Quick Overview " Microwave "

Quick Overview " Microwave "

TRANSMISSION SYSTEMS

Introduction To Transmission Systems
•        Transmission systems form the backbone of any networks.
•        Normally transmission systems include SDH, PDH, ATM, Microwaves, leased lines.
•        In GSM normally the core network is located in the same premises and are mostly interconnected by fixed wireline. In huge network consisting of many MSC located at different places the interconnection may be through any of the transmission systems mentioned above.
•        The Access network consists of BSC’s with many BTS’s connected to them in various transmission topologies. Normal practice is to connect various BSC’c to the MSC via fiber and different BTS’s connected to BSC via microwave in Daisy chain, star or any other topology. However there can be many different ways of implementation.

E1

•      
  2.048 Mbps circuit provides high speed, digital transmission for voice, data, and video signals at 2.048 Mbps.
•        2.048 Mbps transmission systems are based on the ITU-T specifications G.703, G.732 and G.704, and are predominant in Europe, Australia, Africa, South America, and regions of Asia.
•        The primary use of the 2.048 Mbps is in conjunction with multiplexers for the transmission of multiple low speed voice and data signals over one communication path rather then over multiple paths.
•        The most common line code used to transmit the 2.048 Mbps signal is known as HDB3 (High Density Bipolar 3) which is a bipolar code with a specific zero suppression scheme where no more then three consecutive zeros are allowed to occur.

FRAMING FORMAT

Typical implementation of a E1

The 2.048 Mbps Framing Format
•        The 2.048 Mbps signal typically consists of multiplexed data and/or voice which requires a framing structure for receiving equipment to properly associate the appropriate bits in the incoming signal with their corresponding channels.
•        The 2.048 Mbps frame is broken up into 32 timeslots numbered 0-31.
•        Each timeslot contains 8 bits in a frame, and since there are 8000 frames per second, each time slot corresponds to a bandwidth of 8 x 8000 = 64 kbps.
•        Time slot 0 is allocated entirely to the frame alignment signal (FAS) pattern, a remote alarm (FAS Distant Alarm) indication bit, and other spare bits for international and national use.

E1
•        The FAS pattern (0011011) takes up 7 bits (bits 2-8) in timeslot 0 of every other frame.
•        In those frames not containing the FAS pattern, bit 3 is reserved for remote alarm indication (FAS Distant Alarm) which indicates loss of frame alignment when it is set to 1.
•        The remaining bits in timeslot 0 are allocated as shown in the following Figure.
•        If the 2.048 Mbps signal carries no voice channels, there is no need to allocate additional bandwidth to accommodate signaling.

  •        Hence, time slot 1-31 are available to transmit data with an aggregate bandwidth of 2.048 Mbps - 64 kbps (TSO) = 1.984 Mbps.
•        If there are voice channels on the 2.048 Mbps signal, it is necessary to take up additional bandwidth to transmit the signalling information.
•        ITU-T Recommendation G.704 allocates time slot 16 for the transmission of the channel-associated signalling information.
•        The 2.048 Mbps can carry up to thirty 64 kbps voice channels in time slot 1-15 and 17-31.
•        Voice channels are numbered 1-30; voice channels 16-30 are carried in time slot 17-31.
•        However, the 8 bits in time slot 16 are not sufficient for all 30 channels to signal in one frame. Therefore, a multiframe structure is required where channels can take turns using time slot 16.
•        Since two channels can send their ABCD signalling bits in each frame, a total of 15 frames are required to cycle through all of the 30 voice channels.
•        One additional frame is required to transmit the multiframe alignment signal (MFAS) pattern, which allows receiving equipment to align the appropriate ABCD signalling bits with their corresponding voice channels.
•        This results in the TS-16 multiframe structure where each multiframe contains a total of 16 2.048 Mbps, numbered 0-15.
•        Figure  on the previous slide shows the TS-16 multiframe format for the 2.048 Mbps signal as defined by the ITU-T Recommendation G.704.
•        As can be seen in Figure , time slot 16 of frame 0 contains the 4-bit long multiframe alignment signal (MFAS) pattern (0000) in bits 1-4. The “Y” bit is reserved for the remote alarm (MFAS Distant Alarm) which indicates loss of multiframe alignment when it is set to 1.
•        Time slot 16 of frames 1-15 contains the ABCD signalling bits of the voice channels.
•        Time slot 16 of the nth frame carries the signalling bits of the nth and (n+15)th voice channels. For example, frame 1 carries the signalling bits of voice channels 1 and 16, frame 2 carries the signalling bits of channels 2 and 17 etc.
•        It is also important to note that the frame alignment signal (FAS) is transmitted in time slot 0 of the even numbered frames.
T1  Introduction
•        T1 is a digital communications link that enables the transmission of voice, data, and video signals at the rate of 1.544 million bit per second (Mb/s).
•        Introduced in the 1960s, it was initially used by telephone companies who wished to reduce the number of telephone cables in large metropolitan areas.
•        T1 simplifies the task of networking different types of   communications equipment since it can carrz both voice and data on the same link.
•        To illustrate, Figure 1 on the next page shows what a company’s communications network might look like without T1
•        Figure 1 shows that telephone, facsimile, and computer applications would all require separate lines.
•        Typically, voice and low-speed data applications are serviced by analog lines, while high-speed data applications are serviced by digital facilities. •        Figure 2 on the next page depicts the same network with a T1 link installed.

