## 3 Overview of Video Delivery in 4G Mobile Cellular Networks
### Core Network Support for Mobile Video Delivery over 4G LTE
* **ABR manifest file manipulation:** consists of interception of ABR manifest file at the VO platform, parsing it and filtering out the representations from the manifest file that are not possible to reproduce on subscriber side under current network conditions. * **Caching (transparent and selective):** consists of storing popular traffic on the platform in order to make it available for future subscriber's requests [46; 47]. Transparent caching strategy consists of caching of all unencrypted content. As it is resource consuming operators typically uses selective caching such as caching of the content that is previously transcoded. Table 1 provides the applicability of the above-listed techniques on different video traffic types. Whether the traffic is optimized by VO-subsystem depends on its profile configuration, which may depend on: (i) time of the day (e.g., during peak hours more aggressive optimization can be done); (ii) radio access technology (RAT) type of the subscriber (e.g., allowed max bitrate for 4G RAT type is higher than for 3G RAT type) which is received from P-GW via Radius interface (Figure 2); (iii) assigned subscriber policy received from PCRF via Gx interface (Figure 2); (iv) RAN congestion state (e.g., if RAN is in congested state then more aggressive optimization can be used for all subscribers to mitigate congestion); (v) location of subscriber (e.g., if the subscriber is associated to congested cell then more aggressive optimization is applied), and vi) device class (e.g., streams destined to devices with small screens can be optimized in more aggressive way than streams to devices with a wide screens). \begin{table}
\begin{tabular}{c c} \hline \hline
**Video Traffic Type** & **Possible Optimization Method** \\ \hline ABR over HTTP & ABR pacing; JIT pacing; ABR manifest file manipulation \\ ABR over HTTPS & ABR pacing; JIT pacing; \\ ABR over DRM over HTTP & ABR pacing; JIT pacing; \\ ABR over QUIC & ABR pacing; JIT pacing; \\ PD over HTTP & JIT pacing; online/offline transcoding; transrating; caching \\ PD over HTTPS & Not possible to optimize \\ \hline \hline \end{tabular}
\end{table} 
Table 1: VO optimization methods and traffic types. ABR over HTTPS is dominant video format, thus making ABR and JIT pacing the most commonly used optimization methods in VO platforms. We note that in academic studies, video quality-based optimization methods for ABR video delivery is currently very popular research topic [45; 48]. **Core Network Support for Video Multicasting/Broadcasting:** The entry point for eMBMS video content is broadcast/multicast service centre (BM-SC), which schedules and announces eMBMS services to end users. BM-SC is where AL-FEC is applied for packet-level erasure protection, which can be further optimized for eMBMS live streaming services [49; 40]. Based on the content and control inputs from BM-SC, MBMS Gateway (MBMS-GW) establishes IP multicast session towards all eNBs that deliver eMBMS service to end users, supported by control signaling via MME. Single frequency network service (MBSFN) is commonly used, where the content is delivered with tight synchronization requirements enforcing identical physical layer (PHY) configuration at all eNBs to enhance reception at cell edges. Alternatively, single-cell point-to-multipoint service (SCPTM) can be used targeting eMBMS service in a specific cell [9]. An overview of network deployment options for both video unicast and eMBMS services over LTE are provided in [10]. As depicted in Figure 2, the data path (solid red line) traverses BM-SC, MBMS-GW and proceeds via PTM IP Multicast session to the set of eNBs in MBSFN service area in the case of MBSFN service, or directly to a specific eNB in case of SCPTM service. The supporting signaling (dashed red line) traverses MME and Multi-cell/Multicast Coordinating Entity (MCE), where MME is responsible for session-level management, while MCE controls configuration of eNBs in MBSFN area, such as coding and modulation, time synchronization and resource allocation [9].