Teori dasar PDH (Plesiochronous Digital Hierarchy) 

PDH adalah suatu sistem yang selama ini digunakan sebelum SDH ditemukan. Kata plesio berasal dari bahasa yunani yang berarti hampir. Dalam sistem PDH, perbedaan sebesar 50 bit pada kecepatan 2,048 Mbps adalah sesuatu yang wajar. Hal ini dikarenakan PDH tidak menyinkronkan jaringan dalam arti sesungguhnya. PDH hanya menggunakan pulsa detak maksimum pada setiap simpul (switching node) sebagai standar. Jika tidak ada lagi data dalam buffer (dikarenakan sinyal data berikutnya menggunakan pulsa detak yang lambat), maka PDH akan menyisipkan bit-bit tambahan (stuffbit). Sebaliknya, multiplexer penerima harus membuang bit-bit tambahan tersebut. Sistem multiplekser PDH yang sekarang ini paling banyak digunakan adalah multiplekser PDH dari Eropa, Jepang dan Amerika utara. Dari ketiga sistem multiplekser mempunyai perbedaan dalam jumlah bit ratenya. Pada sistem Eropa menggunakan bit rate 2048 Kbit/s, sedangkan pada Amerika utara dan Jepang menggunakan bit rate sebesar 1544 Kbit/s. Multiplekser PDH dari Amerika utara dan Jepang menggunakan sistem PCM 24, sedangkan multiplekser PDH dari Eropa menggunakan PCM 30. Di Negara Indonesia sistem multiplekser PDH menggunakan multiplekser PDH dari Eropa. Untuk melihat lebih jelas perbedaan bit rate antara ketiga Negara tersebut, dapat dilihat pada gambar dibawah ini.

Gambar 1. Struktur PDH Eropa, Jepang dan Amerika Utara

Pada sistem PDH Eropa tiap tingkatan multiplexing diberikan nama, diawali dari E1 (Euro level 1) = 2,048 Mbit/s, E2 = 8,448 Mbit/s, E3 = 34,368 Mbit/s, dan yang terakhir E4 = 139,264 Mbit/s. Pada sistem PDH sebenarnya masih mempunyai nilai yang lebih tinggi lagi dari 139,264 Mbit/s atau biasa disebut 140 Mbit/s, tetapi tidak distandarkan secara internasional oleh CCITT (Consultative Committee International Telegraphy and Telephony). Hirarki pada sistem PDH Eropa dapat dilihat pada gambar dibawah ini.

Gambar 2.  Hirarki pada PDH

Pada PDH sistem demultipleksernya harus dilakukan secara lengkap, dalam hal ini bisa dilihat dari gambar 2. Jika multipleksernya mentransmisikan sinyal 140 Mbit/s, maka pada proses demultiplekser pun harus diawali dari kecepatan 140 Mbit/s terlebih dahulu dan dilanjutkan ke tahap selanjutnya (kebalikan dari multipleksernya). Jika kita menginginkan kecepatan  2 Mbit/s, maka kita harus melalui tahapan-tahapan secara berurutan baru bisa mendapatkan kecepatan yang diinginkan. Jika dilihat dari teori yang telah dibahas maka multiplexing PDH bisa juga disebut multi-steep multiplexing (multiplexing yang mempunyai banyak tahap).

Metode justifikasi pada multiplexing PDH dilakukan secara bit per bit. Metode justifikasi (stuffing) yang dilakukan bit per bit menghasilkan jitter yang tidak terlalu tinggi. Jitter dapat didefinisikan sebagai variasi jangka pendek sinyal digital dari posisi idealnya dalam waktu. Jitter yang berlebihan dapat menyebabkan kerugian terhadap sinyal digital yang ditransmisikan (pembangkitan bit error, slip yang tak terkendali). Oleh karena itu besarnya jitter pada interface jaringan harus dibatasi, untuk menjamin kualitas sinyal transmisi yang memadai.

Introduction of Telecomunication technology

The first radiotelephone service was introduced in the US at the end of the 1940s, and was meant to connect mobile users in cars to the public fixed network. In the 1960s, a new system launched by Bell Systems, called Improved Mobile Telephone Service” (IMTS), brought many improvements like direct dialing and higher bandwidth. The first analog cellular systems were based on IMTS and developed in the late 1960s and early 1970s. The systems were “cellular” because coverage areas were split into smaller areas or “cells”, each of which is served by a low power transmitter and receiver.

This first generation (1G) analog system for mobile communications saw two key improvements during the 1970s: the invention of the microprocessor and the digitization of the control link between the mobilephone and the cell site.

Second generation (2G) digital cellular systems were first developed at the end of the 1980s. These systems digitized not only the control link but also the voice signal. The new system provided better quality and higher capacity at lower cost to consumers.

Third generation (3G) systems promise faster communications services, including voice, fax and Internet, anytime and anywhere with seamless global roaming. ITU’s IMT-2000 global standard for 3G has opened the way to enabling innovative applications and services (e.g. multimedia entertainment, infotainment and location-based services, among others). The first 3G network was deployed in Japan in 2001. 2.5G networks, such as GPRS (Global Packet Radio Service) are already available in some parts of Europe.

Work has already begun on the development of fourth generation (4G) technologies in Japan.

It is to be noted that analog and digital systems, 1G and 2G, still co-exist in many areas.



The Basics of Cellular Technology and the Use of the Radio Spectrum

Mobile operators use radio spectrum to provide their services. Spectrum is generally considered a scarce resource, and has been allocated as such. It has traditionally been shared by a number of industries, including broadcasting, mobile communications and the military. At the World Radio Conference (WRC) in 1993, spectrum allocations for 2G mobile were agreed based on expected demand growth at the time. At WRC 2000, the resolutions of the WRC expanded significantly the spectrum capacity to be used for 3G, by allowing the use of current 2G spectrum blocks for 3G technology and allocating 3G spectrum to an upper limit of 3GHz.

Before the advent of cellular technology, capacity was enhanced through a division of frequencies, and the resulting addition of available channels. However, this reduced the total bandwidth available to each user, affecting the quality of service. Cellular technology allowed for the division of geographical areas, rather than frequencies, leading to a more efficient use of the radio spectrum. This geographical re-use of radio channels is knows as “frequency reuse”.

In a cellular network, cells are generally organized in groups of seven to form a cluster. There is a “cell site” or “ base station” at the centre of each cell, which houses the transmitter/receiver antennae and switching equipment. The size of a cell depends on the density of subscribers in an area: for instance, in a densely populated area, the capacity of the network can be improved by reducing the size of a cell or by adding more overlapping cells. This increases the number of channels available without increasing the actual number of frequencies being used. All base stations of each cell are connected to a central point, called the Mobile Switching Office (MSO), either by fixed lines or microwave. The MSO is generally connected to the PSTN (Public Switched Telephone Network):