The 3GPP LTE architecture is called the System Architecture Evolution (SEA), and its overall goal is to deliver a simplified architecture based on all-IP traffic. It also supports high-speed communication and low latency over Radio Access Networks (RANs). In release 8 of the 3GPP roadmap, LTE was introduced. Since the network is entirely composed of IP packet-switched components, that means that voice data is also sent across as digital IP packets. This is another fundamental difference from the legacy 3G network.

The typical 4G-LTE network has three components: a client, a radio network, and a core network. The client is simply the user's radio device. The radio network represents frontend communication between the client and the core network and includes radio equipment such as the tower. The core network represents the management and control interface of the carrier and may manage one or more radio networks.

The architecture can be decomposed as follows:

• Evolved Universal Terrestrial Radio Access Network (E-UTRAN): This is 4G-LTE air interface to LTE UE devices. E-UTRAN uses OFDMA for the downlink portion and SC-FDMA for the uplink. Subsequently, this makes it incompatible with the legacy 3G W-CDMA technology. The E-UTRAN consists of eNodeBs but can contain several that are interlinked by a channel called the X2 interface.  
• eNodeB: This is the core of the radio network. It handles communications between the UE and the core (EPC). Each eNodeB is a base station that controls eUEs in one or more cellular zones and allocates resources to a specific client in 1 ms chunks called TTI. It will allocate channel resources based on usage conditions to various UEs in its cell proximity. eNodeB systems also have the responsibility of triggering state transitions from IDLE to CONNECTED. It also handles the mobility of the UE such as the handover to other eNodeBs. It is also responsible for transmission and congestion control. The interface out of the eNodeB and into the EPC is the S1 interface. 
• User Equipment (UE): This is the client hardware and is composed of Mobile Terminations (MT), which perform all the communication functions, Terminal Equipment (TE), which manages terminating data streams, and the Universal Integrated Circuit Card (UICC), which is the SIM card for identity management.
• Evolved Packet Core (EPC): In the design of LTE, the 3GPP decided to build a flat architecture, and separate user data (called the user plane) and control data (called the control plane). This is allowed for more efficient scaling. The EPC has five basic components listed here:
  • Mobility Management Equipment (MME): Responsible for control plane traffic, authentication and security, location and tracking, and mobility issue handlers. The MME also needs to recognize mobility in IDLE mode. This is managed using a Tracking Area (TA) code. The MME also governs the Non-Access Stratum (NAS) signaling and bearer control (described later).
  • Home Subscriber Server (HSS): This is a central database associated with the MME that contains information about the network operator subscribers. This may include keys, user data, maximum data rates on the plan, subscriptions, and so on. The HSS is a holdover from the 3G UMTS and GSM networks.
  • Servicing Gateway (SGW): Responsible for user plane and user data flow. Essentially, it acts as a router and forwards packets between the eNodeB and PGW directly. The interface out of the SGW is called the S5/S8 interface. S5 is used if two devices are on the same network and S8 used if they are on different networks.
  • Public Data Network Gateway (PGW): Connects mobile networks to external sources including the internet or other PDN networks. It also allocates the IP address for the mobile devices connected. The PGW manages the Quality of Service (QoS) for various internet services such as video streaming and web browsing. It uses an interface called the SGi to reach into various external services.
  • Policy Control and Charging Rules Function (PCRF): This is another database that stores policies and decision-making rules. It also controls the flow-based charging functions. 
• Public Data Network (PDN): This is the external interface, and for the most part, the internet. It can include other services, data centers, private services, and so on.

In a 4G-LTE service, a customer will have a carrier or operator known as their Public Land Mobile Network (PLMN). If the user is in that carrier's PLMN, he or she is said to be in Home-PLMN. If the user moves to a different PLMN outside their home network (for example, during international travel) then the new network is called the visited-PLMN. The user will connect their EU to a visited-PLMN which requires resources of the E-UTRAN, MME, SGW, and PGW on the new network. The PGW can grant access to a local-breakout (a gateway) to the internet. This, effectively, is where roaming charges start to affect the service plans. Roaming charges are applied by the visited-PLMN and accounted for on the client's bill. Below is a graphic illustrating this architecture. On the left side is a top-level view of the 3GPP System Architecture Evolution for 4G-LTE. In this case, it represents the client UE, the radio node E-UTRAN, and the core network EPC, all residing in a Home-PLMN. On the right is a model of the mobile client moving to a visited-PLMN and distributing the functionality between the visited-PLMN E-UTRAN and EPC, as well as crossing back to the home network. The S5 interconnect is used if the client and carrier reside on the same network and the S8 interface if the client is crossing across different networks.

Non-Access Stratum (NAS) signaling was mentioned in the MME. It is a mechanism for passing messages between UE and core nodes such as switching centers. Examples may include messages such as Authentication Messages, Updates, or Attach Messages. The NAS sits at the top of the SAE protocol stack.

The GPRS Tunneling Protocol (GTP) is an IP/UDP-based protocol used in LTE. GTP is used throughout the LTE communication infrastructure for control data, user data, and charging data. In the figure above, most of the S* channel's connecting components use GTP packets.

The LTE architecture and protocol stack use what is known as bearers. Bearers are a virtual concept used to provide a pipe to carry data from one node to another node. The pipe between the PGW and UE is called the EPS Bearer. As data enters the PGW from the internet, it will package the data in a GTP-U packet and send it to the SGW. The SGW will receive the packet, strip the GTP-U header, and repackage the user data in a new GTP-U packet on its way to the eNB. Again, the eNB repeats the process and will repackage the user data after compressing, encrypting, and routing to logical channels. The message will then transmit to the UE through a radio bearer. One advantage bearers bring to LTE is QoS control. Using bearers, the infrastructure can guarantee certain bitrates dependent on customer, application, or usage.

When a UE attaches to a cellular network the first time, it is assigned a default bearer. Each default bearer has an IP address, and a UE may have several default bearers, each with a unique IP. This is a best-effort-service meaning it wouldn't be used for guaranteed QoS-like voices. A dedicated bearer, in this case, is used for QoS and good user experience. It will be initiated when the default bearer is incapable of fulfilling a service. The dedicated bearer always resides on top of the default bearer. A typical smartphone may have the following bearers running at any time:

• Default Bearer 1: Messaging and SIP signaling
• Dedicated Bearer: Voice data (VOIP) linked to default bearer 1
• Default Bearer 2: All smartphone data services such as email, browser

The switching aspect of LTE is also worth mentioning. We looked at the different generations of 3GPP evolutions from 1G to 5G. One goal of the 3GPP and carriers was to move to a standard and accepted Voice over IP (VOIP) solution. Not only would data be sent over standard IP interfaces but so would voice. After some contention between competing methods, the standards body settled on VoLTE as the architecture. VoLTE uses an extended variant of the Session Initiation Protocol (SIP) to handle voice and text messages. A codec known as the Adaptive Multi-Rate (AMR) codec provides wideband high-quality voice and video communication. Later, we will look at new 3GPP LTE categories that drop VoLTE to support IoT deployments.