Serial PPP Links andModems

• serial link - sequential transport of bits/bytes;  serial interface on routers, PCs, modems and cell phones.
• asynchronous (or synchronous - very expensive)
 
• start, stop bits, inter-byte fill sequences etc, sent along with data
• lower speeds <115.200, cheaper
• parts of the link may go different speeds, compression, flow control
• serial interfaces - RS232 (defines hardware flow control)

Internal modems - interrupt conflicts

Port   Linux device  Base I/O Addr   IRQ
------------------------------------------
COM1      ttyS0           3F8         4
COM2      ttyS1           2F8         3
COM3      ttyS2           3E8         4 [*]  clashes with COM1
COM4      ttyS3           2E8         3 [*]  clashes with COM2

Check the interrupts used on your machine with the command 'cat  /proc/interrupts' .

Once you have set up your modem, using on-board jumpers or DIP switches, you need to tell Linux when it boots to use a different IRQ. Add a line or lines like this to /etc/rc.d/rc.local

    setserial -v /dev/ttyS2 irq 5
    setserial -v /dev/ttyS3 irq 2

External modems - old UARTs

It's easy to check which you have - look at the kernel messages when the system boots up. If you have 8250 or 16450 UARTs, they are unlikely to work at above 9600 bits per second (and even at 9600 may loose characters occasionally). Upgrade your PCs I/O card to one which has 16550A UARTs; this shouldn't cost you more than around US$30.

Most internal modems have 16550A UARTs on board - so this problem doesn't arise for them. Most modern motherboards also have 16550A UARTs on-board.

Cables & connectors

A "straight" RS232 cable is used to connect a modem to a computer. The modem end usually has a 25 pin female RS232 socket, and a computer may have either a 9pin or a 25pin male socket. A "crossed" or "null modem" RS232 cable is used to connect a terminal to a computer, and should not be used to connect a modem.

A four wire RJ11 cable usually connects the phone line to the modem. Ordinary phone lines use just two of the wires, but leased lines may use all four.

Modems and Modem Configuration

Initially, very low transfer rates of only 30 or 120 characters per second (300 or 1200 bits per second (baud)) were the maximum speeds but now even 28.8Kbps is now old fashioned the 56Kbps standard has emerged.

If files are not pre-compressed before transmission, in-transfer modem compression (MNP5 or V42 protocols) can yield even higher effective transmission speeds.

But attaining these connection speeds depends very much on the quality of the telephone line and in some areas of most developing countries it is still necessary to work at lower speeds. In some cases where the signal strength is very low or if there is a lot of interference on the line, it may be necessary to re-dial many times or to drop the modem speed to 2400 or even 1200 baud to ensure a reliable connection.

Modems vary considerably in quality. Ability to handle poor phone lines varies considerably from one modem brand to another, and not necessarily in ways that would be expected - some high-end popular modems are poor at handling bad lines - for example US Robotics Sportsters. Modems that work well under difficult conditions include the Zyxel and the Microcoms, others may be just as good tho. If there are problems connecting it is well worth trying out different modem brands.

Modems have four important settings:

• the modulation speed, i.e. the speed at which they can transmit data over the phone line. This could be anywhere between 300 bps and 56K bps. This is sometimes known as the DCE speed.
• the DTE speed, or the speed at which the modem communicates with the computer. Since modems these days compress the data they send over phone lines, they can actually talk to the computer at a higher rate than they talk over the phone line. The DTE speed is usually fixed by the computer to something like 38400 or 57600, if it has a high-speed (buffered 16550UART) serial port. If not, it may be fixed at a lower speed.
• Flow control. You should always use hardware flow control, also known as RTS/CTS.
• Error correction/compression -  can be used if modems at both ends support this. Most modems these days do. MNP and V.42/V.42bis are some of the standards for error correction and data compression used by modems.

There are also many different parameters you can set, and manufacturers do not agree what is a "good" default setting to send from the factory. For example, U.S. Robotics Sportsters used to ship with software handshaking (XON/XOFF) enabled by default. However this setting interferes with many dial-up protocols such as SLIP and XMODEM, and in general needs to be changed. Different brands of modems have slightly different commands, so you need to refer to your modem manual to find out exactly how this works for your modem.

Dialup and leased line

In dialup mode, the modem dials a telephone number, attempts to connect to a modem which answers, and establishes a connection. In the leased line mode, it expects to be permanently connected to a modem on the remote side.

