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 point-of-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 high-quality 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.