## 3 Overview of Video Delivery in 4G Mobile Cellular Networks
### Core Network Support for Mobile Video Delivery over 4G LTE  
**4G LTE Network Architecture:** Figure 2 illustrates LTE network architecture that consists of two main parts: (i) evolved universal terrestrial radio access network (E-UTRAN); and (ii) evolved packet core (EPC). Due to functional split, EPC separates user plane and control plane elements. User plane elements, Serving-Gateway (S-GW) and Packet Data Network (PDN)-Gateways (P-GW), provide data connectivity between E-UTRAN and external PDN. The S-GW handles the user-plane packet data termination towards E-UTRAN, while P-GW interfaces with the external PDNs performing IP related functions such as IP address allocation, policy enforcement, packet classification and routing. The main control plane element, Mobility Management Entity (MME), is responsible for connection/release of radio bearers to user equipment (UE). Further control plane entities include the Policy and Charging Rules Function (PCRF), enforcing policies and rules that are configured statically or dynamically per subscriber data session, Home Subscriber Server (HSS) that contains subscriber data such as user/QoS/barring profiles, and Online Charging System (OCS) used for real time rating of subscriber data usage and providing subscribers with data usage control. For more details on EPC architecture and elements, we refer interested reader to [42; 43]. Popular over-the-top (OTT) mobile video unicasting services such as ABR or PD do not require additional EPC elements, as they are transmitted transparently over the EPC data bearers. However, as described next, due to massive volume of ABR/PD traffic mobile operators often empower their EPC with video optimization (VO) platforms. In contrast, for video multicasting/broadcasting, significant EPC upgrade is needed to provide eMBMS, as presented in Figure 2.  
Figure 2: EPC network elements supporting mobile video delivery over 4G LTE. **Core Network Support for Video Streaming/Downloading:** To provide support for optimized unicast video delivery, mobile operators introduce VO platforms (Figure 2). VO platform is used to improve subscriber experience by optimizing video content to a format which will provide the best viewing experience for the given subscriber's network conditions. The VO platform consists of traffic classifier (TC) and the system that performs optimization of unicast streams (VO-subsystem). The video traffic, which is sent by content server towards a content consumer (mobile subscriber), is intercepted by the VO platform, where it is first detected by TC, passed to the VO-subsystem, optimized, and sent towards the content consumer. The most frequently used VO optimization methods are:  
* **Transcoding:** converts video content from one format to another by changing e.g., encoding format, resolution, codecs, frame rate, etc. Online (on-the-fly) transcoding is mostly used. In addition, offline transcoding can be used and it is done in a way that some popular videos are downloaded and optimized in advance before being stored in cache [23; 24]. * **Transrating:** converts video by keeping the original video format and resolution and by changing number of bits per pixel. This technique is not widely used [44]. * **ABR pacing:** receives traffic from a content server with one pace and send it towards a consumer with another pace in order to limit the representation quality requested by ABR clients on the subscriber side. This technique changes effective bandwidth perceived by ABR client side in order to affect the representation quality that will be selected by the client [44; 45]. * **JIT (just-in-time) pacing:** receives traffic from a content server with one pace and lowers the downstream pace in \"just in time\" manner. This is done in order to avoid unnecessary filling of video player buffer on subscriber side as well as waste of network resources [24].