Physical Layer:

The Physical layer of B-ISDN can have several different forms but its essential purpose is to collect and organize ATM cells sent down from the ATM layer, transport them to the physical medium and also perform the reverse of the process. It must have an interface with BISDN and provide numerous other functions. It must control mapping and provide some error checking and control.

Interface of the physical layer:

B-ISDN maintains a transmission rate of 155.520Mbps or 622.080Mbps. The physical medium can be optical with an extension capability of 0 - 800m or coaxial cable with an extension capability of 0 - 100m. The SDH-based signal is formed by filling the VC-4 payload space of an STM - frame with ATM cells and the OAM ( Operations and Maintenance ) signals are transported via STMs SOH or POH ( Section overhead, Path overhead ). Since, in case of the transmission speed being 155.520Mbps, only 149.760Mbps is filled with ATM cells and the other 5.760Mbps is filled with STM frame overheads ( SOH, POH, Pointer ). Identifying cell boundaries is done using HEC or an SDH overhead.

Overhead:

The Section Overhead and the Path Overhead of an STM frame provide information on STM frame alignment, STM - 1 and path identification function, section and path error monitoring sections, alarm indication, section and path far-end receive failure, section and path far-end error report and path signaling indication.

Functions of the Physical Layer:

Physical layer consists of two parts - 1. Transmission convergence and 2.Physical medium and each has separate functions.

Transmission convergence:

1.Transmission Frame Generation and Extraction:

This does not apply in cell-based transmission since it does not use a separate frame. However STM-n frames are required in SDH-based transmission.

2. Transmission Frame adaption:

This involves the mapping of ATM cell flow into the payload of the transmission frame, or the ectraction of ATM cell flow from transmission frame.

3. Cell deliniation:

This is the term used to describe identifying ATM cell boundaries in cell flow. It performs the ATM scrambling function and for the receiver performs cell deliniation, confirmation and descrambling.

4. HEC Signal Generation and Confirmation:

Generates the HEC ( Header Error Control ) signal for first 4 bytes of ATM cell header and inserts it into the fifth byte. It applies the reverse procedure in the other direction to detect errors.

5. cell Rate Decoupling:

Adds idle ATM cells to current rate to match overall cell rate to payload capacity and discards them at the receiving end.

Physical Medium:

1. Physical medium:

This depends on the transmission medium - if, for example, optical fibres are used, then the function will be related to that particular medium.

2. Bit-Timing Information:

Conversion of data bit flow into a waveform adapted to a physical medium, or the reverse conversion process, insertion or extraction of timing information and line coding and decoding.

Mapping of an ATM cell:

Before any mapping or transmission, the 48 bytes of information go through a self-synchronous scrambling ( SSS ) based on the polynomial x**43 + 1. So a treated ATM cell can be mapped into the VC-n and then go through synchronous multiplexing. A VC-4 is 2340bytes in size, which is not an even multiple of of an ATM cell ( 53 bytes ). So the cells cross the VC boundary, and as a consequence, the starting location of an ATM cell varies. The VCs POH H4 byte is used for easy detection of the ATM cell boundary.

The distance from the H4 byte to the initial location of the ATM cell is counted. ^ bits are used for this ( 2**6 = 64 ). Figure below shows mapping - page 167.

The receiver detects the ATM cell boundary by searching through STM-n frame to the VC - 4 using AU-4 Ptr., and thenreading the H4. It could also detect the AU-4 starting position and picking out ATM cells, and then searching for the boundary using the H4 byte.

Header Error Control:

The role of the HEC function is to correct any single-bit errors found in any part of the cell header and to detect multi-bit errors. The actions and modes of HEC operation are outlined in the diagram:

After the HEC procedure, the ATM cell is identified either as a valid cell or an invalid cell. Valid cells can also carry errors. This is due to imperfection in Cyclic Redundancy Check or a flaw in error correction stage. These cells and invalid ones are the main cause of degradation in performance of BISDN. The code used for HEC is a cyclic code with generating polynomial - x**8 + x**2 + x + 1. The first 4 bytes, written as a polynomial, are multiplied by x**8 and divided by the generating polynomial. 01010101 is then added. The remainder is recorded in the HEC field, which is subsequently confirmed by the receiving side. At this side the 01010101 is first subtracted before further interpretation.