By Serge-Paul Carrasco on October 9, 2010

Wireless for Dummies 4G LTE EPC and IMS

(T) Each generation of cellular wireless networks has been characterized by new frequency bands e.g. radio waves, higher data rates, and new (but not-compatible!) transmission technology. The first generation of wireless telephony or AMPS enabled the first wireless phone calls over analog transmission.

The second generation or 2G (GSM, CDMAOne) introduced digital transmission, more efficient use of the wireless spectrum (so better coverage), international roaming and the first data services: SMS text messages. The 2.5G (GPRS, EDGE, CDMA 2000, 1xRTT) improves the data rates over 2G without any changes to the radio access network. The third generation or 3G (UMTS, HSPA, CDMA2000, EV-DO) standardized by the 3rd Generation Partnership Project (3GPP) increased again the data rates and introduced a packetized network to enable the wireless Internet with a new range of possibilities for Internet access, instant messaging and mobile TV.

LTE or Long Term Evolution, a 3.5 G technology, and LTE Advanced, a 4 G technology, goes beyond providing the wireless Internet to enable mobile broadband. The world’s first publicly available LTE-service was pioneered by Telia Sonera in Stockholm and Oslo in December 2009. And this year many worldwide carriers have announced their plans to migrate their networks to LTE such as Verizon Wireless this week at the CTIA show in San Francisco.

But note that emerging LTE networks are not backward compatible with 3G networks. So, your 3G iPhone or Android will not work on the soon to be deployed LTE and 4G networks!

LTE provides only IP services and does not support the concept to establish a “voice circuit”. It reduces the time to connect and get onto the network, reduces network latency for the subscriber to 5 ms, increases the network bandwidth from 1.25 Mhz to 20 Mhz and the throughput to 100 Mbps downlink and 50 Mbps uplink.

One of the building blocks of wireless technology is to multiplex the wireless signal e.g. to use the same frequency band between multiple subscribers. All new wireless networks including LTE, WiMax, a broadband wireless access technology defined by IEEE 802.16 and the new WiFi standard IEEE 802.11n are based on OFDMA to multiplex the signals.

And, all new wireless networks LTE, WiMax and 802.11n enable both the transmitter and the receiver of the wireless signal to have two or more antennas, a technique called multiple inputs, multiple output or MIMO, that increases the transmission bandwidth.

Simply put, the architecture of the next generation of telecom networks will include: the LTE radio network or E-UTRAN for evolved Terrestrial Radio Access Network, the Evolving Packet Core (EPC), an all IP network, that will not only support LTE but as well all 3G and 2G wireless access networks and the IP Multimedia Services (IMS) network for VoIP and other media services.

The LTE radio network is simply architected on a mesh of peer eNodeB. The EPC network is built on four key network elements:

The MME (Mobility Management Entity) provides all the mobility services for the network while the HSS (Home Subscriber Services) stores all the subscriber data;
The S-GW (Serving Gateway) is a data gateway for the IP traffic while the P-GW (Packet Gateway) routes the IP traffic.

The protocol to transport the Internet traffic from the eNodeB to the P-GW is an evolved GPRS Tunneling Protocol (GTP) that has been used in 3G networks.

One of the key networks connected to the EPC network is the IMS network. The goal of the IMS network is to provides IP services, in particular, VoIP, conferencing, messaging and presence. The protocol, to control and manage the IMS services, is SIP (Session Initiation Protocol).

The IMS network is basically designed around a few key network elements:

The CSCF (Call Set-up Control Function) that provides the call set-up and control functions;
The HSS (Home Subscriber Services) that stores all subscriber data;
The AS (Application Server) for all the IP services.

The VoIP traffic is converted to pure telephony for the PSTN (Public Switch Telephone Network) through the MGW (Media Gateway) and the MGCF (Media Gateway Control Function).

One important network element shared between the LTE network and the IMS network is the PCRF (Policy and Control Reference Function) server that provides all policies for the subscriber.