Certified Basic Network Support Professional WAN Technologies Leased Lines Circuit and Packet Switching and Cell Relay

WAN Technologies Leased Lines Circuit and Packet Switching and Cell Relay

Circuit switching vs. Packet switching

Circuit switching and packet switching are both communication methods for large networks. The most common example of a circuit switching network is the telephone system: the sender and the receiver establish a dedicated physical path for the entire duration of the call. All of the transmitted information follows the same route and the circuit is available only to the nodes that established it. PSTN (Public Switched Telephone Network), covered in the Internet Connections TechNotes, and ISDN, covered below, both use the circuit switching technology.

In packet switching networks, data is segmented into packets that each take a route independently based on the addressing information their header. In theory, the route can be different for each packet, but also one and the same. The packet is sent from hop to hop whereby each hop (e.g. a router) determines the best next part of the route. Other nodes can send packets, seemingly simultaneously, over the same dynamic route. Most large WANs are largely made up of packet switching networks, the Internet being the most common example.


Integrated Services Digital Network (ISDN) is a circuit-switching network used for voice, data, and video transfer over plain copper telephone lines. ISDN is a bit similar to the normal telephone system but it is faster, more reliable, and requires less time to setup a call. Digital ISDN phones digital faxes are usually provided by the telco. An ISDN modem can be used to convert the signals of non-ISDN equipment to ISDN signals. An ISDN modem can be external or internal and is usually connected to the wall outlet by using an UTP cable with an 8-pin RJ-45 connector on the modem side, and a 6-pin RJ-11 connector on wall jack side. Some ISDN modems use or allow in addition an RJ-45 to RJ45 or RJ-11 to RJ-11 connection. An ISDN modem can also be integrated in a router to provide a shared WAN or Internet connection for multiple users in a network.

An ISDN connection consists of several different types of digital channel. The 64 Kb B-channel used for transferring data, and the D-channel used for transmitting control information are the most common type of channels in use. Most home users or smaller organizations with an ISDN connection usually have an ISDN BRI(Basic-Rate Interface) connection. Two B-channels + one D-channel make up ISDN BRI. Some remote access servers support a feature called multilink allowing the two B-channels to be combined in a single virtual link of 128 Kbps. In reality, often 1 B-channel is used for data (an Internet connection for example) and 1 B-channel is used for voice (connected to a digital telephone for example).

ISDN PRI (Primary-Rate Interface) is more often used by medium to large sized organizations and is made up of 23 B-channels and 1 D-channel. The European version of PRI supports 30 B-channels. A common implementation of these two types of ISDN is a remote access solution with ISDN PRI at the corporate network supporting 23 dial-in connections for employees with ISDN BRI at home.

T1/E1/J1 & T3/E3/J3

A T1 connection is a digital leased line made up of 24 channels (called DS0, 1 DS0 is 64K) that providing transfer rates up to 1.544 Mbps, and is often used to connect corporate networks and ISPs to the Internet. The European version E1 is made up of 30 channels providing rates up to 2.048 Mbps. The Japanese version is made up of 24 channels just like a T1. They all use the DS1 signaling standard and that's why a T1 connections is sometimes also referred to as a DS1 line.

A T3 is an even faster digital leased line providing rates up to 44.736 Mbps (672 channels), and is used for high-speed Internet backbones and large organizations. The European version E3 provides rates up to 34.368 Mbit/s (512 channels ) The Japanese version J3 provides rates up to 34.064 Mbps (480 channels). T3, E3, and J3 use the DS3 signaling standard.

A CSU/DSU (Channel Service Unit/Data Service Unit) is a modem-like device that converts digital data frames from the communications technology used on a LAN into frames appropriate to a WAN and vice versa. This device sits on both ends of the T1/T3 connection, sometimes as an integrated device in a router.


