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Channels:
The " B " (BEARER) channels
Transmit at 64 Kbps chunks
Carries  - USER DATA

The "D" (DATA) channel
Transmits at 16 Kbps for BRI and 64 Kbps for PRI.
Carries call signaling and setup information to establish  connection, request services, route data over B channels, and tear down the call when complete.
This information is designed to travel through a totally separate, dedicated communications network from the bearer channels. Functions at layer 2 and 3 of OSI model. Specifications of Q.931 will flow over a layer 2 Link Access Procedure D-channel (LAPD).

LAPD   (Link Access Protocol - Channel D) is a layer 2 protocol which is used to create / control connections. Defined in CCITT Q.920/921. LAPD works in the Asynchronous Balanced Mode (ABM). This mode is totally balanced (i.e., no master/slave relationship). Each station may initialize, supervise, recover from errors, and send frames at any time. The protocol treats the DTE and DCE as equals.

 LAPD frame format is similar to HDLC and LAPB.

 

BRI -The Basic Rate Interface consists of two B channels and one D channel for signaling (2B+D).

PRI -The Primary Rate Interface is a 23B+D interface.. It is the ISDN equivalent of the 1.544 Mbps over a T1 / E1 line or trunk; the physical layer is identical for both. The D channel is channel 24 of the interface, and it controls the signaling procedures for some or all of the B channels.

Multilink PPP (ML-PPP) is an Internet Engineering Task Force (IETF) standard, specified by RFC 1717. It describes a layer 2 software implementation that opens multiple simultaneous channels between systems, giving users additional bandwidth-on-demand for additional cost. The ML-PPP standard describes how to split, recombine, and sequence datagrams across multiple B channels to create a single logical connection.

BONDing, which approaches synchronization between multiple streams at the bit level. BONDing was designed for video conferencing applications and will most likely require additional hardware for the end system. Because BONDing is hardware oriented, it is efficient; but it is also expensive and inflexible, since once a pipe size is set, it cannot be changed until the session is finished. BONDing has been adopted more readily in the video conferencing arena than in data networking.

TE1: Specialized ISDN terminals are referred to as terminal equipment type 1.
TE2: Non ISDN terminals are referred to as terminal equipment type2. TE2s connect to the ISDN network through a Terminal Adapter.
TA: The ISDN TA allows (TE2s) to operate over ISDN lines. It can either be a stand-alone device or a board inside TE2. If implemented as a stand-alone device, the TE2 connects to the TA via a standard physical-layer interface (for example,V24,V35,....).

Beyond the TE1 and TE2 devices, the connection point in the ISDN network are the Network Termination devices.
NT1: Network termination 1 performs the adaptation of the network signals on the subscriber line.
NT2: Network termination 2 is a more complicated device, typically found in digital private branch exchanges(PBXs).

Beyond the NT1 and NT2, the next connection points are:
LT: Line termination.
ET: Exchange termination.
 

R - The reference point between non ISDN-equipment and a TA.
S - The reference point between NT2 equipment and TE1 or the TA equipment.
T - The reference point between NT1 equipment and NT2 equipment.
U - The reference point between NT1 equipment and ISDN network.
V - The reference point between LT and ET.
 

Defaults to HDLC encapsulation.
Services –
 BRI – 2B+D    =  2 x 64k data plus one 16k for signaling / voice.
 PRI – 23B +D =  23 64K data plus one 64k for signaling / voice.

Router Commands – Global
Isdn switch-type basic-5ess   >BRI
Isdn switch-type primary-5ess >PRI
Most common are 5ESS, DMS100, and NI-1

Router Commands – Interface BRI
Service profile identifiers (SPIDs)
!Encapsulation pap / chap
ISDN  spid1 (spid #) (LD#)
ISDN  spid2 (spid #) (LD#)
Note you can use a dialer group and dialer map commands.

