An Introduction to LTE

You just need to step outside to realize how much mobile phones have changed our lives. People have this need to stay connected all the time and new, faster, better performing mobiles are constantly being released. As a consequence we need to increase what we are able to do with these so-called smartphones: transfer more and more data, be able to download movies, watch a live volleyball match, etc…etc… and as such improved mobile telecommunications networks are needed.

Most of us will have experience with 2G and 3G networks. These include technologies such as GSM, EDGE, UMTS, HSPA and CDMA/EV-DO.  There is an organization known as 3GPP (http://www.3gpp.org/), which has partnerships with more local organizations, that works on the standards in this industry. To give you an idea in the original release of the UMTS specification the maximum download speed was of 384 kbps but nowadays the latest versions  (HSPA+) allow for theoretical speeds of up to 42 Mbit/s.

One of the latest technologies to reach us is known as Long-Term Evolution (LTE) mostly commonly referred to as an early version of 4G. It is already operational in a number of countries around the world. It’s an improvement on UMTS allowing download speeds of 300 Mbit/s with a smaller latency, improved mobility, simplified radio access, the use of smart antennas, support for QoS and it has a different architecture. In UMTS there was a separation between voice and data services referred to as Circuit-Switched and Packet-Switched respectively. The big revolution with LTE is that is all IP based (with a separation between control plane and user plane traffic) which means voice calls (and SMS) need to be implemented in a different way. A few options are:

• When a voice call is initiated have a fallback to 3G/2G
• IMS – IP Multimedia System – a framework to deliver multimedia and voice services with the use of VoLTE (Voice over LTE).
• VoIP

Architecture

The basic architecture for LTE is shown below.

• UEs (User equipment like mobile phones and dongles) – All the big players (Apple, Blackberry, HTC, LG, Nokia, Samsung, Sony) already have compatible mobile phones. The main issue for the consumer is to choose a phone that works on the radio frequency used in their country.
• Radio Access – simplified redesign compared to previous technologies using an eNB (Evolved NodeB). It connects to the Evolved Packet Core (EPC) via the S1 interface using the S1AP protocol to communicate (with SCTP at the transport layer).
• MME – Mobility Management Entity – key component that authenticates user, tracks phone’s location and state, allocates a temporary identity and enforces roaming restrictions
• HSS – Home Subscriber Server – database containing user information for authentication and authorization purposes. Communicates with the MME on the S6a interface
• S-GW – Serving Gateway – routes and forwards user data packets and stores some user equipment parameters, communicates with the MME via the S11 interface and to the P-GW via the S5 interface
• P-GW – Packet Data Network Gateway – exit point for traffic (via SGi interface), packet filtering, charging, lawful interception and acts as link between 3GPP and non 3GPP technologies
• PCRF – Policy Charging and Rules Function – operates in real time as a policy manager for charging purposes and Quality of Service

For the networking folk the protocol stacks, shown below, will be of interest. The first is for the control plane and the second for the user plane.

• The RRC (Radio Resource Control) is responsible for managing the radio resources
• The PDCP (Packet Data Convergence Protocol) provides compression, encryption and integrity protection
• The RLC (Radio Link Control) transfers the PDCPs PDUs and provides error correction
• The S1AP (S1 Application Protocol) managed the signaling between the MME and the eNB.

Another important is the Non Access Stratum (NAS). It’s the highest level of the control plane between the UEs and the MME. It’s main function is to support the mobility of the user equipment as well as security, authentication, session management and IP connectivity.

A key element of LTE is mobility management. Each eNB covers a certain geographical region, so when the UE moves to a different location or is switched on and off, it needs to communicate its new location via a Tracking Area Update.  This is also done at periodic intervals. Another important procedure, known as Paging, happens when a UE is idle and there is data for it, for a change of System Information and lastly in case of ETWS (Earthquake and Tsunami Warning Systems) warnings.

An enhancement to LTE is already in the works – LTE-Advanced. The goal is to offer real 4G speeds with peak rates of 1Gb/s amongst many other improvements.

Do you have any experience with using a live LTE service in your country? Share with us your impressions!