Frequently Asked Questions and Answers

The questions and answers in this section apply to the majority of users. These solutions will generally fix the problems you or your clients are experiencing. If you still have a problem after taking into account everything listed here, search Cisco's web site for more recent troubleshooting information and, if required, open a TAC case for further investigation.

Question:	How can I achieve faster connection speeds?
Answer:	To start, some situations simply cannot be helped or improved. First, make sure that the point of presence (POP) that you are dialing is V.90- or V.92-enabled by asking your provider. Line quality and the distance of the user from the CO limit connection speeds and quality. More than any other factor, this is the most important to remember. With that being said, other reasons exist for slow connection speeds. If applicable , always start by making sure that the connecting modem has the latest drivers and firmware. As with client modems, ensure that the dial servers are running the latest TAC recommended software. Although the modems might not be new, the software running on them is constantly being upgraded to support new features such as modem call waiting and quick connections. The distance to your CO is measured in cable feet. It is good to know how far away you are from the CO. Anything under three miles should achieve an acceptable connection speed with V.90 modulation, unless there is a line quality issue. You can have the telco check your line for the signal-to-noise ratio, which should be at least 40 decibels. If you are dialing a local POP and still receive poor connection speeds, it might be a pad issue. Dial the full 10 digits to see if changing the pad to 6 dB fixes the problem. This is the extent of what you can try to improve a slow connection speed. If you feel there is something still wrong, you can open a case with the TAC to investigate specific issues.
Question:	My clients complain about getting disconnected frequently. What can be done to stop this?
Answer:	The answer to this question is the same as for the previous question. Slow connections and frequent disconnects are both caused by incompatible modems. First, rule out any simple problems such as an idle-timeout, call waiting, a bad phone cable between a jack and PC, or another phone causing the problem. If it is not one of these factors, it is most likely a software problem that can be fixed by upgrading drivers on the client modem or modem firmware on the dial router.
Question:	When dialing in, my clients receive a fast busy signal. How can I fix this?
Answer:	Two possible causes exist for this problem. The easiest way to troubleshoot this is to turn on ISDN Q931 debugs to determine if calls are reaching the router. If the calls are not reaching the router, the fast busy is generated by a problem in the telco, possibly because of an overloaded switch or switch path . If the call successfully reaches the router, this indicates that the router does not know what to do with the call. The first thing to check is that every Primary Rate Interface (PRI) has the line isdn incoming-voice modem in its serial interface configuration. This command instructs the router to send any voice calls that come in over that PRI to a modem. The next thing to check is that there are enough modems in each router for the total number of digital service 0s (DS0s). If you only have 60 modems and 92 DS0s (4 PRI worth), the 61 st voice (modem) call received will get a fast busy signal.
Question:	My clients receive a slow busy signal but plenty of modems are free. How can I fix this?
Answer:	It could be possible that the DS0s are at peak capacity, but that modems are still available. If this is the case, it is time to order more circuits from the telco. If DS0s are available, use the command show isdn status to verify that Layer 2 is MULTIPLE_FRAMES_ESTABLISHED. If not, see Chapter 7, "Dial Troubleshooting," for PRI troubleshooting. If the PRI is up and working, use the command show isdn service to determine if any of the channels are out of service. Channels of a PRI are usually idle or busy. If they are out of service, you must open up a ticket with the provider to clear the out of service channels.
Question:	I have a large Non-Facility Associated Signaling (NFAS) group configured on a dial router. According to the D channel, calls are successfully connected, but some of the NFAS circuits don't ever seem to take calls. How can I troubleshoot what is happening with the calls?
Answer:	Because the D channel is one circuit that controls the incoming calls on all the other circuits, it is possible to have two or more circuits that contain out of order B channels. If they are out of order, the D channel still receives the incoming call notification, but it opens up the wrong B channel for data transfer. This results in the true incoming call being unable to connect because the call is not mapped to a modem. The solution is to verify the correct location for each circuit by any means possible, including tracing cables and getting help from your provider.
Question:	When dialing in, some users do not get IP addresses. How can I troubleshoot this?
Answer:	If you are using a locally configured pool of addresses, perform the command show ip local pool pool name , which shows both the in-use addresses and the addresses not in use. The in-use addresses show the username using that address. The addresses that are not in-use show the username of the last person to get that IP address. If all IP addresses in the pool are in use, the pool range is too small and must be increased. If you are using a Dynamic Host Configuration Protocol (DHCP) server to provide IP addressing, verify that the DHCP server is reachable and verify that it has enough addressing for the number of DS0s in your dial router. Also, verify that leases do not stay active too long. The lease should expire when the client disconnects.
Question:	When some of my clients dial up, they get classful subnet masks. Does this affect anything?
Answer:	No. PPP is a Point-to-Point Protocol, which means that the client modems send everything down the pipe. Even if they have the wrong subnet mask, their peer (the dial router) knows how to route the traffic correctly and this should not adversely affect them.
Question:	How much of a problem do one-time passwords pose for dial-in users?
Answer:	First, having one-time passwords means that the end users have an extra step in dialing up. This generates trouble tickets by itself. Also, you must remember the type of one-time password that you're using because two different types exist: time-based and single-use . Time-based passwords are only good for a certain period of time, so be careful not to limit the time too much because of the 20-30 seconds it takes for the modems to train-up. Using 60- or 120-second passwords seems to work well and anything less generates numerous trouble tickets. Single-use passwords rely on an algorithm that puts passwords in order. These tend to be more user-friendly because the end user can take as much time between getting the password and connecting.
Question:	How does Virtual Private Networking ( VPN ) over dial affect throughput and latency?
Answer:	As always, this depends on the Internet, which can provide both positive and negative aspects. If you connect at any speed above 28,800 bps, the VPN overhead is minimal, especially if you use compression over the VPN connection. Latency is affected by the delay between the POP and the VPN concentrator. Most of the time, this is less than 50 ms, which is hardly noticeable over dial. One factor that usually suffers with VPN over dial is real-time applications, such as Telnet, Secure Shell, and any other command-line interface.