CISCO CCNA - ISDN - Integrated services Digital Network

If you find this page useful check out my other revision pages:

• Last minute revision guide
• Cisco IOS commands
• Layer 2 switching
• VLANS
• Spanning Tree Protocol - STP
• ISDN
• Novell IPX
• Access Control Lists - ACL

For other useful CCNA study guides, CCNA practice exams and technical training, see my certification and networking links page.

ISDN - Integrated services Digital Network

• Carries a variety of traffic. Video, Circuit Switched Data And Telephone Services using normal phone network.
• Call set up and data rates faster than modem.
• Operates at layer 1 to 3.
• ISDN uses SS7 signalling to set up a path with a remote switch.

ISDN Physical layer

• ISDN frames differ depending on inbound (network to terminal NT format) or outbound (terminal to network TE format).
• ISDN frames are 24 bits long.
• ISDN BRI service offers 2 B channels operating at 64kbps which can be used independently from one another for user data or voice traffic.

2*(2*8B + 2D + 6F) sent in pairs

ISDN BRI sent at 8000/sec

(1*8B + 2D + 6F) = 24*8000 = 192kbps

but effective data rate = 2*8B = 16*8000 = 128kbps

ISDN Data link layer

• ISDN datalink layer is LAPD - Link Access Procedure D channel - ensures control and signalling information flows properly.
• D channel operates at 16kbps and is used to carry control and signalling information.
• Terminals cannot transmit into D channel unless they first detect a specific number of ones (indicating no signal). If the TE detects a bit in the echo (E) channel that is different from it's D bits then it stops transmitting immediately.

ISDN SPIDs - Service Profile Identifiers

You also need to know your assigned SPID to identify your configuration at the CO.

• SPIDs allow multiple ISDN devices to share local loop.
• SPIDs are processed during each call set up.
• SPIDs assigned by your service provider.
• A SPID is required for each B channel.

ISDN Devices

TE1 - Device compatible with ISDN network, connects to NT1 or NT2
TE2 - Device not compatible with ISDN requires TA
TA - (terminal Adapter) Converts signals so non-ISDN devices can use ISDN
NT1 - Connects 4-wire ISDN to 2-wire local loop
NT2 - Directs traffic to and from different subscriber devices and NT1

ISDN Reference Points

R - References points between non-ISDN device and TA
S - References points that connect into NT2 (Inbound)
T - References Outbound connections from NT2 into ISDN network or NT1
U - References connection between NT1 and ISDN network (North America only)

ISDN Protocols

E - Telephone network standards
I - Concepts, terminology, general methods
Q - How switching should operate

Dial on Demand Routing - DDR

Before setting up a dial on demand connection you must consider what kind of traffic will initiate the dial up connection, consider the following points:

• You must first establish how ISDN connections will be initiated, established and maintained.
• You must establish which sites are involved, who can initiate and terminate connections.
• Applications supported over DDR, which applications can initiate a connection.
• What unexpected protocols could cause a connection to be made.

ISDN switch types

For proper ISDN configuration you have to configure the correct switch type. Most common type in US is At&T's 5ESS and Nortel DMS-100. In UK it's Net3 and Net5.

Nik Makris is certified
under the CCNA® program