CEI CLC/TR 50510 PDF

Introduction to Broadband

Communication today is digital and therefore sound, pictures, voice, data carried by networks are data expressed in terms of bit, bytes and their multiples (kilo (k), mega (M), giga (G), tera (T)). A bit (binary digit) is the smallest digital unit and has only two values: 0 or 1. A byte includes 8 bits and defines the size of a data file. Transmission capacity of networks and terminals is not expressed by using bytes, but using bit per second (Bit/s). When a data file is transferred in a network two supplementary bits are necessary. It means that 10 bits are required for 1 byte.

A subscriber who wants to download or upload a large file must wait for data transfer to be accomplished. This time depends on the file size; say 100 MB, and transmission speed. In most networks, like VDSL or cable, transmission speeds are asymmetrical, with download (data transfer towards the user) being faster than upload (data transfer from the user), e.g. 100 Mbit/s and 4 Mbit/s, respectively commonly offered using DOCSIS 3.0 technology. At these speeds, it takes just 8 s to download a 100 MB file, but as much as 200 s (3 min 20 s) to upload it. In ADSL networks, typical download and upload speeds are 15 Mbit/s and 1 Mbit/s, corresponding file transfer times being 53 s and 800 s (13 min 20 s) respectively.

Actual transmission speed in many networks is lower than advertised and vary with traffic load due to oversubscription of shared system capacity. This problem does not exist in point-to-point (P2P) fibre networks without capacity sharing, where 1 Gbit/s symmetrical transmission speed is currently possible.

While users of passively split FTTx network do share common capacity, it is large, for example 2,5 Gbit/s in a GPON system, so fast and reasonably symmetrical service can be offered, like 100/50 Mbit/s or 100/25 Mbit/s.

The following table gives examples of transmission times for downloading a 6,25 GB file with high quality DVD movie.

We do not know all future applications, however existing technologies of today, such as video on demand and the exchange of medical data e.g. for Ambient Assisted Living (AAL), offer an outlook on the future usage of broadband networks.

For the purpose of this document the following classifications will be used:

- low bit rate transmission: up to 1 Mbit/s

- medium bit rate transmission: 1 up to 10 Mbit/s

- high bit rate transmission: 10 up to 100 Mbit/s

- Very high bit rate transmission: 1 Gbit/s and more.