What is YateBTS?
YateBTS is a software implementation of a GSM/GPRS radio access network based on Yate and is compatible with both 2.5G and 4G core networks comprised in our YateUCN unified core network server.
Resiliency, customization and technology independence are the main attributes of YateBTS.
YateBTS takes a GSM signal from your cell-phone or any other machine, and sends it through a VoIP connection anywhere in the world.
We offer an extended range of software solutions:
We offer two main hardware products:
It is intended for mobile network operator labs, M2M application development, mobile phone vendors, academics and security researchers.
When connected to a tower mounted booster, the YateBTS SatSite can reach a power output ranging from 10 to 50W.
YateBTS SatSite is a simplified solution that implements both the radio access network and the core network, offering the complete functionality of a 2.5G network.
It is highly power efficient, and can be charged by solar panels.
SatSite also comes with the great advantage of being based on commodity hardware that can be serviced locally, in the carrier’s operating country.
How It Works
When you use your cell in a YateBTS network, the GSM signal reaches the antenna of the YateBTS.
Afterwards, the signal passes up to Layers 1 and 2 where the GSM signal is processed and is fed through the socket to Yate.
Yate renders the received connection with the protocol needed (SIP or another) to communicate with the outside server of the VoIP provider, for example, that links you to the person or machine that you want to communicate with.
YateBTS has been created with the purpose of delivering an enhanced and combined solution between L1 PHY, L2 Link Layer and L3 Radio Resource Manager, also know as MBTS, and Yate’s multiple features, such as the IAX over satellite, SS7 and Diameter, USSD, RManager, roaming or local phone switch.
YateBTS is a fundamental element in the YateUCN™, a single core solution for both LTE and 2/2.5 G networks.
It is is usually configured to operate in one of two modes:
- YateUCN™ for for 2G SS7/MAP or 4G IMS/VoLTE mobile operate networks or
The Conventional Network Architecture Vs. the Unified Core Network™
Here is the Unified Core Network solution proposed by Legba and SS7Ware:
How the Unified Core Network Works
For the transmission of data, through GPRS, the YateBTS base station communicates with OpenSAE, which acts as a typical 2.5G GGSN element, over a GTP interface, thus routing traffic to and from the Internet. YateBTS covers all frequencies between 70 Mhz up to 3 GHZ.
Second of all, the MBTS connects to the radio transceiver through a socket interface, the same way the MBTS does to the network layer.
Network in a PC
It performs registering, routing calls, SMS, USSD messages and authenticates users to the public release of YateBTS.
NiPC consists of an HLR, an AuC and a VLR/MSC.
Features of public and private versions of YateBTS
- a radio transceiver
- MBTS, the L1 PHY, L2 link layer and L3 radio resource manager.
- YBTS, a Yate module, is connected to the lower layers, namely Layer 1, 2 and 3, by a hard socket.
The Advantages of YateBTS Architecture
Yate and YBTS
In other words, MBTS sets up the radio channel and forwards all information received on that channel to YBTS. YBTS then sees if it’s a call/SMS/USSD/registration/etc. request and sends a specific Yate message for that type of request. Other Yate modules or custom applications handle this messages. An example of this lays in the Network in a PC application that comes by default with YateBTS. YBTS is the link between MBTS, Yate and its many functionalities.
The architecture of Yate is based on a message passing system and can be divided into four main parts as you can see below:
- dynamic libraries loaded as plugins in the engine (YBTS is one such module)
- or external applications started by a specific module (extmodule) that allows them to talk to the engine and other modules.
It is worth mentioning that VoIP or PBX is just one of the implemented modules and not the core functionality.
In fact Yate is a multifunctional type of product, that can be used in many situations, where distance communication is to take place.
Some types of modules that could bring great value when used together with YateBTS are: billing, monitoring and the various software drivers that Yate offers, such as SIP, IAX, Jabber (public version) and SS7 (in the commercial version of YateBTS).
For a more in-depth description of Yate modules functionalities please see: Modules
Layer 3 Radio Resource Management
Layer 2 Functions
Layer 1 Functions
Since adjacent channels overlap, the standard does not allow adjacent channels to be used in the same cell.
