The development of VoIP (Voice over Internet Protocol) technologies have been widely accepted by companies seeking lower costs for voice communications mainly in SMEs (Small and Medium Enterprises) environments. The term VoIP should not be confused with ToIP (Telephony over IP), VoIP refers to the technology required for voice communication over IP, while ToIP is a telephone service for users, which uses VoIP technology to give such service.

VoIP allows voice transmission via an IP network, including those connected to the Internet. It involves digitizing voice signals, via a codec. The bandwidth consumption of such communication is directly related to the codec as shown in Table # 1.

Tabla # 1: Bandwidth consumption for different codec

Furthermore, VoIP uses various types of techniques for call signaling, not having a defined protocol in this field. SIP (Session Initiation Protocol) is one of the protocols used for this purpose, also deployments with H.323 (a recommendation from the ITU Telecommunication Standardization Sector, ITU-T) or IAX (Inter-Asterisk eXchange protocol, native to the Asterisk private branch exchange, PBX) can be found.

SIP is one of the most widespread protocol in the implementation of ToIP. This protocol is responsible for the end-to-end communication signaling, call establishment procedures, communication modification and its termination.For transmitting real-time data, VoIP uses the RTP (Real time Transport Protocol) protocol, which is responsible for transmission control in the multimedia sessions and uses UDP (User Datagram Protocol) as the transport protocol. In Figure # 1, the header distribution for each VoIP packet is observed.

Figure # 1: VoIP Packet transmitted by the stations.

The telecommunications network of an ISP (Internet Service Provider) is actually an interaction of various networks types, within LTE (4G) can be highlighted as an access network to the end user. The information flow reaches to the user through several infrastructures, on which the ISP carries data traffic, from this perspective, some necessary points will be discussed in order to ensure the Quality of Service (QoS) in such environments.

From the point of view of LTE management, it is permitted to define profiles and classes of services, which are key points when negotiating QoS mobile requirements during the communication establishment, the transit of packets, even in handovers. However, it is necessary to consider the access and transport network if we want to ensure end-to-end QoS.

Transport Network

A typical scenario of an Internet service provider could be described as shown in Figure # 1, it can be seen that LTE is only a part of the provider's access network. The ISP has different types of access networks depending on the provided services. All the necessary infrastructure to transport information from the access networks, must transit through the core network. The core is also connected with other different network types, where the end service is usually located (Internet, PSTN, ftp services, video streaming, voice or other), which the user wants to access.

Figure # 1. ISP's simplified topology [2].

With so much information that comes every day around the fourth generation mobile (4G), it is very common that we like to be updated, we care about general aspects of the architecture of this new technology. Well, "System Architecture Evolution" (SAE) or also known as EPC ("Evolved Packed Core") has its development from the year 2004 until 2009, during that time many studies have been deployed which allowed to define the standards that describes the architecture of the core network. The specifications are in chronological order in the following figure:

Figura # 1: SAE evolution [1].

The Propagation of radio signals has a number of inherent problems of electromagnetic waves, of which LTE or 4G is not exempt. Aspects such as absorption, refraction, diffraction, fading, and other properties can cause serious problems of coverage, interference, and general degradation of the transmitted signal. Some of these parameters are largely unpredictable, mainly those arising from meteorological phenomena or the ground, and therefore, fewer steps may be taken in such cases.

Moreover, the free space loss (FSL) is directly related to the system operating frequency and the distance between transmitter and receiver. The case of frequency is a parameter already defined for this type of technology by the International Telecommunication Union [1], also depends on the frequency assignments of each country, and therefore it is not something that operators can easily handle to increase the coverage of their mobile systems.

The case of multipath is a problem that is associated with the spread of radio beam by several pathways, supported by the phenomena of reflection, diffraction and scattering. This type of phenomenon produces a difference in the distance that a beam that travels with sight line over another who has suffered the effect of reflection (to name one example) and consequently there is a difference between the arrival times of the signals and the receiver, which may cause the overlap of the signals at the receiving antenna causing inter-symbol interference (ISI), or too low power to be demodulated, it is also possible that even a given beam incident on the receiving antenna as shown in the following Figure # 1.

Figure # 1: Multipath example.