• Top Level Network Model

This is one way to view the network.  The Core can be based on GSM MAP (Mobile Application Part), ANSI 41 or IP for example.  Composite cores accommodating both GSM MAP and ANSI 41 are now evolving (GAIT).  Similarly, Next Generation network cores are in development that accommodate emerging switching technologies (NNI Cores).  Moving out from the center, the core can be connected to the Edge via the Radio Network Connection (RNC) element.  The Edge can be composed of the  Base Station Controllers (BSCs), which connect to the Distribution ring.  The Outer Distribution ring is composed of the Base Transceiver Stations (BTSs), in the case of a mobile system, or Hubs (in the case of Point-to-Multipoint systems).  Beyond the Distribution ring, in the case of Wireless systems, the Air Interface supports the transport of information via various protocols to the subscriber terminals (mobiles or CPE).  There are several common conventions for describing the interfaces between the different elements.   One common naming convention is the GSM nomenclature.

• GSM Interfaces

"Um, Abis and A" are just three of the defined interfaces in a GSM network, but they illustrate the point.  Communication across interfaces is accomplished via control plane signaling.

Mobile<--Um--> BTS<-- Abis-->BSC<-- A-->Core (MSC, etc.)-->PSTN or IP network

The "wireline" side of the network encompassing the BTS, BSC and Core (MSC, etc.) comprises the Radio Access Network (RAN).

The IMT-2000 UMTS standard expands upon the basic GSM specification and adds new interfaces, network elements and protocols, but the ideas of abstraction and encapsulation are the same.

The specific function assigned to each layer in the stack is dependent upon what network interface you are looking at.  For example, at the mobile station:

Layer 1, the Physical Layer, maps the logical traffic channels onto the physical channels and is responsible for all of the RF processing.

Layer 2 is the DataLink layer and is subdivided into two individual layers.  The MAC (Media Access Control) layer is responsible for framing up the bits from layer 1 and for mapping logical channels onto transport channels for passage up the stack.  The Radio Link Control (RLC) component resides on top of the MAC layer and is responsible for error detection and correction (which provides an assessment of the communications link by monitoring the frames before passing them up).   Additionally, with the advent of 3G systems, the RLC is receiving a lot of attention as it directly impacts the QoS aspects of the system.  Quite frequently, people simply refer to the MAC when describing the MAC + RLC.

Layer 3 is the Network Layer and can be subdivided as follows:  The lowest component of Layer 3 is called the Radio Resource Control Layer (RCC).  The RCC is responsible for the setup and teardown of calls and is also responsible for Handoffs.  The layer on top of the RCC is broken up into two areas.  The Call Control (CC) part handles housekeeping chores and ancillary services like Short Message Service (SMS).   The Mobility Management (MM) part handles Location Updates, Registration, Authentication and other security aspects.

• Channel Types

In general there is some confusion as to the meaning of the various types of channels specified at the various layers.  A common characterization is as follows:

Transport Channels are Uni-directional from BTS to UE (User Equipment).  They specify "how" data is transferred over the radio interface.  Dedicated Channels are one type of Transport Channel.  They are UE specific, assigned on a per call basis and last only as long as the call.  Common Channels are another type of Transport Channel, but they are Cell specific.

Logical Channels specify "what" data is transferred.  Examples of Logical Channels are Dedicated Traffic, Dedicated Control and Common Control.

Physical Channels are defined by the Air Interface and specify the modulation, coding and all of the other RF characteristics of the transmitted signal.