The standard defines several bands ranging from 400 MHz to 1990 MHz.
Uplink and downlink bands are generally separated by 45 or 50 MHz (at the low-frequency end of the GSM spectrum) and 85 or 90 MHz (at the high-frequency end of the GSM spectrum).
Uplink/downlink channel pairs are identified by an index called the ARFCN.
Within the BTS, these ARFCNs are given arbitrary carrier indexes C0..Cn-1, with C0 designated as a Beacon Channel and always operated at constant power.
GSM has physical and logical channels.
The logical channel is time-multiplexed into 8 timeslots, with each timeslot lasting for 0.577ms and having 156.25 symbol periods.
These 8 timeslots form a frame of 1,250 symbol periods.
Channels are defined by the number and position of their corresponding burst period.
The capacity associated with a single timeslot on a single ARFCN is called a physical channel (PCH) and referred to as “CnTm” where n is a carrier index and m is a timeslot index (0-7).
Each timeslot is occupied by a radio burst with a guard interval, two payload fields, tail bits, and a midamble (or training sequence).
The lengths of these fields vary with the burst type but the total burst length is 156.25 symbol periods.
The most commonly used burst is the Normal Burst (NB).
Multiplexing and Timing
One logical channel constitute of 8 burst periods (or physical channels) which is called a Frame.
Traffic channel multiplexing follows a 26-frame (0.12 second) cycle called a “multiframe”.
Control channels follow a 51-frame multiframe cycle.
The C0T0 physical channel carries the SCH, which encodes the timing state of the BTS to facilitate synchronization to the TDMA pattern.
GSM timing is driven by the serving BTS through the SCH and FCCH.
All clocks in the handset, including the symbol clock and local oscillator, are slaved to signals received from the BTS, as described in GSM 05.10.
BTSs in the GSM network can be asynchronous and all timing requirements in the GSM standard can be derived from a stratum-3 OCXO.
As a general rule, each GSM channel uses a block parity code (usually a Fire code), a rate-1/2, 4th-order convolutional code and a 4-burst or 8-burst interleaver.
Notable exceptions are the synchronization channel (SCH) and random access channel (RACH) that use single-burst transmissions and thus have no interleavers.
For speech channels, vocoder bits are sorted into importance classes with different degrees of encoding protection applied to each class (GSM 05.03).
Both 260-bit vocoder frames and 184-bit L2 control frames are coded into 456 bit L1 frames.
On channels with 4-burst interleaving (BCCH, CCCH, SDCCH, SACCH), these 456 bits are interleaved into 4 radio bursts with 114 payload bits per burst.
On channels with 8-burst interleaving (TCH, FACCH), these 456 bits are interleaved over 8 radio bursts so that each radio burst carries 57 bits from the current L1 frame and 57 bits from the previous L1 frame.
Interleaving algorithms for the most common traffic and control channels are described in GSM 05.03 Sections 3.1.3, 3.2.3 and 4.1.4.
Closed Loop Power Control
The reason that you have such a function is because the power level is a critical component for communication.
Without a power level management, the nicest thing that can happen is the voice call will have a poor quality.
What it usually happens is that the voice call will be dropped or you cannot establish one due to the saturation of the YateBTS receiver.
It’s called a loop because twice a second there is a message exchange between the mobile station and YateBTS in the following order:
Pcommnand = Ptx + (Ptarget – Prx)
In the lower bands (850 and 900), the power control range is 5-33 dBm.
In the upped bands (1800 and 1900) the range is usually 5-30 dBm.
The difference between transmitted and received power is due to path loss, (Lpath).
The formula relating transmitted and received power to path loss is:
Prx = Ptx – Lpath + Ga
This is expressed in decibels, so this is actually a logarithmic equation.
Lpath is typically in the range of 120 to 170 dB.
For a correct setup of the RSSI target parameter please see the subsection about “Radio.RSSITarget” in the “gsm_advanced” configuration section.
Closed Loop Timing Control
To solve this problem, GSM using a timing correction called timing advance, which is controlled in a closed-loop process similar to the process used for closed loop power contro