in 3G UMTS parlance as a “Node Bs”—a rather non-descriptive name!). Each cell’s wireless link operates between the mobile nodes and a base transceiver station, just as in 2G networks. The RNC connects to both the circuit-switched cellular voice network via an MSC, and to the packet-switched Internet via an SGSN. Thus, while 3G cellular voice and cellular data services use different core networks, they share a common first/last-hop radio access network.

A significant change in 3G UMTS over 2G networks is that rather than using GSM’s FDMA/TDMA scheme, UMTS uses a CDMA technique known as Direct Sequence Wideband CDMA (DS-WCDMA) [Dahlman 1998] within TDMA slots; TDMA slots, in turn, are available on multiple frequencies—an interesting use of all three dedicated channel-sharing approaches that we earlier identified in Chapter 6 and similar to the approach taken in wired cable access networks (see Section 6.3.4). This change requires a new 3G cellular wireless-access network operating in parallel with the 2G BSS radio network shown in Figure 7.19. The data service associated with the WCDMA specification is known as HSPA (High Speed Packet Access) and promises downlink data rates of up to 14 Mbps. Details regarding 3G networks can be found at the 3rd Generation Partnership Project (3GPP) Web site [3GPP 2016].

Figure 7.20 shows the overall 4G network architecture, which (unfortunately) introduces yet another (rather impenetrable) new vocabulary and set of acronyms for network components. But let’s not get lost in these acronyms! There are two important high-level observations about the 4G architecture:

Figure 7.20 4G network architecture

• A unified, all-IP network architecture. Unlike the 3G network shown in Figure 7.19, which has separate network components and paths for voice and data traffic, the 4G architecture shown in Figure 7.20 is “all-IP”—both voice and data are carried in IP datagrams to/from the wireless device (the User Equipment, UE in 4G parlance) to the gateway to the packet gateway (P-GW) that connects the 4G edge network to the rest of the network. With 4G, the last vestiges of cellular networks’ roots in the telephony have disappeared, giving way to universal IP service!

• A clear separation of the 4G data plane and 4G control plane. Mirroring our distinction between the data and control planes for IP’s network layer in Chapters 4 and 5 respectively, the 4G network architecture also clearly separates the data and control planes. We’ll discuss their functionality below.

• A clear separation between the radio access network, and the all-IP-core ­network. IP datagrams carrying user data are forwarded between the user (UE) and the gateway (P-GW in Figure 7.20) over a 4G-internal IP network to the external Internet. Control packets are exchanged over this same internal network among the 4G’s control services components, whose roles are described below.

The principal components of the 4G architecture are as follows.

• The eNodeB is the logical descendant of the 2G base station and the 3G Radio Network Controller (a.k.a Node B) and again plays a central role here. Its data-plane role is to forward datagrams between UE (over the LTE radio access ­network) and the P-GW.

UE datagrams are encapsulated at the eNodeB and tunneled to the P-GW through the 4G network’s all-IP enhanced packet core (EPC). This tunneling between the eNodeB and P-GW is similar the tunneling we saw in Section 4.3 of IPv6 datagrams between two IPv6 endpoints through a network of IPv4 routers. These tunnels may have associated quality of service (QoS) guarantees. For example, a 4G network may guarantee that voice traffic experiences no more than a 100 msec delay between UE and P-GW, and has a packet loss rate of less than 1%; TCP traffic might have a guarantee of 300 msec and a packet loss rate of less than .0001% [Palat 2009]. We’ll cover QoS in Chapter 9.

In the control plane, the eNodeB handles registration and mobility signaling traffic on behalf of the UE.

• The Packet Data Network Gateway (P-GW) allocates IP addresses to the UEs and performs QoS enforcement. As a tunnel endpoint it also performs datagram encapsulation/decapsulation when forwarding a datagram to/from a UE.

• The Serving Gateway (S-GW) is the data-plane mobility anchor point—all UE traffic will pass through the S-GW. The S-GW also performs charging/billing functions and lawful traffic interception.

• The Mobility Management Entity (MME) performs connection and mobility management on behalf of the UEs resident in the cell it controls. It receives UE subscription information from the HHS. We cover mobility in cellular networks in detail in Section 7.7.

• The Home Subscriber Server (HSS) contains UE information including roaming access capabilities, quality of service profiles, and authentication information. As we’ll see in Section 7.7, the HSS obtains this information from the UE’s home cellular provider.

Very readable introductions to 4G network architecture and its EPC are [Motorola 2007; Palat 2009; Sauter 2014].

LTE uses a combination of frequency division multiplexing and time division multiplexing on the downstream channel, known as orthogonal frequency division multiplexing (OFDM) [Rohde 2008; Ericsson 2011]. (The term “orthogonal” comes from the fact the signals being sent on different frequency channels are created so that they interfere very little with each other, even when channel frequencies are tightly spaced). In LTE, each active mobile node is allocated one or more 0.5 ms time slots in one or more of the channel frequencies. Figure 7.21 shows an allocation of eight time slots over four frequencies. By being allocated increasingly more time slots (whether on the same frequency or on different frequencies), a mobile node is able to achieve increasingly higher transmission rates. Slot (re)allocation among mobile nodes can be performed as often as once every millisecond. Different modulation schemes can also be used to change the transmission rate; see our earlier discussion of Figure 7.3 and dynamic selection of modulation schemes in WiFi networks.

Figure 7.21 Twenty 0.5 ms slots organized into 10 ms frames at each frequency. An eight-slot allocation is shown shaded.

The particular allocation of time slots to mobile nodes is not mandated by the LTE standard. Instead, the decision of which mobile nodes will be allowed to transmit in a given time slot on a given frequency is determined by the scheduling algorithms provided by the LTE equipment vendor and/or the network operator. With opportunistic scheduling [Bender 2000; Kolding 2003; Kulkarni 2005], matching the physical-layer protocol to the channel conditions between the sender and receiver and choosing the receivers to which packets will be sent based on channel conditions allow the radio network controller to make best use of the wireless medium. In addition, user priorities and contracted levels of service (e.g., silver, gold, or platinum) can be used in scheduling downstream packet transmissions. In addition to the LTE capabilities described above, LTE-Advanced allows for downstream bandwidths of hundreds of Mbps by allocating aggregated channels to a mobile node [Akyildiz 2010].

An additional 4G wireless technology—WiMAX (World Interoperability for Microwave Access)—is a family of IEEE 802.16 standards that differ significantly from LTE. WiMAX has not yet been able to enjoy the widespread deployment of LTE. A detailed discussion of WiMAX can be found on this book’s Web site.