Exercises

Setting up the Point-to-Point Protocol (PPP)

The pppd daemon carries out the following activities after it is run:

1. establishes the physical link
2. negotiating the ppp Link Control Protocol (LCP)
3. negotiating IP parameters
4. creating a ppp device (ppp0, ppp1...)
5. configuring the ppp device
6. adding  a route to the ppp device.

You can run ppp over an RS232 link directly connecting two PCs, like this: (in one long line)
/usr/sbin/pppd ttyS0 57600 crtscts local passive persist
[your IP address]:[remote IP address]  netmask 255.255.255.252

You can run kppp that is possible to configure and use from X-Windows environment. The interface is very similar with Dial-up Networking of Microsoft Windows 95/98.

Configure and Test your Modem

The most common method of configuring modems is using the Hayes AT commands. For this, you need a program that lets you talk to the modem over the serial port. On Unix machines, this is the cu command:

cu -l /dev/ttyS0 -s 57600 (if you get permission denied, do: chown uucp /dev/ttyS0)

1. AT
2. OK
3. AT &F1 &W
4. OK
5. at&f1
6. OK
7. ati4 [displays the current settings for a USRobotics Courier V.]


   B0  C1  E1  F1  M1  Q0  V1  X7
   BAUD=57600  PARITY=N  WORDLEN=8
   DIAL=PULSE  ON HOOK   TIMER

   &A3  &B1  &C1  &D2  &G0  &H1  &I0  &K1  &L0  &M4  &N0
   &P0  &R2  &S0  &T5  &X0  &Y1  %N6

   S00=000  S01=000  S02=043  S03=013  S04=010  S05=008  S06=002 S07=060
   S08=002  S09=006  S10=007  S11=070  S12=050  S13=000  S14=001 S15=000
   S16=000  S17=000  S18=000  S19=000  S20=000  S21=010  S22=017 S23=019
   S24=150  S25=005  S26=001  S27=000  S28=008  S29=020  S30=000 S31=000
   S32=009  S33=000  S34=000  S35=000  S36=000  S37=000  S38=000 S39=000
   S40=000  S41=000  S42=126  S43=200  S44=015  S45=000  S46=000 S47=000
   S48=000  S49=000  S50=000  S51=000  S52=000  S53=000  S54=064 S55=000
   S56=000  S57=000

   LAST DIALED #: T2020

OK

The important settings shown above are:

&B1  -- Fix the DTE speed to current speed and lock it (57600 in our case)
&C1  -- Normal CD (i.e. CD goes on when connected only)
&D2  -- DTR normal (i.e. not forced high always)
&H1  -- Use hardware flow control
&K1  -- Negotiate modem compression and use if both modems agree
&M4  -- Negotiate modem error correction and use if both modems agree
S0=1 -- Auto-answer incoming calls
ATX0 -- Set for dumb dialling when the modem cannot detect your
        dialtone, busy tone etc.
ATX4 -- Modem can detect dialtone, busy tone etc.

If any of these are not present, you will have to set them and save the configuration using the AT&W command. e.g. To set &K1, you would type AT&K1&W.
 
• On USR modems some settings are set by DIP switches under the modem (e.g. &D and &H settings are overridden by the DIP switches)
• Also AT$H gives you some online help on the commands, and AT&$ and AT%$ give you help on the & and % commands respectively (USRobotics only)
• Use ATDPphonenumber to dial out (ATDT is for tone dial lines). You should see the modem go off-hook (the OH light comes on) and hear it dial. Once it connects, the CD light comes on. You should then receive a login prompt from the terminal server.
• Login using the username of ppp with password ppp. You will receive some garbage, which is actually the terminal server trying to start up a PPP chat to you.
• Use <return>~. to exit from cu. You should see both the TR and the CD lights go off.

