Switching Methods in WAN Technology
Switching is a component of a network topology that determines the connection created between nodes. Some of the common types of switching methods are packet switching and circuit switching. Every network transport system relies on either of these switching mechanisms.
The rapid growth of the Internet and the abundance of computer hardware and software available to people has placed an increasing demand on telecommunications providers to supply faster data rates for private use. One of the currently popular solutions is Digital Subscriber Line ( DSL ) technology. These standards are often called xDSL because of many variations, permit rapid data communications over common telephone lines, often simultaneously permitting voice conversations. Switching methods are mainly techniques to move data through a network of links and switches.
A major network company Aber, 2001 defines xDSL as the dedicated, point-to-point, public network access technologies which allow multiple forms of data, voice, and video to be carried over the twisted-pair copper wire on the local loop between a network service provider’s central office and the customer site.
Common switching Methods
Switching methods refers to the routing process used to move data throughout the wide-area network. This method divides messages into packets and sends each packet individually. Switching methods influence the rapid process of routing. Some of the common switching methods are
1. Packet Switching
2. Circuit Switching
Packet Switching Methods
Packet switching involves the breaking up of messages into smaller components called packets. Depending on the system involved, the packet size often ranges from about 600 bytes to 4000 bytes. Each packet contains source and destination information and is treated as an individual message. The small messages are received and routed through optimal pathways by various nodes on a wide area network. The type of packet to be switched is called a datagram. Datagrams are simply broadcasting to a remote node. There is never an assurance that the message will remain intact or a fact that could be worsened by a packet-switched network.
When directing datagrams, there may be more than one route to pass the data. Each individual packet is directed down to be the optimal path at transmit time. Therefore, the pathways can become better or worse depending on their congestion levels or whether or not they are operating at all. So, a datagram from a message may end up taking a different pathway from another datagram from the same message.
Packet Switching Method between the router and PAD
Packet switching is ideal for digital data because the information is grouped into frames or packets, which are simply a collection of bytes of data. Packet switching networks treat each packet as an individual message to be routed. Messages are broken into packets and reassembled via the Packet Assembler/Disassembler device (PADS).
In packet switching, information inside the packets is read as to where the packet is sent. Then each packet is individually routed to its destination. Since there may be more than one pathway to the same destination, packets may be routed to more than one path to the destination.
To ease the process of directing the datagrams, packet-switched networks incorporate a special device known as a Packet Assembler/Disassembler device (PAD). The main function of a PAD device is to make sure the packets are placed in the right order as they are received. In order to follow the right order, the sequence number is placed in each packet designating the data packets. The PAD merely looks at that number in the packet and is able to subsequently reassemble the message that was originally sent. The pad is also responsible for taking messages coming into the network, breaking them up into packets, and then assigning sequence numbers to each packet. Datagrams do not utilize any sort of association between the sender and receiver, such as agreeing on packet size. Datagrams also do not typically use acknowledgments, sent from the receiver to sender acknowledging the receipt of a particular datagram.
Advantages of Packet Switching
1. Packet switching is quite faster because messages are not stored in their entirety for later recovery.
2. It allows the avoidance of pathway failure due to excessive traffic loads or mechanical problems.
3. Packet switching allows us to use pathways that may not normally get much traffic. Instead of concentrating on a few paths that are always busy, packet switching spreads the load of communication across several paths.
Circuit Switching Methods
Circuit switching involves the formation of a physical path for data flow between a sender and receiver. This method creates a link between the callers using the phone system. The whole connection of sender to receiver is called a circuit. Circuit switching has advantages associated with a physical pathway like the reliability of transfer. The problem associated with circuit switching is that overhead is required to create the physical pathway.
The circuit offers the desirable bandwidth to the sender and receiver. This process is well only if each of the pair is actively sending and receiving, but when the channel becomes idle, all that bandwidth is wasted. In circuit-switched networks, a single pathway is set up at the outset of communication and used throughout. Therefore, circuit switching is mainly the dedicated communication channel between senders and receivers. But it has difficulty creating the channel as well as maintain it even after the transmission is completed.
Integrated Services Digital Network
Integrated Services Digital Network (ISDN) is a circuit-switched telephone network system, intended to allow digital transmission of voice and data over ordinary telephone copper wires, resulting in better quality and higher speeds. It is a set of protocols for establishing and breaking circuit-switched connections. ISDN consists of digital lines that are broken up into two types of channels Data and Signaling.
The data-bearing B channels or bearer channels support data transfer rates up to 64Kbps per channel. The B channels can be grouped together to support higher data rates.
The Signaling D channel or control channel uses a separate circuit to pass signaling and control information. The standard dial-up connections include signaling and control information on the same circuit as the user data. This is called the in-band signaling and takes up the bits for the purpose of control and management. Therefore, the standard analog circuits cannot support 64Kbps data rate known as a clean channel. Instead, they can support a maximum of 56Kbps.
ISDN supports two major service types. They are:
1. Basic Rate Interface (BRI)
2. Primary Rate Interface (PRI)
Basic Rate Interface (BRI)
Basic Rate Interface (BRI) is made up of two B channels and one D channel (2B+D) for transmitting control information. It is also called as ‘S’ or ‘T’ interface. The BRI B-channel service operates at 64Kbps and its main function is to carry the user data. BRI D-channel service operates at 16Kbps and is intended to carry control and signaling information. The Remote Access servers support a feature called the multilink allowing both the B channels to be combined in a single virtual link of 128Kbps.
Primary Rate Interface (PRI)
Primary Rate Interface (PRI) consists of 23 B-channels and one D-channel (23B+D or 30B+D depending on the bandwidth. It can also handle 23 and 30 voice channels respectively. The 23B+D delivers throughputs of 1.544 Mbps while 30B+D delivers 2.040 Mbps. These arrangements feature separate 16 Kbps D channels for handling control information.
Fiber Distributed Data Interface (FDDI)
Fiber Distributed Data Interface is a media access control protocol with token-ring architecture which has a communication bandwidth of 100 Mbps. It is supported on a fiber network medium and is fast compared to standard token ring and Ethernet.