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Introducing ISDN
ā¢ Telephone companies developed^ ISDN (Integrated Services Digital
Network) as part of an effort to standardize subscriber services.
ā¢ This included the^ User-Network Interface (UNI),^ better known as the
local loop.
ā¢ The ISDN standards define the hardware and call setup schemes for
end-to-end digital connectivity.
ā¢ These standards help achieve the goal of worldwide connectivity by
ensuring that ISDN networks easily communicate with one another.
ā¢ In an ISDN network, the digitizing function is done at the user site
rather than the telephone company.
Introducing ISDN
ā¢ Unlike POTS, ISDN is digital from end to end.
ā¢ With asynchronous connections (POTS) the local loop is analog and
requires PCM (Pulse Code Modulation) - explained later.
ā¢ Benefits of ISDN include:
- (^) Carries a variety of user traffic signals, including data, voice, and video
- (^) Offers much faster call setup than modem connections
- (^) B channels provide a faster data transfer rate than modems
- (^) B channels are suitable for negotiated Point-to-Point Protocol (PPP) links
ISDN Disadvantages
ā¢ BRI is slower than DSL and cable
ā¢ More expensive than DSL and cable
ā¢ Bottom line: ISDN, in its current form, is no longer a āfirst-
choiceā technology.
Why 64Kbps channels and what is PCM?
ā¢ For now, 64,000 bps is whatās required to carry a single phone call
over a link (an analog call which has been digitized).
ā¢ PCM (Pulse Code Modulation) is how the analog signal is translated to
digital and visa versa.
ISDN standards and access methods
ISDN standards define two main channel types
ā¢ The bearer channel, or B channel, is defined as a clear digital path of
64 kbps
ā¢ The second channel type is called a delta channel, or D channel.
- (^) There can either be 16 kbps for the Basic Rate Interface (BRI) or 64 kbps for the Primary Rate Interface (PRI).
ISDN standards and access methods
ā¢ ISDN is widely available in two flavors:
- (^) BRI: Basic Rate Interface
- (^) 2 64 Kbps Bearer Channels,16 Kbps Delta Channel (for control information), 48 Kbps for framing and synchronization
- (^) 2B + 1D (2B+D)
- (^) 192 Kbps = 128+16+
- (^) PRI: Primary Rate Interface
- (^) 23B + 1D (T1), the D channel is 64-kbps
- (^) 30B + 1D (E1), European E
- (^) 1.544 Mbps (North America) or 2.048 Mbps (E1)
D Channel
ā¢ When a^ TCP connection is established, there is an exchange of
information called the connection setup.
- (^) This information is exchanged over the path on which the data will eventually be transmitted.
- (^) Both the control information and the data share the same pathway.
- (^) This is called in-band signaling.
ā¢ ISDN^ however, uses a^ separate channel for control information, the^ D
channel.
- (^) This is called out-of-band signaling.
ā¢ The D channel carries signaling messages, such as call^ setup and
teardown, to control calls on B channels.
ā¢ Traffic over the D channel employs the Link Access Procedure on
the D Channel (LAPD) protocol.
ā¢ LAPD is a data link layer protocol based on HDLC.
ISDN 3-layer model and protocols
ā¢ ISDN utilizes a suite of ITU-T standards spanning the physical, data
link, and network layers of the OSI reference model.
ā¢ The ISDN BRI and PRI^ physical layer specifications^ are defined in
ITU-T I.430 and I.431, respectively.
ā¢ The ISDN^ data link specification^ is based on LAPD and is formally
specified in the following, ITU-T Q.920, ITU-T Q.921, ITU-T Q.922, ITU-T Q.
ā¢ The ISDN^ network layer^ is defined in ITU-T Q.930, also known as
I.450 and ITU-T Q.931, also known as I.451.
ā¢ These standards specify user-to-user, circuit-switched, and packet-
switched connections. I like the āolderā chart. Layer 3 Q.9 31 Layer 2 Q.9 21 Short Term Memory
BRI Physical Layer
ā¢ If the frame is outbound, it is sent from the terminal to the
network.
ā Outbound frames use the TE frame format.
ā¢ If the frame is inbound, it is sent from the network to the
terminal.
ā Inbound frames use the NT frame format.
These Reference Points will be discussed in a moment, but this is where they get TE and NT from. Short Term Memory
BRI Physical Layer
ā¢ ISDN BRI frames contain 48 bits.
ā¢ Four thousand of these frames are transmitted every second, 4,000 x
48 = 192,000 bps.
- (^) Each B channel , B1 and B2, have a capacity of 2(8*4000) = 64 kbps, 128 kbps for both B channels (B1 and B2)
- (^) The D channel has a capacity of 4*4000 = 16 kbps (D)
- (^) Framing and overhead 124,000 = 48,000 kbps. (F, L, E, A, S) 64k (164,000) - B1 channel 64k (164,000) - B2 channel 16k (44,000) - D channel 48k (12*4,000) ā Framing/Overhead
192 kbps BRI Total 144 kbps = B1 + B2 + D (2B+D) 4,000 frames per second B1, B2, D and Framing Bits
ISDN Data Link
Layer
- The LAPD flag and control fields are identical to those of HDLC.
- The LAPD address field is 2 bytes long.
- Service access point identifier (SAPI),^ which^ identifies the portal at which LAPD services are provided to Layer 3.
- The command/response bit (C/R), indicates whether the frame contains a command or a response.
- The second byte contains the^ terminal endpoint identifier (TEI).
- (^) Each piece of terminal equipment on the customer premises needs a unique identifier.
- (^) The TEI may be statically assigned at installation, or the switch may dynamically assign it when the equipment is started up.
- (^) Statically assigned TEIs range from 0 to 63.
- (^) Dynamically assigned TEIs range from 64 to 126.
- (^) A TEI of 127, or all 1s, indicates a broadcast. Short Term Memory
Call Setup
ā¢ To establish an ISDN call, the^ D channel is used between the router
and the ISDN switch to control functions such as call setup, signaling, and termination.
ā¢ Signal System 7 (SS7) signaling is used between the switches within
the service provider network.
ā¢ These functions are^ implemented in the Q.931 protocol.
ā¢ The Q.931 standard recommends a network layer connection between
the terminal endpoint and the local ISDN switch, but it does not impose an end-to-end recommendation.
- Not an end-to-end function but processed by the switch.
- Depending upon the switch type, you may or may not get all of the steps show above. Short Term Memory
ISDN Interfaces
- To connect devices that perform specific functions, the interface between the two devices needs to be well defined.
- R ā References the connection between a non-ISDN compatible device Terminal Equipment type 2 (TE2) and a Terminal Adapter (TA), for example an RS-232 serial interface.
- S^ ā References the points that connect into the customer switching device Network Termination type 2 (NT2) and enables calls between the various types of customer premises equipment.
- T^ ā Electrically identical to the S interface, it references the outbound connection from the NT2 to the ISDN network or Network Termination type 1 (NT1).
- U^ ā References the connection between the NT1 and the ISDN network owned by the telephone company. Short Term Memory
CAUTION : Some routers contain NT1ās. Never connect a
router with a U interface into a NT1. It will most likely ruin
the interface. Know what type of interface your router has!
ISDN reference points
ā¢ Because the S and T references are electrically similar, some
interfaces are labeled S/T interfaces. Although they perform different functions, the port is electrically the same and can be used for either function.