FIGURE.1

 

FIGURE.2

PDH Overview
•        Long-established analog transmission systems that proved inadequate were gradually replaced by digital communications networks.
•        In many countries, digital transmission networks were developed based upon standards collectively known today as the Plesiochronous Digital Hierarchy (PDH).
•        Although it has numerous advantages over analog, PDH has some shortcomings: provisioning circuits can be labor-intensive and time-consuming, automation and centralized control capabilities of telecommunication networks are limited, and upgrading to emerging services can be cumbersome.
•        A major disadvantage is that standards exist for electrical line interfaces at PDH rates, but there is no standard for optical line equipment at any PDH rate, which is specific to each manufacturer.
•        This means that fiber optic transmission equipment from one manufacturer may not be able to interface with other manufacturers’ equipment.
•        As a result, service providers are often required to select a single vendor for deployment in areas of the network, and are locked into using the network control and monitoring capabilities of that vendor.
•        Reconfiguring PDH networks can be difficult and labor-intensive - resulting in costly, time-consuming modifications to the network whenever new services are introduced or when more bandwidth is required.
SDH Overview
•        Bellcore (the research affiliate of the Bell operating companies in the United States) proposed a new transmission hierarchy in 1985.
•        Bellcore’s major goal was to create a synchronous system with an optical interface compatible with multiple vendors, but the standardization also included a flexible frame structure capable of handling either existing or new signals and also numerous facilities built into the signal overhead for embedded operations, administration, maintenance and provisioning (OAM&P) purposes.
•        The new transmission hierarchy was named Synchronous Optical Network (SONET).
•        The International Telecommunication Union (ITU) established an international standard based on the SONET specifications, known as the Synchronous Digital Hierarchy (SDH), in 1988.
•        The SDH specifications define optical interfaces that allow transmission of lower-rate (e.g., PDH) signals at a common synchronous rate.
•         A benefit of SDH is that it allows multiple vendors’ optical transmission equipment to be compatible in the same span.
•        SDH also enables dynamic drop-and-insert capabilities on the payload; PDH operators would have to demultiplex and remultiplex the higher-rate signal, causing delays and requiring additional hardware.
•        Since the overhead is relatively independent of the payload, SDH easily integrates new services, such as Asynchronous Transfer Mode (ATM) and Fiber Distributed Data Interface (FDDI), along with existing European 2, 34, and 140 Mbit/s PDH signals, and North American 1.5, 6.3, and 45 Mbit/s signals.

FIGURE.1

FIGURE.2

SDH Overview STM1 data rate calculation
•        SDH multiplexing combines low-speed digital signals such as 2, 34, and 140 Mbit/s signals with required overhead to form a frame called Synchronous Transport Module at level one (STM-1).

•        Figure 1 shows the STM-1 frame, which is created by 9 segments of 270 bytes each.
•        The first 9 bytes of each segment carry overhead information; the remaining 261 bytes carry payload.
•        When visualized as a block, the STM-1 frame appears as 9 rows by 270 columns of bytes.
•        The STM-1 frame is transmitted row #1 first, with the most significant bit (MSB) of each byte transmitted first.
•        This formula calculates the bit rate of a framed digital signal:
•        bit rate = frame rate x frame capacity
•        In order for SDH to easily integrate existing digital services into its hierarchy, it operates at the basic rate of 8 kHz, or 125 microseconds per frame, so the frame rate is 8,000 frames per second.
•        The frame capacity of a signal is the number of bits contained within a single frame.
•        Figure 2 shows: frame capacity = 270 bytes/row x 9 rows/frame x 8 bits/byte = 19,440 bits/frame
•        The bit rate of the STM-1 signal is calculated as follows:         
bit rate = 8,000 frames/second x 19,440 bits/frame = 155.52 Mbit/s
SDH Overview Multiplexing of STM frames

•        As the Figure coming on the next slide shows, the ITU has specified that an STM-4 signal should be created by byte interleaving four STM-1 signals.
•        The basic frame rate remains 8,000 frames per second, but the capacity is quadrupled, resulting in a bit rate of 4 x 155.52 Mbit/s, or 622.08 Mbit/s.
•        The STM-4 signal can then be further multiplexed with three additional STM-4s to form an STM-16 signal.
•        Table 1 lists the defined SDH frame formats, their bit rates, and the maximum number of 64 kbit/s telephony channels that can be carried at each rate.
•        Normally used for point to point transmission
•        Used mainly in the GHz range.
•        Normally distance between radios is less than 50Kms.....