Let us see in brief what telecommunication network look like in general.
Refer a diagram below.
User device is connected to network via a network entity, we can call it 1st level Network Access device. This device in turn can connect to 2nd level Network Access device. 2nd level network access device will handle mainly Network layer functions. If user device requested (or required) connection to another user device present in another network, 2nd level network access device will need to take help of Gateway Access device. Gateway access device will typically be knowlegeable in both network protocol and gateway or common protocol with another network. Network may also contain other devices for authentication, location mapping (required if user is mobile) etc. All devices will have layers for signaling/transmission planes as specified in corresponding standards.
Let us take an example of GSM mobile.
User device is of course Mobile (or Cell), supporting GSM and GPRS. Access would be over air (Wireless access) through "Access protocol/interface", called as "Um/Air interface".
1st level Network Access device is known here as BSS (Base Station Subsystem). BSS takes care of access over air. When user dial a number for making a voice call, BSS will provide all required access functions (Physical layer, Data Link layer functions). It transparently passes rest of the higher layer information to 2nd level Network Access device over "Network protocol/interface", called as "A interface".
2nd level Network Access device is known as MSC (Mobile Switching Centre). MSC takes care of higher layer functions (Network layer, Transport/Session/Presentation layer functions). For these functions in GSM, MSC may need to take of additional Network devices (like VLR (for location information), AuC (for authentication), EIR (for equipment validation) etc.), but not included here for simplicity. If user is trying to make a call to user in another network (let us say landline number of public telephone network), MSC need to take help of GMSC (Gateway MSC) connected to public telephone network. GMSC talks to MSC over "E interface".
This complete the brief on OSI model.
© Copyright Samir Amberkar 2010| Part 4 « | OSI Index | . |