Setup dialout PPP

Objective: To configure your computer for dialout PPP.
1. Bring down the default route through the ethernet interface

route del default
2. Add debugging output to syslogd - edit /etc/syslog.conf and add:

*.=debug /var/log/debug and then restart syslogd - "killall -1 syslogd", syslogd
 
3. Configure your modem for dialout
4. Dial out using cu and ATDTphone and manually login using the account ppp and password ppp. Note the prompts that you get, since you will use them to construct your chat script. Each of you uses the phone number based on the diagram.
5. Dial out using pppd

pppd /dev/ttyS0 57600 \
connect 'chat -v "" AT OK ATDTPHONE ogin: ppp-user word: \\qpasswd' \
modem crtscts debug defaultroute
The \\q ensures that your password is not echoed in the /var/log/messages.
Check that ppp is running. You can use tail -f /var/log/messages to view what is happening.
• To bring down the ppp connection by killing the pppd process. You can also run 'ppp-off' if you have installed it from the /usr/doc/ppp* directory. ppp will negotiate and bring down the link gracefully.
• Tracing the ppp handshaking: When you use the debug option to pppd, it logs the IPCP and LCP handshaking via syslog; by default RedHat linux prints this information to /var/log/messages. You can also add an option of "kdebug 1" (or more) to see debugging messages from the kernel ppp driver. Similarly, the "-v" argument to chat makes it log the entire dialogue to syslog.
• You can have the whole chatscript on the commandline as shown above, but if you use it frequently, it is more convenient to put it into a script, e.g. chatscript, and use the command as:

pppd connect 'chat -f /etc/ppp/chatscript' /dev/ttyS0 modem crtscts defaultroute and /etc/ppp/chatscript would contain:

        TIMEOUT         3                               
        ABORT           '\nBUSY\r'                      
        ABORT           '\nNO ANSWER\r'                 
        ABORT           '\nRINGING\r\n\r\nRINGING\r'    
        ''              \rAT                            
        'OK-+++\c-OK'   ATH0                            
        TIMEOUT         60                              
        OK              ATDT1234                        
        CONNECT         ''                              
        ogin:--ogin:    ppp                             
        assword:        \qppp

• There are also default scripts to start ppp, look in /usr/doc/ppp*/scripts. You can copy the ppp-on, ppp-off and ppp-on-dialer to /etc/ppp and use them (after entering the required information into /etc/ppp).
• pppd has innumerable options -- look at the man page for full details. Some of the more commonly used ones are:
 
• defaultroute -- adds an entry to the routing table making this ppp link your default route. However, it does not replace a default route if you already have one.
• debug -- sends debug info to syslog (with facility daemon, level debug)
• [local]:[remote] -- assigns addresses to the link
• netmask -- assigns netmask to the link
• pppd allows you to specify options in a variety of ways -- you can have a file called /etc/ppp/options with global options, overridden by /etc/ppp/options.ttyS0 with options for ppp on /dev/ttyS0 only, overridden by options in ~/.ppprc, which can all be overridden by commandline options.
Authentication:

PAP authentication is done through the /etc/ppp/chap-secrets file
the format is:

login domain password
normally 'domain' is not used unless required by the server, so a typical example is:

js * alePest

Note passwords are not encrypted. The file is the same on the server side, but it is also it is possible to tell pppd to use the /etc/passwd file

---

How to set up your machine as a dialin PPP server

1. Setup your computer to answer incoming calls
• Install mgetty if /sbin/mgetty is not available on your machine.
• Edit /etc/inittab to add a line for your modem:

d1:2345:respawn:/sbin/mgetty ttyS0
• Reset init, using init q
• Check, using ps -ax|grep mgetty that mgetty is running on the port
2. Create a user for ppp
• Add a line into /etc/passwd as follows (use vipw):

ppp::501:501:PPP user:/:/usr/sbin/pppd
• Then use passwd ppp to set the password of the ppp user to ppp.
3. Setup ppp to accept incoming calls
• Edit the /etc/options/ppp.ttyS1 to have the address that you will give your client. :169.222.N.N modem crtscts lock debug
4. Wait for a call
5. Check /var/log/messages and ifconfig to see what happens when you receive a call.
6. We have setup routing for the network that you are willing to serve, so that all traffic for your client is routed via you. Once the connection is established, ask your neighbour to try pinging you and using traceroute to an outside machine to check if this works.

Additional Notes:
 

• There are two ways of making your modem answer incoming calls -- one is to set the modem with ATS0 =1 (to answer after one ring), and the other is to leave S0 at 0, but have an intelligent getty program that notices the phone ring and sends an ATA command to the modem to answer. We prefer the second.
• Dynamic addressing: Note that the options.ttyXX is being used to dynamically give an IP address to whoever dials into that port. You could also have static addresses.
• A much nicer terminal program than cu is minicom. Apart from having a pretty screen display, you can use it to dial up to BBSes and transfer files using X/Y/Zmodem. Beware, however, that in its normal configuration minicom itself sends initialisation strings to the modem when you enter it, and again when you leave. You can view and edit these with 'minicom -s'

 

You should create or edit /etc/minirc.dfl (Red Hat) or /var/lib/minicom/minirc.dfl (Slackware) so it looks like this:

pr port             /dev/ttyS0
pu baudrate         38400
pu bits             8
pu parity           N

Note that you should never use /dev/modem to refer to your modem - it creates problems with lock files as one program might refer to the device as "/dev/modem" while another refers to it as "/dev/ttyS0", for example.

