7.4 – Cellular Internet Access:

7.4.1 An Overview of Cellular Network Architecture

  • Global System for Mobile Communications (GSM) standards:
    • In the 1980s, Europeans recognized the need for a pan-European digital cellular telephony system that would replace the numerous incompatible analogue cellular telephony systems, leading to the GSM standard.
    • Europeans deployed GSM technology with great success in the early 1990s, and since then GSM has grown to be the 800-pound gorilla of the cellular telephone world, with more than 80% of all cellular subscriber worldwide using GSM.
  • Generations:
    • First generation (1G): Systems were analog FDMA systems designed exclusively for voice-only communication.
    • Second generation (2G): were designed for voice, but later extended (2.5G) to support data as well as voice services.
    • Third generation (3G): also support voice and data, but with an emphasis on data capabilitites and higher-speed radio access links.
    • Fourth generation (4G): deployed today and are based on LTE technology, feature an all-ip core network, and provide integrated voice and data at multi-Megabit speeds.
  • 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, 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.
  • Many systems today place the BTS at corners where 3 cells intersect, so that a single BTS with directional antennas can service 3 cells.
  • 2G cellular systems uses combined FDM/TDM for the air interface.
    • 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 chanel will be able to support F.T simultaneous calls.
    • GSM systems consists of 200-Khz frequency bands with each band supporting eight TDM calls. GSM encodes speech at 14 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 ratdio channels to mobile subscribers, perform paging (finding the cell in which a mobile user is resident), and perform handoff of mobile users.
    • The base station controller and its controlled base transceiver stations collectively constitute a GSM base station subsystem (BSS).
  • The mobile switching center (MSC) plays the central role in user authorization and accounting, call establishment and teardown, and handoff.
    • A single MSC will typically contain up to 5 BSCs, resulting in approximately 200k subscribers per MSC. A cellular provider’s network will have a number of MSCs, with a special MSCs known as gateway MSCs connecting the provider’s cellular network to the larger public telephone network.

7.4.2 – 3G Cellular Data Networks: Extending the Internet to cellular Subscribers

  • The 3G core cellular data network connects radio access networks to the public net. The core network interoperates with components of the existing cellular voice network that we previously encountered.
  • Given the considerable amount of existing infrastructure in the existing cellular voice network, the approach taken by the designers of 3G data service 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):
      • 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 GGSN.
    • Gateway GPRS support nodes (GGSNs):
      • The GGSN is 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 (officially known in 3G UMTS parlance as a “Node Bs”)
  • 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 FDMS/TDMA scheme, UTMS uses a CDMA technique known as Direct sequence Wideband CDMA 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 ot the approach taken in wired cable access networks.
    • This change requires a new 3G cellular wireless-access network operating in parallel with the 2G BSS radio network.
    • The data service associated with the WCDMA specification is known as HSPA and prmises downlink data rates of up to 14 Mbps.

7.4.3 – On to 4G: LTE

  • In 2015 more than 50 countries had 4G coverage exceeding 50%
  • Two important high-level observations about the 4G architecture:
    • A unified, all-IP network architecture:
      • The 4G network is “all-IP”, both voice and data are carried in IP datagrams to/from the wireless deviceto 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 4G data plane and 4G control plane:
      • The 4G network architecture clearly separates the data and control plane just like the data and control plane in IP’s network layer.
    • 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 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.
  • 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 and again plays a central role here. Its data-plane role is to forward datagrams between UE 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.
      • F.ex. A 4G network may guarantee that voice traffic experiences no more than a 100msec delay between UE and P-GW, and has a packet loss rate of less than 1%; TCP traffic might have a guarantee of 300msec and a packet loss rate of less than 0.0001%
    • In the control plane, the eNodeB handels 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.
    • The Home Subscriber Server (HSS) contains UE information including roaming access capabilities, quality of service profiles, and authentication information. The HSS obtains this information from the UE’s home cellular provider.
  • LTE uses a combination of frequency division multiplexing and time division multiplexing on the downstream channel, known as orthogonal frequency division multiplexing (OFDM)
  • In LTE, each active mobile node is allocated one or more 0.5ms time slots in one or more of the channel frequencies. By allocated increasingly more time slots, a mobile node is able to achieve increasingly higher transmission rate.
    • 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.
  • 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, matching the physical layer protocol to the channel conditions between the sender and receiver and choosing the receiver 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 can be used in scheduling downstream packet transmissions.
  • LTE-advanced allows for downstream bandwidth of hundreds of Mbps by allocating aggregated channels to a mobile node.
  • An additional wireless technology, WiMAX (World Interoperability for Microwave access), is a family of IEE 802.16 standards that differ significantly from LTE. WiMAX has not yet been able to enjoy the widespread deployment of LTE.

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