Long Distance Codes:

MCI - 1010222

Sprint - 1010333

ATT - 1010288

Router Commands – Interface PRI -
Controller t1 x/y  slot/port
Pri-group timeslot 1-23 speed 64k
Framing esf
Linecode b8zs
 

Configuration Example
 
 

hostname branch1   username main password secret1   isdn switch-type basic-dms100 !   interface Ethernet 0   ip address 131.108.64.190 255.255.255.0 !   interface BRI 0   encapsulation PPP   ip address 131.108.157.1  255.255.255.0 !   isdn spid1 415988488501 9884885   isdn spid2 415988488602 9884886   ppp authenticate chap   dialer idle-timeout 300   dialer map IP 131.108.157.2 name  main 4883   dialer-group 1 !  ip route 131.108.0.0 255.255.0.0 131.108.157.2 ! dialer-list 1 protocol ip permit !

hostname main  username branch1 password secret1  username branch2 password secret2  !  isdn switch-type basic-dms100  ! interface Ethernet 0 ip address 131.108.38.1    255.255.255.0 ! interface BRI 0 encapsulation PPP  ip address 131.108.157.2  255.255.255.0  isdn spid1 415988488201 9884882  isdn spid2 415988488302 9884883  ppp authenticate chap  dialer idle-timeout 300 ! dialer map IP 131.108.157.1 name branch1 4885 dialer-group 1 !  ip route 131.108.64.0    255.255.255.0 131.108.157.1 dialer-list 1 prot ip list 101  access-list 110 deny igrp any any  access-list 110 permit ip any any

Cisco Commands

The hostname of the router is used to identify the router to another router when sending Challenge Handshake Authentication Protocol (CHAP) messages.

username name password secret
The username command is required when used to specify the CHAP secret message to use when challenged by another router. The two routers that need to talk must share the same password.

isdn switch-type switch-type
Phone company CO equipment software.

isdn spid1 spid-number [ldn]
isdn spid2 spid-number [ldn]
Bell assigned numbers - LDN is required for calling long distance.

dialer-group group number
dialer-list dialer-group protocol protocol-name {permit | deny}
dialer-list dialer-group protocol protocol-name list access-list-number
Dialer-group is associted with Dialer-list, They work together to control protocol access and connection activation.
Will close on TIMEOUT value.

ppp authentication chap
Specifies that CHAP authentication is to be used on this interface

dialer idle-timeout seconds
This command makes the router disconnect calls that haven't had any allowed traffic.
Only outbound packets reset the idle timer.

dialer map protocol name remote-name broadcast speed 56 phone-number
The dialer map is used to distinguish between different remote sites, which can be connected at various times. A map is required for each site and each protocol used. The remote name is the hostname of the other router. The broadcast flag specifies whether broadcast packets, such as routing updates, are sent for this protocol to this site. The number can be left out to create a "receive-only" dialer map.
Dialer maps provide the network-layer-to-link-layer mapping required by the router to successfully encapsulate, or send packets out, an interface.

ip route network [mask] {address | interface} [distance]

Provides the static routing information necessary in order for the packets to route packets to the proper next hop. Routing of packets to a DDR interface is what creates the Demand which, as the name implies, is an essential part of DDR.

NOTE: On NAT implementations - Use the "ip route X.X.X.X Null0" command to spoof netowrks into being advertised.
 

To monitor  - use the show controller, show int commands and
 BRI = debug isdn q921
 PRI = debug isdn q931

show isdn status
The current ISDN Switchtype = basic-ni1
ISDN BRI0 interface
       Layer 1 Status:
               ACTIVE
       Layer 2 Status:
               TEI = 109, State = MULTIPLE_FRAME_ESTABLISHED
               TEI = 110, State = MULTIPLE_FRAME_ESTABLISHED
       Spid Status:
               TEI 109, ces = 1, state = 8(established)
                       spid1 configured, spid1 sent, spid1 valid
                       Endpoint ID Info: epsf = 0, usid = 1, tid = 1
               TEI 110, ces = 2, state = 8(established)
                       spid2 configured, spid2 sent, spid2 valid
                       Endpoint ID Info: epsf = 0, usid = 3, tid = 1
       Layer 3 Status:
               0 Active Layer 3 Call(s)
       Activated dsl 0 CCBs = 0
       Total Allocated ISDN CCBs = 0
 
 

