Frame Relay - for Faster and More Efficient Data Communications ...

40Fig. 9Combination of layering and partitioning; exampleLRDXCLocal networkRegional networkDigital cross connectFig. 10PDH-based Transport Network Layered Model(CEPT hierarchy)local level, NET 2 at the district, NET 3 atthe regional and NET 4 at the national level.Signal transport in real networks leads toa combination of layering and partitioning,Fig. 9. A 2 Mbit/s signal passes throughdigital cross connects (DXC) located in alocal network, is transmitted via a 140Mbit/s line between the cross connectsand via an STM-1 system between the localnetworks, and then passes againthrough DXCs in a local region. Three ofthe DXCs use the 2 Mbit/s and one of themthe 140 Mbit/s switching level. The exam-pie shows that all Reference Points andTransport Processing Functions involvedin the transport of information can be identifiedas belonging to a certain layer of thetransport network and a certain level of abstraction.In CCITT draft Recommendation G.snal,the transport network is divided into threelayers, which in turn can be divided intosublayers. The layers are as follows:-The Circuit Layer network is concernedwith the transfer of information betweenusers, in direct support of telecommunicationservices. This circuit definition isspecifically used to describe network architectureand differs from CCITT circuitdefinitions used for other purposes-The Path Layer network supports theCircuit Layer network and may consist ofvarious sublayers- The Transmission Media Layer networksupports the Path Layer network andconsists of a function for transfer of informationbetween two nodes and afunction concerned with the physical media.The concepts introduced above are applicableto transport networks based on PDHand SDH.Plesiochronous Digital Hierarchy(PDH)In this article, the following PDH characteristicsignal types are considered:DSn Binary digital signals with datarates and framing according to theorder n in Rec. G.702. (Signalsgenerated and processed in terminalequipment)Dn Digital signals with data rates andpulse masks according to Rec.G.703. (Signals transmitted viastandardised interfaces).ERICSSON REVIEW No. 1-2, 1992