In previous Generations, the base stations were controlled by a central device.
In 2G, it was the base station controller, and in 3G it was the RNC.
These controllers were responsible - For setting up the radio links to wireless
devices via the base stations, For controlling the connections while in use,
For ensuring QoS and for handing over a connection to another
base station when required.
In LTE, this concept was abandoned, as it required significant resources, because the
task was concentrated in few network nodes.
Most applications on the device only transmit and receive information in bursts with long
timeouts in between.
During these times of inactivity, the air interface connection to the mobile device
has to be changed to use the available bandwidth efficiently and to reduce the power consumption
of mobile devices.
So The packet-switched connections generate a lot of signaling load because of the frequent
switching of the air interface state.
So these management task were distributed, to speed up the connection setup time and
reduce the time required for handover, which is very crucial for real time services.
Thus making the LTE access network a simple flat network of interconnected Base Stations
without a centralised controller.
Hello everyone, Welcome back to the world of LTE today we will discuss the Radio Access
Network of LTE which is also called evolved UMTS Terresterial Radio Access Network or
eUTRAN.
The most complex node in the LTE network is the base station, referred to as eNode-B.
It is derived from the name which was originally given to the UMTS base station (Node-B) with
an ‘e’ referring to ‘evolved’.
eNode-Bs consist of 2 major elements: • RRU or the Remote Radio Unit consists
of the antennas, It is also called as remote radio head, which are the most visible parts of a mobile network.
They are also responsible for Modulation and Demodulation of all signals transmitted or
received on the air interface;
•The BBU or Base Band Unit consists of digital modules that process all signals transmitted
and received on the air interface and acts as an interface to the core network over a
high-speed backhaul connection.
Now,In LTE base stations are autonomous units.
Most functions of the RNC were integrated into the eNodeB.
Hence, the eNode-B is not only responsible for the air interface but also for
Radio Resource Management that includes - Radio Bearer Control
- Radio Admission Control - Connection Mobility Control
- Scheduling i.e Dynamic allocation of resources to UEs in both uplink and downlink
It is also responsible for, - IP header compression and encryption of user data stream
- Selection of an MME at UE attachment when no routing to an MME can be determined from the information provided by the UE
- Routing of User Plane data towards Serving Gateway
- Scheduling and transmission of paging messages (originated from the MME)
- Scheduling and transmission of broadcast information (originated from the MME or O&M)
- Measurement and reporting configuration for mobility and scheduling
Now, The interface between the base station and
the core network is referred to as the S1 interface.
It is usually carried either over a high-speed copper or fiber cable, or alternatively over
a high-speed microwave link.
The S1 interface is split into two logical parts, which are both transported over the
same physical connection- User data is transported over the S1 User
Plane part of the interface.
IP packets of a user are tunneled through an IP link in a manner similar to that already
described for GPRS to enable seamless handovers between different LTE base stations and UMTS
or GPRS/EDGE.
In fact, the GPRS Tunneling Protocol (GTP) is reused for this purpose.
Only the destination IP address on layer 3 (the tunneling IP layer) is changed, while
the user’s IP address remains the same.
The S1 Control Plane (S1-CP) protocol, as defined in 3GPP (TS 36.413, [7])
is required for two purposes: Firstly, the eNode-B uses it to interact with
the core network for its own purposes, that is, to make itself known to the network, to
send status and connection keep-alive information and for receiving configuration information
from the core network.
Secondly, the S1-CP interface is used for transferring signaling messages that concern
the users of the system.
For example, when a device wants to communicate using the LTE network, an individual logical
connection has to be established and the core network is responsible for authentication,
for supplying keys for encrypting data on the air interface and for the establishment
of a tunnel for the user’s data between the eNode-B and the core network.
Once the user’s data tunnel is in place, the S1-CP protocol is used to maintain the
connection, to organize a handover of the connection to another LTE, UMTS or GSM base
station as required.
In S1-CP protocol stack, Instead of the commonly known TCP and UDP protocols on layer 4, the
Telecom-specific Stream Control Transmission Protocol (SCTP) is used as defined in RFC 4960.
It ensures that a large number of independent signaling connections can be established simultaneously
with in-sequence transport, congestion management and flow control.
Now, As mentioned earlier, Unlike the previous generation technologies, LTE integrates the
radio controller function into the eNodeB.
This allows tight interaction between the different protocol layers of the radio access
network, thus reducing latency and improving efficiency.
Such distributed control eliminates the need for a high-availability, processing-intensive
controller, which in turn has the potential to reduce costs and avoid ‘single points
of failure’.
One consequence of the lack of a centralized controller node is that, as the UE moves,
the network must transfer all information related
to a UE, i.e. the UE context, together with any
buffered data, from one eNodeB to another.
So to avoid data loss during handover, X2 interface was introduced.
In LTE, base stations can communicate directly with each other over the X2 interface for
two purposes: First, handovers are now controlled by the
base stations themselves.
If the target cell is known and reachable over the X2 interface, the cells communicate
directly with each other.
Otherwise, the S1 interface and a core network are employed to perform the handover.
The second use of the X2 interface is for interference coordination.
As in UMTS, neighboring LTE base stations use the same carrier frequency, there are
areas in the network where mobile devices can receive the signals of several base stations.
If the signals of two or more base stations have a similar strength, the signals of the
base stations that the mobile device does not communicate with at that moment, are perceived
as noise and the resulting throughput suffers significantly.
As mobile devices can report the noise level at their current location and the perceived
source to their serving base station, the X2 interface can then be used by that base
station to contact the neighboring base station and agree on methods to mitigate or reduce
the problem.
So friends, in this video we have seen how limitations of previous generations were mitigated
by Decentralizing the Radio access network in LTE.
In our next video we will be discussing about the Evolved Packet Core.
Happy Learning.