Packet Switching
When packets are send over a network they consist of a header and raw data. The data in the packet never changes but the header is different dependant on a number of key components:
The header is generated by a number of applications that fall within the OSI 7 layer model.
- Source Address
- Destination Address
- Route
- Size
- Length
- Type
- Error Management
- Priority
The header is generated by a number of applications that fall within the OSI 7 layer model.
Public switched telephone network (pstn)
A PSTN is the backbone of cabled telecommunications in the UK and is provided by British Telecom. PSTN is a circuit switched network able to direct any source to any other destination on the network irrespective of location or distance.
The equipment below is analogue and connected by copper cable.
- DP (Distribution Point) - Situated on street furniture these provide a spur to individual consumers for the PSTN.
- CAB (Street Cabinets) - Situated in many streets around the UK these cabinets receive the copper cables coming from the consumers and their DP's and are connected to multicore cables that draw further into the network.
- DLE (Digital Local Exchange) - These contain are the boundary of the digital ISDN (Integrated Services Distribution Network) network and contain ADC (Analogue to Digital Converter) and DAC (Digital to Analogue Converter) to transform the analogue circuits information into data and back again. The DLE's provide connection and tones to the call maker and receiver when needed. They are referred to as processor switches by BT.
- DMSU (Digital Main Switching Units) / Tandem Switch - These are the major exchange's of the UK's network. They are connected together in a mesh network via optical cable. DLE's are always connected to DMSU's via 2 paths due to redundancy and protection of the infrastructure.
- ISC (International Switching Centre) - When the network detects a number beginning in 00 they are routed to the ISC. The ISC then liaises with various paths worldwide to negotiate the cheapest possible route at that time for international connectivity.
PDH & SDH
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PDN (Plesiochronous Digital Hierarchy) is used in a system of non synchronous nature, the paths will have the same bit rate but operate from different crystal timing oscillators. The signals are combined at each point gradually multiplexed to larger bitrates as the levels of the ISDN are achieved.
The PDH hierarchy must b demultiplexed to obtain lower level bit-rates. SDH (Synchronous Digital Hierarchy) works with the fact that the exact rates that are used to transport the data on SDH are tightly synchronised across the entire network, using atomic clocks. This synchronisation system allows entire inter-country networks to operate synchronously, greatly reducing the amount of buffering required between elements in the network. The SDH Hierarchical levels are shown on the image on the right. The bit rates conform to ITU G.738 worldwide.
The PDH hierarchy must b demultiplexed to obtain lower level bit-rates. SDH (Synchronous Digital Hierarchy) works with the fact that the exact rates that are used to transport the data on SDH are tightly synchronised across the entire network, using atomic clocks. This synchronisation system allows entire inter-country networks to operate synchronously, greatly reducing the amount of buffering required between elements in the network. The SDH Hierarchical levels are shown on the image on the right. The bit rates conform to ITU G.738 worldwide.