System information

Chapter 2. Functional comparisons 21
• Constant bit rate (CBR)
• Unspecified bit rate (UBR)
• Variable bit rate (VBR)
In the case of IBM 8265, VBR is supported as CBR.
Traffic shaping and policing is required when interconnecting ATM switches
over a public carrier where the contracted quality of service is less that the
physical speed of the connection.
2.2.1.5 LAN emulation services
The basic LAN emulation functions provided by an ATM backbone comprise:
• LAN emulation configuration server (LECS)
• LAN emulation server (LES)
• Broadcast and unknown server (BUS)
LECS/LES/BUS are the basic functions provided by IBM 8210 Multiprotocol
Switch Server (MSS). For small networks a LES/BUS integrated in the IBM
8265 Control Point is sometimes used to avoid the cost of an MSS.
LECS/LES/BUS are functions that are critical to the operation of an ATM LAN,
and therefore various schemes are implemented to provide redundancy.
These schemes are fairly complex, and it is not recommended that
LECS/LES/BUS servers of different brands be deployed for redundancy.
LAN emulation is based on strong standards. Other than for the complexity of
redundant operation of LECS/LES/BUS, LAN emulation is generally
interoperable between different vendor equipment.
Cisco Systems provides several implementations of the LECS/LES/BUS
function, for example in high-end IOS products such as Cisco 7200 and on
layer-3 modules for some Catalyst switches. In the case of the OC-12 LANE
module for Catalyst 6500, a complete LANE server implementation is
provided. For most purposes a Cisco 7200 IOS router with a single OC-3 ATM
interface can be equated to an IBM 8210 MSS.
2.2.1.6 Layer-3 services
Since ATM technology lends itself to innovations in the area of layer-3 routing,
it is desirable to closely integrate these with the layer-2 hardware. For this
reason the MSS provides such functions together with support for a
comprehensive set of traditional routing protocols. As mentioned, Cisco IOS
provides almost the same functionality as the MSS.