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This chapter provides physical and functional overviews of the data compression service adapter (CSA). The chapter contains the following sections:
The CSA provides high performance, hardware-based, data compression capabilities for Cisco 7200 series routers and VIP2 in the Cisco 7000 series and Cisco 7500 series routers. Following are the two CSA models:
There are no media interfaces on the CSA faceplate. The CSA off-loads all compression and decompression-related packet handling from host processors by compressing and decompressing packets passing through interfaces on serial port adapters installed in the host chassis.
The CSA supports simultaneous Stacker data compression algorithms, with independent full-duplex compression and decompression capabilities. The CSA supports only PPP and Frame Relay encapsulation.
The basic function of data compression is to reduce the size of a data frame to be transmitted over a network link. Reducing the size of the frame reduces the time required to transmit the frame across the network. Data compression works by providing a coding scheme at each end of a transmission link that allows characters to be removed from the frames of data at the sending side of the link and then replaced correctly at the receiving side. Because the condensed frames take up less bandwidth, greater numbers of them can be transmitted per unit of time.
Data compression schemes used in internetworking devices are referred to as lossless compression algorithms. These schemes reproduce the original bit streams exactly, with no degradation or loss, a feature required by routers and other devices to transport data across the network. The two most commonly used compression algorithms on internetworking devices are the Stacker compression and the Predictor data compression algorithms.
Stacker compression was developed by STAC Electronics and is based on the Lempel-Ziv compression algorithm. The Stacker algorithm uses an encoded dictionary that replaces a continuous stream of characters with codes. The symbols represented by the codes are stored in memory in a dictionary-style list. Because the relationship between a code and the original symbol varies as the data varies, this approach is more responsive to the variations in the data. This flexibility is particularly important for LAN data, because many different applications can be transmitting over the WAN at any one time. In addition, as the data varies, the dictionary changes to accommodate and adapt to the varying needs of the traffic.
Predictor compression was developed by Novell. The Predictor compression algorithm tries to predict the next sequence of characters in a data stream by using an index to look up a sequence in the compression dictionary. It then examines the next sequence in the data stream to see if it matches. If it does, that sequence replaces the looked-up sequence in the dictionary. If there is no match, the algorithm locates the next character sequence in the index and the process begins again. The index updates itself by hashing a few of the most recent character sequences from the input stream.
Cisco internetworking devices use the Stacker and Predictor data compression algorithms. The CSA supports only the Stacker algorithm.
The CSA has an enabled LED, standard on all service adapters, plus the active and error LEDs. After system initialization, the enabled LED goes on to indicate that the CSA has been enabled for operation. (The LEDs are shown in Figure 1-3.)
The following conditions must be met before the enabled LED goes on:
If either of these conditions is not met, or if the router initialization fails for other reasons, the enabled LED does not go on.
The CSA has the following LEDs and indications:
| LED Label | Color | State | Function |
Enabled | Green | On | Compression service adapter is enabled for operation. |
Active | Green | On | Indicates the CSA is active and ready to process data. This LED goes on when the service adapter's boot process is complete and remains off during normal system operation. |
Error | Amber | On | Indicates an error was found, and if it remains on, it indicates the error might prevent accurate compression. Error codes are generated by software. This LED remains off during normal system operation. |
This section provides information about where you can install the CSA on the VIP2 and in the Cisco 7200 series routers, and on the VIP2.
Figure 1-4 shows a VIP2-40 with installed service and port adapters. Figure 1-5 shows a VIP2-50 with installed service and port adapters. With the VIP2 oriented as shown, the left adapter is in port adapter slot 0, and the right adapter is in port adapter slot 1.
In Cisco 7200 series routers, port adapter slots are numbered from the lower left to the upper right, beginning with port adapter slot 1 and continuing through port adapter slot 2 for the Cisco 7202, slot 4 for the Cisco 7204 and Cisco 7204VXR, and slot 6 for the Cisco 7206 and Cisco 7206VXR. Port adapter slot 0 is reserved for the optional Fast Ethernet port on the I/O controller---if present.
Figure 1-6 shows a Cisco 7206 with installed port adapters and an I/O controller with a Fast Ethernet port. Not shown are the Cisco 7202, which has two port adapter slots, the Cisco 7204 and Cisco 7204VXR, which have four port adapter slots, and the Cisco 7206VXR, which has six port adapter slots. The CSA can be installed in any available port adapter slot in Cisco 7200 series routers.
7200 Series Port Adapter Slots---Cisco 7206
Posted: Mon Jul 12 15:15:21 PDT 1999
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