Transcript Circuit Switching - Universitas Hasanuddin

Circuit Switching
• Blocking occurs when the network is
unable to connect to stations because all
possible paths between them are already
in use. Non-blocking network permits all
stations to be connected (in pairs) at once
and grants all possible connection
requests as long as the called party is free
ISDN
• Integrated Services Digital Network (ISDN)
is a network that provides end-to-end
digital connectivity to support a wide range
of services including voice and data
services. ISDN allows multiple digital
channels to operate simultaneously
through the same regular phone wiring
used for analog lines, but ISDN transmits
a digital signal rather than analog
• ISDN standards define two main channel types,
each with a different transmission rate. The
bearer channel, or B channel, is defined as a
clear digital path of 64 kbps. It is said to be clear
because it can be used to transmit any type of
digitized data in full-duplex mode. For example,
a digitized voice call can be transmitted on a
single B channel. 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). The D channel is used to carry control
information for the B channel.
The following sequence of events occurs during the establishment of a BRI or PRI call:
1.The D channel is used to send the called number to the local ISDN switch.
2.The local switch uses the SS7 signaling protocol to set up a path and pass the called
number to the remote ISDN switch.
3.The remote ISDN switch signals the destination over the D channel.
4.The destination ISDN NT-1 device sends the remote ISDN switch a call-connect message.
5.The remote ISDN switch uses SS7 to send a call-connect message to the local switch.
6.The local ISDN switch connects one B channel end-to-end, leaving the other B channel
available for a new conversation or data transfer. Both B channels can be used
simultaneously.
• 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. ISDN however, uses a separate
channel for control information, the D
channel. This is called out-of-band
signaling.
• ISDN specifies two standard access
methods, BRI and PRI. A single BRI or
PRI interface provides a multiplexed
bundle of B and D channels.
• BRI uses two 64 kbps B channels plus one
16kbps D channel. BRI operates with
many Cisco routers. Because it uses two
B channels and one D channel, BRI is
sometimes referred to as 2B+D.
• In North America and Japan, PRI offers
twenty-three 64 kbps B channels and one
64 kbps D channel. A PRI offers the same
service as a T1 or DS1 connection. In
Europe and much of the rest of the world,
PRI offers 30 B channels and one D
channel in order to offer the same level of
service as an E1 circuit. PRI uses a Data
Service Unit/Channel Service Unit
(DSU/CSU) for T1/E1 connections.
Frame Relay
• Frame Relay is an International
Telecommunication Union
Telecommunications Standardization
Sector (ITU-T) and American National
Standards Institute (ANSI) standard.
Frame Relay is a packet-switched,
connection-oriented, WAN service. It
operates at the data link layer of the OSI
reference model.
• Frames carry data between user devices
called data terminal equipment (DTE), and
the data communications equipment
(DCE) at the edge of the WAN.
• A Frame Relay network may be privately
owned, but it is more commonly provided
as a service by a public carrier. It typically
consists of many geographically scattered
Frame Relay switches interconnected by
trunk lines.
• Frame Relay is often used to interconnect
LANs. When this is the case, a router on
each LAN will be the DTE. A serial
connection, such as a T1/E1 leased line,
will connect the router to a Frame Relay
switch of the carrier at the nearest pointof-presence for the carrier. The Frame
Relay switch is a DCE device. Frames
from one DTE will be moved across the
network and delivered to other DTEs by
way of DCEs.
• The connection through the Frame Relay
network between two DTEs is called a
virtual circuit (VC). Virtual circuits may be
established dynamically by sending
signaling messages to the network. In this
case they are called switched virtual
circuits (SVCs). However, SVCs are not
very common. Generally permanent virtual
circuits (PVCs) that have been
preconfigured by the carrier are used. The
switching information for a VC is stored in
the memory of the switch.
• Because it was designed to operate on highquality digital lines, Frame Relay provides no
error recovery mechanism. If there is an error in
a frame it is discarded without notification.
• The various virtual circuits on a single access
line can be distinguished because each VC has
its own Data Link Channel Identifier (DLCI).
The DLCI is stored in the address field of every
frame transmitted. The DLCI usually has only
local significance and may be different at each
end of a VC.