Module 4 – ISDN and DDR
Index
ISDN Concepts
Traditional PSTN is based on an analog connection between the local exchange and the customer site. Analog links are limited in the available bandwidth.
ISDN was made possible by telcos upgrading switches to support digital signals over the local loop.
Some of the benefits include:
- Carries several different types of data
- Faster to connect than analog
- B channels offer more bandwidth than dialup
- B channels are able to be used with PPP
The D channel is used to setup calls and signaling, and is either 16 or 64kbps.
Each B channel offers 64kbps of bandwidth.
ISDN standards and access methods
ISDN is made up by three series of protocols:
- E protocols – defines telephone network standards
- I protocols – defines concepts, terminology, and general methods. I.100 includes general ISDN concepts. I.200 defines service aspects. I.300 defines network aspects. I.400 defines how UNI is provided.
- Q protocols – defines how call setup and switching is handled.
Unlike a standard TCP connection, ISDN uses out-of-band signalling, meaning all connections are negotiatied over the D channel, while data goes through the B channels.
B channels can carry digitized speech signals. They use either HDLC or PPP. PPP is more robust as it allows for authentication and negotiation of link and protocol configuration.
The D channel uses Link Access Procedure on the D Channel (LAPD) as its data link layer protocol.
North America and Japan PRI connections are 23B+D (giving T1), and most other countries use 30B+D (giving E1).
ISDN 3-layer model and related standards
BRI and PRI pysical layer specifications are defined in ITU-T I.430 and I.431
ISDN data link layer is based on LAPD and is specified in ITU-T Q.920, Q.921, Q.922, and Q.923
The network layer is defined by ITU-T Q.930 and Q.931, also known as I.450 and I.451, respectively.
With a BRI connection, a standard phone line local loop is used. Although there is only one physical link, the 3 ISDN channels are multiplexed as to not require any extra physical links.
Each BRI frame on the physical layer has:
- 8 bits from B1
- 8 bits from B2
- 2 bits from D
- 6 bits overhead
4,000 frames are sent per second, giving a total of 192kbps, though 48kb of this is overhead, which makes for 144kbps of throughput.
The overhead bits are used for these purposes:
- Framing – provides synchronization
- Load balancing – changes the average bit value
- Echo of previous D channel bits – used for contention resolution
- Activation bit – activates devices
- Spare bit – unassigned
The ISDN signaling channel uses LAPD for Layer 2. The flag and control fields for LAPD are the same as in HDLC. The address field is 2 bytes long, made up of the following:
- Service Address Point Identifier (SAPI) – identifies the portal that is providing LAPD services to layer 3. One byte long
- Command/Response (C/R) bit – specifies if the command is a command or response
- Terminal Endpoint Identifier (TEI) – identifies the terminal, ranges between 0 and 63 if static, 64-126 if dynamic, and 127 for a broadcast. 7b long.
The D channel is always up, so that ISDN calls can be started.
The following steps are used to connect an ISDN call:
- Called number sent to local ISDN switch via D channel
- The local switch sets up a connection to the remote ISDN switch using the SS7 protocol
- The remote switch signals the remote site via the D channel
- The remote site’s NT-1 device sends a call-connect message to the remote switch
- The remote switch uses SS7 to forward the call-connect message to the local switch
- The local switch connects one B channel to the remote site, leaving the other free for another conversation or data link, as both channels can be used at the same time.
The following devices are used to facilitate ISDN connectivity:
- Terminal Equipment 1 (TE1) – native ISDN device. eg. ISDN router or ISDN phone
- Terminal Equipment 2 (TE2) - non-ISDN device. eg. workstation or router. Requires TA to connect to ISDN service
- Terminal Adapter (TA) – Converts serial link into ISDN BRI
- Network Termination 2 – point at which the ISDN lines at the customer site are aggregated and switched using a customer switching device
- Network Termination 1 – controls the physical/electrical termination at the customer premises. Converts from 4 wire ISDN BRI to 2 line used over local loop
The following are reference points used to identify connections between ISDN devices:
- R – connection between TA and TE2
- S – connection between NT2 and customer premises equipment
- T – electrically identical to the S interface, referencing the outbound connection from the NT2 to the ISDN network or NT1
- U – link between the NT1 and the telco owned ISDN network
As S and T connections are electrically identical, interfaces are often labeled as S/T, meaning they can perform either role.
Determining the router ISDN interface
For North America and Japan, a router should use an ISDN interface with the NT1 built in, as the NT1 is a customer premises device in these countries. These interfaces are labeled with a ‘U’
In other countries, the NT1 is usually provided by the telco, so the interface should not include an NT1. These interfaces are labeled with ‘S/T’
If the router doesn’t have a way to fit an ISDN interface, a TA is required, this will connect to a serial interface on the router.