To get minicom to give a nice colour display, add '-c on' to the command line, or put the following line in /etc/profile:
export MINICOM='-c on'
 

There is another way of configuring a dialin ppp link - to use an intelligent getty program that auto-senses PPP. (see below)

Regardless of the way the link is initiated, the options are usually located in the ppp options file. The important things for a PPP server to do are to allocate an IP address for the remote end of the link (if it uses dynamic addresses, which is common for dialup PPP) and to handle routing for the link when it comes up. Routing can be handled by adding the proxyarp option to pppd, where the server responds to ARP queries on the local LAN for the remote machine, and then routes packets down the serial link. More common these days however, is to use a small network for this link and route this network to the ppp link. The pppd program automatically adds a host route for the machine at the other end of the PPP link when the link comes up and deletes it when the link goes down.

For a dialout link, pppd needs to know the name of a connection program (like chat), and (usually) a phone number, username and password that the server recognises it by. These are usually part of the chat script. Also needed are the serial port and speed to be used for the modem connection.

The PPP HOWTO available with all Linux distributions covers PPP setup, both as client and server, in detail.

mgetty and auto-sensing ppp:

For a Unix machine to accept logins on a serial port (either a directly connected terminal or a modem) there is a program called getty that runs on each line and waits for some action (either a telephone ringing, or a user pressing enter on a directly connected terminal). mgetty is a "smart" version of getty that can do a few more things than just detect a phone ring and start the login program. mgetty is started by init (from inittab) and waits for the phone to ring. It can then detect whether this is a data or fax call, and answer appropriately. All this is controlled by a config file (mgetty.config).


When PPP clients use the PAP (or CHAP) protocol for user authentication and so do not need a separate login/password authentication at startup then mgetty has a feature whereby it can sense the PPP start-up packets and automatically start pppd (at the server end). The client end maintains the username and password in the pap-secrets file (under unix pppd) and authentication happens during the initial ppp chat. This simplifies the chat script since you no longer need to worry about what exactly the login prompt will look like.

By default mgetty disables this feature -- to enable it, you have to recompile mgetty with -DAUTO_PPP (as documented) and uncomment the line for AutoPPP in the login.config file. On the server side, pppd can either use a pap-secrets file to authenticate the client, or, if given the "login" option, uses the regular login (/etc/password) database.

Other programmes:
 

uugetty

An option to mgetty is uugetty, which can do many of the same things (allow use of a modem on a serial port for both dial in and out). uugetty requires a config file for each port that it answers on, usually

dip

dip is another program that handles dialup IP connections. It can do both slip and ppp (but actually just calls pppd with the appropriate arguments if you want ppp).

diald -- dial on demand

Another very useful feature is the ability to dial out automatically when required and hang up the line when not in use. The diald program provides this facility.

Link Quality Monitoring (LQM)

To ensure that the link can successfully transfer data, PPP monitors its quality LQM quality report (LQR) packets. For link quality monitoring to be operational, it only has to be enabled on one side of the link. If the two machines are configured for different link quality reporting periods, they negotiate to the lower value, so that the LQR period will be the same on both sides of the link.

LQR packets contain counters of incoming and outgoing data packets for the routers on each side of the link. Each time an LQR packet is received, PPP uses that packet to calculate the outbound link quality (the percentage of packets the router transmitted that its peer successfully receives) and the inbound link quality (the percentage of packets that the peer transmitted that this router successfully receives).

After five LQR reporting periods, PPP averages the inbound and outbound link quality and compares these values against a user-specified threshold. This is a rolling average. After the first five LQR reporting periods, PPP acquires the data from the next period and drops the oldest data. Then it computes the link quality average for that set of five LQR periods, and so on.

If either the inbound or outbound link quality average drops below the threshold, PPP brings down the link. The driver software automatically brings the link back up and renegotiates the connection. PPP monitors the link control packets flowing over the connection and resumes network control protocol packet traffic when the link quality improves.