Behind Mobile LTE: Core Network

LTE mobile broadband networks might look like an obvious job for a Wireless Network Engineer. And in fact, wireless knowledge is essential when optimizing the service itself. But underneath the antennas, the Core Network Engineer is the one that ensures full connectivity in the entire mobile network.

Although 3G was the first technology to give real mobile access to the Web, it wasn’t until LTE “Long Term Evolution” or sometimes called 4G, that our lives became completely mobile. Full access to mobile multimedia at high speed is an offer very hard to resist for the common Internet consumers.

Even though 5G is on the horizon, the growth of LTE is still inevitable. According to GSMIntelligence, more than 580 operators have launched LTE networks in 188 countries as of December 2016. 

And not all those networks are small. Reliance Jio, a large ISP in India has created a superfast LTE network that can reach up to 135 Mbps and serve 90% of the population in India, an outstanding 1.12 billion people!

Deploying an LTE network of this scale involves a combination of wireless, infrastructure and networking experience.

LTE’s architecture

How does this amazing technology works? LTE designers decided to architect the technology with very few network elements. Having fewer hops helps in lowering the network latency and increases flexibility.

LTE uses a very simple flat network architecture, as shown in Fig. 1. At a high level, this architecture is composed of four basic elements:

Figure 1

1. The User Equipment (UE): This is the device that requests connectivity to the network and downloads/uploads any data, in other words, “your smartphone”. A UE must be compatible with the technology and must be attached and registered to a network, for example, AT&T’s LTE network. When the user starts an Internet session, the device activates a Packet Data Protocol “PDP” context and waits for a response.
2. The Evolved UMTS Terrestrial Radio Access Network (E-UTRAN): The network of antennas or Evolved Node B (EnodeB), gives radio access to the UE anywhere there is coverage. When an EnodeB receives the request from the UE, it will forward it to the core network.
3. The Evolved Packet Core (EPC) The EPC is the brain of the network. It validates the session request from the UE, generates a PDP context and gives access to the Public Data Network “PDN”.
4. The Public Data Network (PDN): Is a shared network that is accessed by users that belong to different organizations. In this case, it could be an Internet Service Provider (ISP) that provides data transmission to the public.

The Evolved Packet Core (EPC)

The EPC is a centralized set of elements that collaborate with each other to provide LTE service to millions of subscribers at very high speeds. It is the job of a network engineer to ensure that an EPC is a functional, scalable and available network.

The EPC consists of five different elements that play an important role in LTE as shown in Fig 2.

Figure 2

1. The Mobility Management Entity (MME): It does all the signaling for the mobile devices but does not process any user data traffic. An MME will provide session and mobility management for users. A local MME can connect to another (remote) MME to provide data roaming exchange. For more information on MME platforms, check out the Cisco’s MME ASR 5×00 Hardware.
2. Home Subscriber Server (HSS): A device used for user authentication. It can also provide subscriber data management and mobility information. To get an idea of an HSS hardware platform, check out Huawei’s IMS/HSS 9860.
3. The Serving SAE Gateway and PDN Gateway (S-GW and P-GW): can be combined into one or SAE-GW. The S-GW connects to the eNodeB (antenna) and provides user data path for the UEs. The P-GW provides access to the PDN, allocates IP addresses and does all the packet filtering. For more information on SAE-GW hardware platform, check out Cisco’s ASR 5000 Series and its configuration guide.
4. Domain Name Server (DNS): EPC depends on DNS to find critical services. Usually, EPC services are referenced by names, such as an APN “Access-Point Name”. DNS will provide the mapping of these names to gateway IP address so that users can have access to the Internet. LTE DNS computations are much lower than SAEGW or MME, so DNS software can be configured in an independent hardware platform, for example, an HP Proliant DL380 Gen9 Server will do perfectly fine.
5. Policy and Charging Rules Function (PCRF): This server controls the service policy. It uses QoS information for managing each user’s session and billing rule information. Check out Cisco’s Policy Suite Mobile including PCRF.

When a mobile device connects to the Internet

When a user wants to browse the Internet, the UE needs to be registered to the LTE network by following a procedure called Network Attachment. The UE connects to the antenna “eNodeB” which in turn forwards the request to the MME.

The user is first authenticated by the HSS. The MME will then send a session request which forwards it to the SAE-GW, which in turn assigns an available IP. The SAE-GW will return the session response to the MME and the MME will send all the information back to the UE.

The UE can start browsing online. For more information on the technical LTE specification, visit 3GPP website.

Designing and Installing an EPC

Although EPC elements have routing capabilities, their main focus is on the LTE service itself. Another device, such as a Layer 3 switch should interconnect all elements and provide a high-availability network design. Either a Master/Slave or Load-Balanced failover configuration should be set between servers. The layer 3 switch should also be able to handle large sums of LTE traffic coming from the E-UTRAN and going to the PDN.

When designing an EPC, the network engineer needs to take care of the IP address allocation, taking into account future expansion. Choosing the wrong subnet mask in a fast-growing network such as LTE could easily lead to rapid IP address exhaustion.

EPC elements connect with each other using defined interfaces so that control and user traffic can be separated. Each interface on the EPC elements should be configured using VLANs to keep strong security and simple routing. For example, it is highly advisable to separate billing information from user traffic.

Knowing about infrastructure installation is critical when racking and cabling an EPC. Although not mandatory, fiber optic cabling is mostly recommended to be able to handle high speeds and large sums of user traffic.

An EPC should be located in a data center following standards, such as physical security, anti-seismic flooring and fire detection systems.

How to expand your knowledge on LTE

Running an EPC and LTE network is supposed to be a business for Internet Service Providers. But with the recent release of private LTE-EPC networks and SDN, we could see an increased demand for LTE expertise.

There are some certifications you can take to expand your knowledge on LTE. Among the many Cisco certifications, SPLTE, “Service Provider Mobility LTE Networks” focuses on learning about the functionality of EPC. 

Another great LTE networking certification is HCNP-LTE or Huawei Certified Network Professional-LTE. Huawei is one of the leaders in LTE deployments and their experience makes them qualified to offer this certification. HCNP-LTEP targets many areas of the LTE, from the air interface to the Core network.

Summary

Today, LTE is the ruling technology for Internet access and for many reasons. It does not only reach high speeds, from 100Mbit/s-300Mbit/s, but it will also provide Internet access while moving at high speeds. Imagine going at 200 Km/hr on a high-speed train while still being able to have a normal VoIP conversation… That is LTE!

This degree of mobility is only possible with LTE. And it will go high lengths to keep us online. Although 5G is already in the headlines, it will still take years to standardize and finalize.