Arsitektur 5G: Radio Access Network
We now describe the RAN by sketching the role each base station plays. Keep in mind this is kind of like describing the Internet by explaining how a router works—a not unreasonable place to start, but it doesn’t fully do justice to the end-to-end story.
First, each base station establishes the wireless channel for a subscriber’s UE upon power-up or upon handover when the UE is active. This channel is released when the UE remains idle for a predetermined period of time. Using 3GPP terminology, this wireless channel is said to provide a bearer service. The term “bearer” has historically been used in telecommunications (including early wireline technologies like ISDN) to denote a data channel, as opposed to a channel that carries signaling information.
_images/Slide03.png Figure 8. Base Station detects (and connects to) active UEs.
Second, each base station establishes “3GPP Control Plane” connectivity between the UE and the corresponding Mobile Core Control Plane component, and forwards signaling traffic between the two. This signaling traffic enables UE authentication, registration, and mobility tracking.
_images/Slide04.png Figure 9. Base Station establishes control plane connectivity between each UE and the Mobile Core.
Third, for each active UE, the base station establishes one or more tunnels between the corresponding Mobile Core User Plane component.
_images/Slide05.png Figure 10. Base station establishes one or more tunnels between each UE and the Mobile Core’s User Plane.
Fourth, the base station forwards both control and user plane packets between the Mobile Core and the UE. These packets are tunnelled over SCTP/IP and GTP/UDP/IP, respectively. SCTP (Stream Control Transport Protocol) is an alternative reliable transport to TCP, tailored to carry signaling (control) information for telephony services. GTP (a nested acronym corresponding to (General Packet Radio Service) Tunneling Protocol) is a 3GPP-specific tunneling protocol designed to run over UDP.
As an aside, it is noteworthy that connectivity between the RAN and the Mobile Core is IP-based. This was introduced as one of the main changes between 3G and 4G. Prior to 4G, the internals of the cellular network were circuit-based, which is not surprising given its origins as a voice network.
_images/Slide06.png Figure 11. Base Station to Mobile Core (and Base Station to Base Station) control plane tunneled over SCTP/IP and user plane tunneled over GTP/UDP/IP.
Fifth, each base station coordinates UE handovers with neighboring base stations, using direct station-to-station links. Exactly like the station-to-core connectivity shown in the previous figure, these links are used to transfer both control plane (SCTP over IP) and user plane (GTP over UDP/IP) packets.
_images/Slide07.png Figure 12. Base Stations cooperate to implement UE hand over.
Sixth, the base stations coordinate wireless multi-point transmission to a UE from multiple base stations, which may or may not be part of a UE handover from one base station to another.
_images/Slide08.png Figure 13. Base Stations cooperate to implement multipath transmission (link aggregation) to UEs.
The main takeaway is that the base station can be viewed as a specialized forwarder. In the Internet-to-UE direction, it fragments outgoing IP packets into physical layer segments and schedules them for transmission over the available radio spectrum, and in the UE-to-Internet direction it assembles physical layer segments into IP packets and forwards them (over a GTP/UDP/IP tunnel) to the upstream user plane of the Mobile Core. Also, based on observations of the wireless channel quality and per-subscriber policies, it decides whether to (a) forward outgoing packets directly to the UE, (b) indirectly forward packets to the UE via a neighboring base station, or (c) utilize multiple paths to reach the UE. The third case has the option of either spreading the physical payloads across multiple base stations or across multiple carrier frequencies of a single base station (including Wi-Fi).
Note that as outlined in Chapter 2, scheduling is complex and multi-faceted, even when viewed as a localized decision at a single base station. What we now see is that there is also a global element, whereby it’s possible to forward traffic to a different base station (or to multiple base stations) in an effort to make efficient use of the radio spectrum over a larger geographic area.
In other words, the RAN as a whole (i.e., not just a single base station) not only supports handovers (an obvious requirement for mobility), but also link aggregation and load balancing, mechanisms that are familiar to anyone who understands the Internet. We will revisit how such RAN-wide (global) decisions can be made using SDN techniques in a later chapter.