Sonet(Synchronous Optical NETwork) is a hierarchy of standardized digital data rates for optical transmission interfaces proposed by Bellcore. The data rates in these fiber optic networks are divided in OC-levels. The following table lists the speeds for all OC levels:

OC-1 = 51.85 Mbps
OC-3 = 155.52 Mbps
OC-9 = 466.56 Mbps
OC-12 = 622.08 Mbps
OC-18 = 933.12 Mbps
OC-24 = 1.244 Gbps
OC-36 = 1.866 Gbps
OC-48 = 2.488 Gbps
OC-192 = 9.952 Gbps
OC-768 = 40 Gbps
OC-3072 = 160 Gbps

Obviously, you only need to remember the speed of OC-1, for example: OC-192 is simply 192 times the speed of OC-1.


X.25 is a classic packet-switching standard from ITU-T that operates at the Physical, Data Link, and Network layers of the OSI model. It uses PSTN and ISDN connections to allow large scale WANs. X.25 was mainly used in older environments with remote terminals connected through a simple PSTN modem connection. Because the older telephone lines were prone to error and interference, X.25 is mainly concerned with error-correction to allow a more reliable connection. The main part of an X.25 network usually belongs to a public carrier, and subscribers connected to it usually pay for the bandwidth they use.


ATM is short for Asynchronous Transfer Mode, a packet-switching network that is commonly used for high-speed backbones in large network environments such as the Internet, for voice, data and video transfer. Data is transmitted in small 53-byte fixed length cells , and that is why ATM is also referred to as a cell-switching network . Partly because of the fixed length cell approach, ATM is able to reach data rates up to 622 Mbps. Also, an ATM switch uses integrated hardware circuits that switch cells between incoming and outgoing ports which significantly increase data throughput compared to software based switching. Every cell with the same source and destination address travels over the same route if possible.

ATM supports several innovative features such as Bandwidth on demand and QoS (Quality of Service). The latter allows data to be prioritized based on the content. For example, real-time video transfer could have a higher priority than file transfer, to allow the user to watch the video without interruptions. ATM uses its own reference model, which corresponds roughly to both the OSI Data Link and the Physical Layer. ATM supports different types of physical media including, OC-3, OC-12, TE/E3, UTP and FDDI.

Frame Relay

Frame Relay, one of today's most common examples of a packet-switching network, is a high-performance WAN protocol that operates at the physical and data link layers of the OSI model. An advantage of using Frame Relay is that the physical network medium and the available bandwidth are dynamically shared between the connected end nodes. Common use of Frame Relay is to interconnect LANs in a WAN and provide centralized shared Internet connectivity to remote offices. Frame Relay can be very cost-effective because generally you often only pay for the bandwidth usage.


Leased Lines

Typically referred to as a point-to-point connection or dedicated connection.  Is is a pre-established WAN communications path from the CPE, through the DCE switch, to the CPE of the remote site, allowing DTE networks to communicate at any time with no setup procedures before transmitting date.  It uses synchronous serial lines up to 45Mbps.

Circuit Switching

Sets up like a phone call.  No data can transfer before the end-to-end connection is established.  Uses dial-up modems and ISDN.  It is used for low-bandwidth data transfers.

Packet Switching

WAN switching method that allows you to share bandwidth with other companies to save money.  As long as you are not constantly transmitting data and are instead using bursty data data transfers, packet switching can save you a lot of money.  However, if you have a constant data transfers, you will need to use a leased line.  Frame Relay and X.25 are packet switching technologies.  Speeds can vary from 56Kbps to 2.048Mbps.

cell relay
A data transmission technology based on transmitting data in relatively small, fixed-size packets or cells. Each cell contains only basic path information that allows switching devices to route the cell quickly. Cell relay systems can reliably carry live video and audio because cells of fixed size arrive in a more predictable way than systems with packets or frames of varying size.

Asynchronous Transfer Mode (ATM) is the cell relay standard set by the CCITT organization. ATM uses a cell of 53 bytes.

Cell relay uses data cells of a constant size. Frames are similar data packets, but they differ from cells in that they may vary in size according to the requirement or situation. This technology is not secure because its protocols do not support error handling or data recovery. As such, all sensitive and important transmissions may be delivered faster using fixed-sized cells, which are easier to carry compared to variable-sized frames or packets.

Cell relay is very reliable for delivering sensitive information. Switching devices give the exact route to cells as per the destination address embedded in a cell. One example of cell relay is ATM, a popular form used to transmit a cell with fixed size of 53 bytes.

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