As there are no fixed standards for D channel communication with ISDN switches, it is necessary to specify the type on the router. The type of switch generally varies from country to country, though it can change within a country as well.
With some switches, a service profile identifier (SPID) is required. An SPID sets the line configuration, and allows for voice and data to both be carried over the local loop.
SPIDs are only used in North America and Japan.
Below is a table of the different switch types by country.
| Country | Switch Type |
| US and Canada | AT&T 5ESS and 4ESS; Northern Telecom DMS-100 |
| France | VN2, VN3 |
| Japan | NTT |
| UK | Net3 and Net5 |
| Europe | Net3 |
ISDN Configuration
To setup a BRI connection, use the following commands:
Router(config)# isdn switch-type [type]
Router(config)# interface bri [number]
If required: Router(config-if)# spid1 [spid-number] [ldn (optional)]
If required: Router(config-if)# spid2 [spid-number] [ldn (optional)]
Router(config-if)# no shutdown
To setup a PRI connection, use the following commands:
Router(config)# controller [t1 | e1]
For T1 lines: Router(config-controller)# framing [sf | esf]
For E1 lines: Router(config-controller)# framing [crc4 | no-crc4] [australia (optional)]
Router(config-controller)# linecode [ami | b8zs | hdb3] - Nth America usually uses B8ZS, Europe usually uses HDB3.
Router(config-controller)# pri-group [timeslots range (1-24 for T1, 1-31 for E1)]
Router(config)# interface serial [slot/port: | unit:] [23 (T1) | 15 (E1)]
Router(config-if)# isdn switch-type [type]
Router# show isdn status allows the isdn link to the local switch to be checked. When the link is working correctly, the layer 1 status will be ‘ACTIVE’, and the layer 2 status ‘MULTIPLE_FRAME_ESTABLISHED’
Router# show isdn active will give information regarding:
- called number
- time until disconnect
- Advice of Charge (AOC)
- charging units used
- whether AOC is provided during or after calls
Router# show dialer shows the following details:
- current call status
- dial timer config
- dial reason
- remote device that is connected
Router# show interface bri 0/0 will display interface statistics.
Troubleshooting ISDN configurations
Router# debug isdn q921 displays layer 2 information related to the D channel. This command should be used if show isdn status doesn’t show layer 1 as ‘ACTIVE’ and layer 2 as ‘MULTIPLE_FRAME_ESTABLISHED’
Router# debug isdn q931 shows exchanged messages for call setup and teardown
Router# debug ppp authentication displays messages regarding PPP authentication
Router# debug ppp negotiation displays messages related to the PPP link configuration when the connection is started
Router# debug ppp error shows errors relating to PPP.
The debug ppp commands are for use when there are layer 2 issues and show isdn status doesn’t show any problems with the ISDN connection.
DDR Configuration
Dial-on-demand routing (DDR) is used to make a connection when an ‘interesting packet’ needs to cross the link. Packets that are not interesting are forwarded only if the connection is already active. The protocols of which the packets are ‘interesing’ are set with dialer-list.
The following steps are used for an interesting packet to be forwarded using DDR:
- Router recieves packet, checks there is a valid route to the destination, and identifies the outbound interface
- If the interface is connected to the next hop router, the packet is forwarded, if the interface is not connected, the packet is identified as interesting.
- The router connects the the remote site. Now that the link is up, the packet is forwarded.
- All packets to the remote site, whether interesting or not are forwarded over the link.
- Once the set time limit has passed without an interesting packet, the call is terminated.
Legacy DDR refers to having one connection set per interface.
Follow these steps to set up legacy DDR:
- Set the static routes
- Define the interesting traffic
- Set up the dialer information
Static routes should be used over for connections with DDR, as using a dynamic routing protocol could cause the connection to be dialed more often than it is needed.
Using dialer-list [no.] protocol ip permit allows all IP traffic, if more control is needed, an ACL can be used. Create the ACL and then use dialer-list [no.] protocol ip list [ACL no.]
Dialer profiles allow for a single ISDN interface to connect to different links as required. Several interfaces can also be used, so if, for example, there were three possible connections, but only 2 were ever required at once, the dialer could have 3 profiles in a pool with 2 interfaces.
When configuring dialer interfaces, the dialer pool membership is specified for the physical interface, with the other settings configured for each logical interface.
debug dialer [events | packets] will help diagnose problems with the DDR configuration.
