X.25

Read Tananbaum (pp 350 - 359)

Highlights:

   * Virtual Circuits
   * Permanent Virtual Circuits
   * Switched Virtual Circuits
   * Packet Switching
   * Digital Signalling

Advantages of X.25

   * End-to-End digital signalling
   * `packets' used for control and data
   * Virtual Circuits utilize resources better

Disadvantages of X.25

   * Can be very slow
   * Terrible for telephony

Informal Overview of X.25

Host is called DTE (Data Terminal Equipment).
Carrier's equipment which interacts with the DTE is called DCE (Data
Circuit-terminating Equipment).
X.25 defines the DTE/DCE interface.

There are 3 distinct phases: Call Setup, Data Transfer, and Call Clearing.
The interfaces are reminiscient of the Request-Indicate-Response-Confirm
events.
Call Setup has the following events.
Call Request, Call Indicate, Call Accept, Call Confirm.
Data Transfer has the following events.
Data request, Data indicate.
(Connection oriented, guarantees delivery) Call Clearing has the following
events.
Clear Request, Clear Indicate, Clear Response, Clear Confirm. (Note the
4-way clearing as well).

There used to be an explicit entity called the PAD (Packet Assembly and
Disassembly) which waited for 128 bytes from the terminal before it sent off
a packet, and likewise, broke the packet up at the receiver side to give the
illusion of a stream. Nowadays, its functionality is performed at the DTE.

Note, that the actual DTE/DCE interface is usually a modem.

An X.25 Phone Call

  1. Initiator punches in a number.
  2. The number is packetized and given to the DCE as a packet.
  3. The DCE establishes a LCN (Logical Channel Number) between itself and
     the DTE (LCN is a local channel number).
  4. The DCE establishes a "Virtual Circuit" through the carrier's network.
     Note, that the LCNs for each leg of the Virtual Circuit will be
     different.
  5. The Virtual Circuit is established at call time, and is call-specific
     (different calls may have different routes). It is therefore called a
     Switched Virtual Circuit (SVC) as opposed to a Permanent Virtual
     Circuit (PVC).
  6. The receiver DTE will eventually be contacted with the sender's phone
     number, its phone number and a LCN.
  7. Switching in the intermediate DCEs is Store-and-Forward, with
     acknowledgements between DCE-DCE links (to guarantee reliable
     delivery).
  8. It's called Virtual Circuit because the circuit is not physically made.
     Packets are stored and forwarded based on tables of LCN mappings.
  9. You can guess how slow this thing may end up being...
 10. Call clearing basically goes and destroys the table entries for the
     Virtual Circuit (except for PVC), and is a full 2-way exchange.

ISDN

Two physical layer options: Basic and Primary.

Basic Rate

The subscriber-loop is physically unchanged.
The connection to the home/office is a 4-wire interface as opposed to the
2-wire interface, that has to change.
Provides 2 64Kbps B-channels, the Bearer lines for user data.
Providers say Basic ISDN provides 128Kbps. Strictly, that is not true. It
specifies 2 64Kbps channels, and usually carriers will combine them using
some kind of ad-hoc technology. Effectively though, the user will see
128Kbps.
There is a 3rd 16Kbps channel for signalling, the D-line.
Unlike analog telephony, ISDN separates data channels from control channels.

The Basic standard for the subscriber loop is defined for 3 miles. Beyond
that, you need repeaters. So providing ISDN to rural areas is very very
expensive.

Primary Rate

The Primary Rate is designed for T1 and E1.
It offers 23 64Kbps B channels and 1 64 Kbps D channel for T1.
It offers 30 64Kbps B channels and 1 64Kbps D channel for E1. (The 32nd
channel is still used for timing).
The specifications define combining multiple B channels into a higher
bandwidth H channel. H channels are H0 (384 Kbps), H11 (1.536 Mbps) and H12
(1.92 Mbps).

ISDN Data Link

B Channel is equivalent to the X.25 virtual circuit, except that it is used
only for data. All the control is through the D Channel. The B Channel and D
Channel are Time Division Multiplexed on the same wire. Thus now, the
telephone wire has to do analog/digital conversions as well as TDM. Since
each ISDN line provides 2 B Channels, there is a capability to identify the
call per-B Channel as well.