AS5200-1#sh contr t1

T1 0 is up.
   No alarms detected.
   Version info of slot 0:   HW: 2, Firmware: 14, NEAT PLD: 14, NR Bus PLD: 22
   Framing is ESF, Line Code is B8ZS, Clock Source is Line Primary.
   Data in current interval (685 seconds elapsed):
         0 Line Code Violations, 0 Path Code Violations
         0 Slip Secs, 0 Fr Loss Secs, 0 Line Err Secs, 0 Degraded Mins
         0 Errored Secs, 0 Bursty Err Secs, 0 Severely Err Secs, 0 Unavail Secs
   Total Data (last 24 hours)
         0 Line Code Violations, 0 Path Code Violations,
         0 Slip Secs, 0 Fr Loss Secs, 0 Line Err Secs, 8 Degraded Mins,
         0 Errored Secs, 0 Bursty Err Secs, 0 Severely Err Secs, 0 Unavail S

AS5200-1#sh isdn service
PRI Channel Statistics:
ISDN Se0:23, Channel (1-31)
   Activated dsl 0
   State (0=Idle 1=Propose 2=Busy 3=Reserved 4=Restart 5=Maint)
   0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 3 3 3 3 3 3 3 3
   Channel (1-31) Service (0=Inservice 1=Maint 2=Outofservice)
   0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
ISDN Se1:23, Channel (1-31)
   Activated dsl 1
   State (0=Idle 1=Propose 2=Busy 3=Reserved 4=Restart 5=Maint)
   0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 3 3 3 3 3 3 3 3
   Channel (1-31) Service (0=Inservice 1=Maint 2=Outofservice)
   0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
 

• Use dialer-group, dialer-list and access-list  for Dial on Demand routing. Interesting Packet data will initiate dial if serial is down. (watch NAT configs). Can be set with timers time-range command.
• Use the static - backup interface command for immediate DBU.
• Use static routes with adjusted AD weights for control, NAT may need the route to Null0.
• Note to test you can not ADMIN shut the primary - (Mis-configure), remove the cable or the  backup interface statement.

Commands for DBU
2600#sho backup
Primary Interface   Secondary Interface   Status
-----------------   -------------------      ------
Serial0/0               BRI0/0                        normal operation  (Not up)

2600#sho backup
Primary Interface   Secondary Interface   Status
-----------------   -------------------     ------
Serial0/0               BRI0/0                       backup mode  (Up on Backup)
 

2600#sho dialer (Up on Backup)

BRI0/0 - dialer type = ISDN

Dial String      Successes   Failures    Last DNIS   Last status
18005551212              7          3    00:01:04       successful
0 incoming call(s) have been screened.
0 incoming call(s) rejected for callback.

BRI0/0:1 - dialer type = ISDN
Idle timer (121 secs), Fast idle timer (20 secs)
Wait for carrier (30 secs), Re-enable (15 secs)
Dialer state is data link layer up
Dial reason: ip (s=44.26.91.43, d=224.0.0.5)
Time until disconnect 120 secs
Connected to 18005551212 (pri_4500)

BRI0/0:2 - dialer type = ISDN
Idle timer (121 secs), Fast idle timer (20 secs)
Wait for carrier (30 secs), Re-enable (15 secs)
Dialer state is idle

2600#sho int bri 0/0  (Up on Backup)
BRI0/0 is up, line protocol is up (spoofing)
  Hardware is PQUICC BRI with U interface
  Description: ISDN backup to
  Internet address is 44.26.91.43/28
  MTU 1500 bytes, BW 64 Kbit, DLY 20000 usec,
     reliability 255/255, txload 1/255, rxload 1/255
  Encapsulation PPP, loopback not set
  Last input 00:00:27, output never, output hang never
  Last clearing of "show interface" counters 05:15:03
  Queueing strategy: fifo
  Output queue 0/40, 0 drops; input queue 0/75, 0 drops
  5 minute input rate 0 bits/sec, 0 packets/sec
  5 minute output rate 0 bits/sec, 0 packets/sec
     76 packets input, 986 bytes, 0 no buffer
     Received 0 broadcasts, 0 runts, 0 giants, 0 throttles
     0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort
     76 packets output, 486 bytes, 0 underruns
     0 output errors, 0 collisions, 2 interface resets
     0 output buffer failures, 0 output buffers swapped out
     3 carrier transitions