Dedicated Lines: An example of a dedicated line is a leased line or a point-to-point link, is a communications line with ongoing transmission, rather than having an on/off status as transmission is required. These lines run over a dedicated analog or digital point-to-point connection that can interconnect different types of networks. Synchronous circuits require the same clock so that the receiving circuit knows exactly when each frame bit is received.
There are several dedicated line speeds used, and they’re based on the standard digital signal level 0 (DS-0) rate of 64 kbps. The T-carriers are the most common dedicated lines in North America. The T1 carrier can carry 24 DS-0s for a capacity of 1.544 Mbps. The T3 carrier is a dedicated phone connection. It consists of 672 individual DS-0 channels and supports data rates of approximately 45 Mbps. The T3 is also commonly called DS-3 and carries 28 T1 lines. The E1 carrier is the most common dedicated line in Europe and other countries, and can carry 30 DS- 0s for a capacity of 2.048 Mbps.
WAN Switching: WAN switching is used with networks that operate beyond the single point-to-point connection. There are two types of WAN switching: Circuit Switching and Packet Switching.
Circuit-Switched Networks
In a circuit-switched network, a dedicated point-to-point connection or circuit is required for transmission between the sender and receiver. Circuit-Switch networking is commonly used in telephone companies. Integrated Services Digital Network (ISDN) is an example of a circuit-switched network. It provides permanent, ongoing WAN connectivity and is the most widely used connectivity between routers.
Digital signals are employed with ISDN to support faster speeds than analog. Transmission speeds run up to 64 kbps. With Internet connectivity, ISDN has been bumped down the ranks by concurrent technologies such as Digital Subscriber Line (DSL), Asymmetric Digital Subscriber Line (ADSL) cable modems, and faster analog modems. Still, ISDN remains a common method for short-term connectivity between routers and is frequently used to create a backup link when the primary leased line or Frame Relay connection goes down.
Packet-Switched Networks (PSN)
In a packet-switched network (PSN), nodes share bandwidth with each other by sending small data units called packets. One difference from circuit-switched networks is information in packet-switched networks is diced up into packets and then forwarded to the next destination based on the router’s routing table. There the packets are reassembled based on their originating sequence numbers. PSNs are more economical than dedicated circuits because they create virtual circuits, which are used as needed.
Examples of PSNs:
- X.25: A connection-oriented packet-switching network, in which the packets are transmitted over virtual circuits and is defined by the International Telecommunications Union (ITU-T). The ITU-T specifications identifies the point-to-point communication between Data Terminal Equipment (DTE), Data Circuit-Terminating Equipment (DCE), or a Data Service Unit/Channel Service Unit (DSU/CSU), which supports both switched virtual circuits (SVCs) and permanent virtual circuits (PVCs). Data terminal equipment (DTE) and data circuit-terminating equipment (DCE) are processed through routers and other devices. Routers are typically DTEs that are linked-up with modems or packet switches, which perform the DCE function. X.25 was designed to support most systems that are connected to the network. It has evolved to an international standard and is more widely used outside the United States.
- Link Access Procedure-Balanced (LAPB): this was developed for usage with X.25. LAPB defines methods for exchanging frames, monitoring frame sequence and absent frames, and carrying out frame acknowledgements and retransmission when necessary.
- Frame Relay: an advanced-performance, connection-based WAN technology. It is the follow-up to X.25 and LAPB and functions at speeds from 56 Kbps to 45 Mbps. It’s versatile in its deployment options. It operates by statistically multiplexing several data streams over a single link. Each data stream is called a virtual circuit (VC). The two models of Frame Relay VCs: Permanent Virtual Circuits (PVCs) and Switched Virtual Circuits (SVCs). Each VC is assigned an identifier to keep it unique. This identifier is called Data Link Connection Identifier (DLCI), and established on a per-leg basis over the transmission. It must be unique and accepted by two adjacent Frame Relay devices. As long as the two are in agreement, the value can be any valid number that doesn’t have to be the same end to end. Valid DLCI numbers are 16-1007. For DLCI purposes, 0-15 and 1008-1023 are reserved. The logical connection between the Frame Relay (FR) switch and the customer premises equipment (CPE) is also established by DLCI.
- Switched Multimegabit Data Service (SMDS): a high-speed, connectionless, packet-switched public network service. It is transmitted over a SONET ring with a maximum service area of 30 miles. It provides bandwidth to organizations that transmit massive amounts of data over WANs on a bursty or incremental basis.
- Asynchronous Transfer Mode (ATM): a connection-oriented high-bandwidth, low-delay transport technology that uses both switching and multiplexing. It handles the transmission of voice, data, and video across service provider networks and uses 53-byte, fixed size cells rather than frames. It can provide bandwidth on demand, making it ideal for bursty applications. ATMs are reliant on high speed, high-bandwidth mediums like fiber optics.
- Voice over IP (VoIP): a multi-service digital access technology that integrates various types of data into a single IP packet, including data, voice, audio and video. Multiple-processing is an advantage in terms of expense, functionality and interoperability.
