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The command 'ppp callback accept' is used on the Access Server side to enable callback to clients. On the other side, the callback client is configured with 'ppp callback request' command to request a callback from the Access Server.
The command 'dialer callback-secure' is used to disconnect calls that are not properly configured for callback.
Following are important commands often used while configuring
PPP:
1. Encapsulation ppp - specifies ppp authentication method.
2. PPP authentication PAP - Specifies that PAP be used for authentication
during the PPP negotiation.
3. PPP authentication CHAP - Specifies
that CHAP be used for authentication during PPP negotiation.
11.0 Compression
The broad category of compression techniques is as given below:
1. Header compression - Here the TCP/IP header is compressed. This type of compression is suitable for small packets since the TCP/IP header constitute significant fraction of the overall packet.
2. Payload compression - Here the data portion of the packet gets compressed. By using payload compression, the header of the packet is left intact (uncompressed) and hence useful for virtual circuit services such as X.25, Frame Relay. Payload compression uses STAC compression method. This is because the header can readily be read for determining the route that the packet will take.
The following are some important points regarding payload compression:
1. When you use layer 2 payload compression, the frame header is not compressed. The entire contents of the frame except the header is compressed, including higher layer headers.
2. Payload compression
is achieved by using either Stacker algorithm or Predictor algorithm.
3. It is not recommended to use both TCP header compression and
payload compression simultaneously, as this may result in poor performance
and sometimes the link may not come up at all.
4. WAN protocols,
such as Point-to-Point Protocol (PPP), Frame Relay, High-Level Data
Link Control (HDLC), X.25, and Link Access Procedure, Balanced (LAPB)
use layer 2 payload compression.
3. Link compression - This compression method compresses both the header and the payload. Link compression is also known as per-interface compression. Link compression uses any of two types of algorithms:
Predictor - Predictor is memory intensive
Stacker - Stacker
is CPU intensive.
12.0 Cisco routers typically support the following compression methods:
1. Predictor
2. Stacker
3. MPPC, and
4. TCP header
compression.
Predictor is more memory intensive, whereas Stacker,
and MPPC are more CPU intensive.
Note that you must enable compression
on both ends of the link. Compressing and already compressed data
does not yield any better throughput. In fact, it can cause performance
degradation due more strain on system resources.
Keep in mind the following points when enabling compression:
1. In dial-up environment, if compression is done at the modem level, it is not recommended to configure router level compression, as this would result in expansion rather than compression. Compressing a compressed data usually results in expansion.
2. When data is encrypted by an application, it is not efficient to enable data compression at layer 2. The recommended practice is to compress data before it is encrypted and send it. This is because, encryption occurs at layer 3 and compression at layer 2 only results in expansion of data. Hence, it is recommended to compress the data before it is encrypted, where possible.
3. Use Predictor compression when you want the compression to be memory intensive rather than CPU intensive.
4. The command 'frame-relay payload-compress' is used to enable compression on a Frame Relay interface. Note that it is not recommended to use link compression in Frame Relay environment, as you do not want the header to be compressed.
13.0 Win98:
To configure Dial-Up Networking in WIndows '98, double click My Computer on the Desk Top, and then double click Dial-Up Networking.
The total bandwidth of an ISDN BRI can be calculated as below:
2 B (Bearer) channels: 2X64 = 128 Kbps
1 D (Data) channel: 16Kbps
Framing and Synchronization: 48 Kbps
Total bandwidth: 128+16+48
= 192 Kbps.
Also, note that the bandwidth excluding framing and
synchronization is 128+16 Kbps -= 144 Kbps.
ISDN can be mapped to bottom 3 layers of OSI reference model.
Layer 1 is the physical layer and the following protocols are defined at this level:
1. I.430: Defines the communication across S/T reference point.
2. I.431: Defines a full-duplex, point-to-point, serial, synchronous connection and applicable for ISDN PRI.
3. ANSI T1.601: Defines the communication across the U interface (North America only).
Layer 2 and layer 3 work for D channel only.
Layer 2 of ISDN
uses LAPD (Q.921), and Layer 3 of D channel uses Q.931 which is
a subset of DSS1.
The B channel is where actual user data is transmitted or received. B channel uses IP/IPX with HDLC/PPP/FR/LAPB protocol.
ISDN BRI Switch types that are widely used in various countries:
basic-5ess: AT&T basic rate switches (US)
basic-1tr6: Used
in Germany
basic-net3: Used in UK and Europe
ntt: Widely used
in Japan
15.0 ISDN PRI:
15.1 Some of the important channel characteristics of ISDN PRI are given below:
1. ISDN PRI (Primary Rate Interface) has 23 B channels , one D channel at 64 Kbps, and one Framing and Synchronization channel at 8 Kbps in North America. The corresponding values in Europe are: 30 B Channels, one D channel at 64 Kbps, and one framing and synchronization channel at 64 Kbps. Note that D channel speed is same for both in Europe and N.America.
2. The D channel is N.America is carried in timeslot 24, whereas the D channel in Europe is carried in timeslot 16.
3. The aggregate speed of ISDN PRI in N.America is equivalent to T1 (1.544 Mbps), and in Europe it is equivalent to E1 (2.048 Mbps)
15.2 The command used for specifying the D-channel for ISDN PRI
is:
Router(config)# interface serial {slot / port: | unit} {23
| 15} slot / port correspond to Cisco 7000 and 3600 series of routers.
Unit number correspond to Cisco 4000 or AS5000 series routers.
The channel numbers for T1 range from 1 to 24. The serial interfaces on Cisco routers start numbering from 0. Therefore channel 24 correspond to sub-interface 23. Also, D channel on T1 signaling always correspond to channel 24.
The channel number for E1 range from 1 to 31. The D channel on E1 signaling is always channel 16. Therefore, the corresponding sub-interface will be 15
The command 'isdn switch-type primary-5ess' configures an ISDN PRI controller to switch type 5ess.
15.3 Some more of the important commands used in ISDN PRI are as below:
1. Isdn switch-type <switch-type>: This command configures the controller for ISDN PRI operation. The command can be used at global level as well as interface level. If used at global level, the specified switch-type applies to all controllers.
2. Controller {t1 / e1} slot/port: This command is used to identify Cisco 7000 or 36000 series controller. For Cisco 4000 or AS5000 series use 'Controller {t1 / e1} unit#' command.
3. Pri-group: This command configures the specifies interface for PRI operation. Also, number of times slots allocated to the PRI is specified using this command. Syntax: pri-group [timeslot range].
4. Interface serial: This command is used for ISDN PRI D-channel selection.
5. Isdn incoming-voice modem: This command is used to allow incoming analog calls to be switched to internal modems that are installed on a digital network module.
15.4 ISDN Show commands that are used extensively while configuring and troubleshooting ISDN:
1. show isdn status - This command outputs ISDN switch type, whether the ISDN connection is working properly. It also displays the status of Layer 1, Layer 2, and layer 3.
2. Show interface bri 0 - This command shows whether BRI 0 is up and whether line protocol is up (spoofing). The command also displays encapsulation method used (PPP, HDLC, or X.25).
3. Debug isdn q921: Turns of ISDN Q.921 debugging. To turn-off, use the command no debug isdn q921 or undebug all command.
4. Show controller t1 - This command is used to verify whether the ISDN PRI controller is working properly or not.
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