LTE Overview

 LTE 4G 

Components of RAN (Radio Access Network)

It includes base stations and antennas that cover a specific region based on their capacity.

Radio Access Network consists of Baseband Unit(BBU), Radio Unit/RRH or Remote Radio Unit, Antennas(BS), and Software Interfaces(UEs). 

The user request will be received by Radio Unit and transformed into a digital from signal format by a baseband unit.

BBU provides a set of signal processing functions that make the wireless communication possible. 

In 5G RANs, including 5G cloud-based RANs, the BBU is broken up into the distributed unit (DU) and the central unit (CU).

Opting for a DU and CU architecture can reduce some of the cost of deployments, allow for flexibility in the design of RAN infrastructure, and can be used in a cloud-RAN infrastructure.

The DU runs the radio link control and medium access control (MAC) layers in addition to some of the physical layer at a base station. It in turn is controlled by the CU.

The CU runs the radio resource control protocol, which conducts many functions, including information broadcasting, establishing and releasing connections between the user equipment and the RAN, and controlling the quality of service.

The future is 5G RANs or open source RANs. 

Components of Core Network  

It is also called as Evolved Packet Core(EPC).

MME (Mobility Management Entity)

It talks to the E-nodeB and it has a signaling connection (s1-c) so it is in dotted lines(not solid lines). S1-c denotes control-plane link. 

Responsibilities of MME

1. NAS (Non Access Stratum) signaling and its security.

Attachment of bearer setup and deletion. 

It is responsible to have the continuous communication, 

2. Area List Management 

It keeps track of Area Lists which is E-UTRAN. A network can have multiple E-UTRAN, and MME tracks it with area code for each E-UTRAN. 

3. PGW & SGW Selection

It selects PGW and SGW from the pool. Small operators can have only on of these but it can be many with big operators. 

4. Roaming & Authentication Subscriber

User/Subscriber/User Equipment authentication is managed by MME with the help of HSS.

5. EPS Bearer Management

6. Signaling for Mobility between 3GPP RANs

MME controls when the user swithces between 2G and 4G or 3G and 4G or viceversa. 

Eg: You are Youtube in 4G network coverage, but you want to switch to 3G, without losing/dropping the signaling, it retains the youtube data while switching. 

HSS (Home Subscription Server)

It is a storage of sub data (Auth Keys, QoS Profile, APN Profile etc.,), address of currently serving MME. 

Responsibilities of HSS 

1.  User Authentication

In LTE, there is a big emphasis on security. It allocates a key for every subscriber/User Equipment. HSS is a sort of bank, so it stores all the keys in this bank.

2.  Subscription Profile Management

Profile is a list of services, and the profile is managed per UE.  When UE is registered to the MME, MME gets the profile information for this UE from the HSS. 

Services can be he following:

a) Roaming Control 

Roaming - When a mobile phone is used outside range of its native network and connecting to another cellular network.

It manages where the UE should get the roaming service. Eg: If you are traveling from Leicester to London, it should not roam whereas if the user travels to Ireland, it should roam. So here the native network is United Kingdom, so UE didn't roam.

b) Speed/Throughput Limits

Network operator sets the max throughput limit, so all the users can get upto the set limit. Eg: If the limit is 15Mbps, user/UE will not be able to access the data more than this 15Mbps speed. 

In this way, UEs roaming criteria and speed limit is respected by other network elements(E-UTran/SGW/PGW) in the architecture.

SGW  (Serving Gateway)

It has the userplane link (S1-U) to Enode-B.

Responsibilities of SGW

1. Packet Routing & Forwarding 

SGW ensures that the packet which is meant for UE is delivered to the UE or viceversa in the opposite direction with the help of S1-U link.

2. E-UTRAN Idle Mode DL Packet Buffering

If the UE is not connected to the network (no connection between UE and Enode-B), all the DL packets will be buffered in the SGW until the UE is connected to network. 

Eg: Consider UE/Mobile phone doesn't have internet, but my friend sent a whatsapp message. It will not reach my mobile, but where it will be stored. It will be buffered in SGW because SGW somehow figures out that UE is not connected to the network. Once UE is connected, it will send it via S1-U link. 

3. E-UTRAN and Inter-3GPP MobilityAnchoring

When UE is handing over from one Enode-B to another Enode-B, SGW buffers all the data. Once the handover is successful, it delivers all the data. This is specific E-UTRAN Anchoring.

When the overlap happens from 3G to 4G, the SGW buffers the data in that instance as well. 

4. UL & DL Charging per UE, PDN and QCI

It keeps track of how much data is transmitted to UE and how much data is received from UE. So, all the billing information happens in SGW. 

It deals the accounting for inter-operator charging, and charging per UE.

PGW (Packet Data Network Gateway)

Responsibilities of PDN Gw

1. IP Address Allocation 

UE's IP address is determined by PGW. 

2. Packet Filtering and Policy Enforcement  

It keeps track of the following:

a) Which data needs to be sent to the UE

b) Which service is used by UE

It generates CDR (Call Detail Records) which has the detail information of when the data is transmitted, how much is transmitted, which UE, which service system, etc. 

3. Transport Level QoS Marking and Mapping 

QoS stands for Quality of service. All the services have latency and priority. 

PGW ensures that QoS is respected for all services, if it is video/voice, the PGW knows that the packet needs to reach UE really fast by mapping the QoS.

3. User Info Anchoring for 3GPP and non 3GPP Handovers

QoS stands for Quality of service. All the services have latency and priority. 

Threats to Packet Core Network

https://positive-tech.com/expert-lab/research/epc-research/