A link between any two switches is divided into multiple channels by using multiplexing. These virtual channel links are numbered with a number called a virtual channel identifier (VCI). That is, any given physical copper cable, optical fibre or other link between two ATM switches consists of multiple (possibly thousands of) virtual channels. When a connection is established it is necessary to find a series of virtual channels (each obviously on a physical link) that connects the two endpoints of the connection. A series of virtual channel links are known as a virtual path, which is identified by a Virtual Path Identifier (VPI). The VPI and VCI are fields in the header of a cell. Whenever a cell arrives at a switch, the switch in essence performs a table lookup using the VPI/VCI pair, as well as the interface on which the cell arrived. From the table it determines the VPI and VCI that should be used by the next switch; it replaces the VPI/VCI fields in the cell header and then forwards the cell through the appropriate network interface.
Given the description above we can now look at an ATM switch. The switch we will use is FORE System's ForeRunner ASX 200. It looks as follows:
|ForeRunner ASX 200|
Just below the unit's name are four slots, labelled A, B, C and D. The network media will be connected to the modules in these slots. Slot A is currently empty with a blank plate covering it. Slots C and D contain identical modules and slot C contains a different module. We will return to these modules later.
Just underneath these four slots is a 'drawer' that contains the switch board or switch fabric. It contains the VPI/VCI lookup tables. However, doing a simple table lookup for every cell won't enable the switch to maintain the speed it should. Therefore the fabric is more than just a set of tables.
Below the switch board is another 'drawer' which is the processor (or computer) that controls the switch. It is therefore aptly called the switch control processor (SCP). It contains a SPARC processor, some memory, control logic and a hard disc. On the front panel of the processor are a couple of connectors. The two serial ports may be used to connect a terminal to the switch. One may then interact with the SCP via the terminal. There are two ports so that one may connect a local terminal, as well as a remote terminal via a modem to the SCP. The Ethernet connector enables one to connect the SCP to a network (via an Ethernet MAU) and then manage the switch via the network. In addition the front panel contains a reset facility and some status LEDs.
In the next picture the 'drawer' containing the SCP has been partially opened. (Also note that the network modules that were in slots B, C and D have been removed.)
|Switch with SCP partially opened.|
|The SCP from above. Note the hard disc towards the rear|
and the processor below the transparent glass.
|A closer view of the Sun SPARC processor|
|The switching fabric removed from the chassis.|
Now that the supporting infrastructure has been discussed the four slots that contain network modules can be revisited. This switch can maintain a speed of 2.5Gbps. The capacity is spread equally over the four slots, meaning that every slot can contain a module that will be able to communicate at a sustained 2.5Gbps/4 = 625Mbps. One may either install a module that uses all this bandwidth for a single high-speed medium, or one may install a module that distributes the capacity over several links. The module in slot B contains six (actually six transmit/receive pairs of) connections that each is able to maintain a speed of 625Mbps/6 = 105Mbps. The modules in slots B and D each has four connectors. Each connector should therefore be able to maintain a speed of 665Mbps/4 = 166Mbps.
The modules in slots B and D are labelled NM-4/155UTP5EC. Reading this label from the left identifies it as a (network) module with 4 ports each operating at 155Mbps over UTP category 5 cable. Somehow the EC indicates that it uses SONET framing (in contrast to a similar model that has only a C in this position and that uses SDH framing). The differences between the SONET and SDH standards are small, but technical and not important for the purposes of this museum. Note that the actual SONET/SDH speed used is 155.52Mbps.
|The NM-4/155UTP5EC modules in slots B and D|
|An NM-4/155UTP5EC module partially removed.|
|An NM-4/155UTP5EC module viewed from the top.|
|The NM-6MM/125B module in slot C (as well as the covered empty slot A).|
|ForeRunner ASX 200 powered up|
|ForeRunner ASX 200 with network modules.|