Network Access Layer (3): WAN Technologies

Abstract

Crossing the close spatial borders of a LAN, WANs (Wide Area Network) offer the possibility to bridge very large distances between individual computers and local computer networks. To do this, special transmission media is needed as well as new, additional network technologies and protocols. Our access to the global Internet is carried out mostly via WAN technologies, e.g., over the telephone network, cable television network or a mobile radio network. In this chapter, the most important concepts and technologies for the establishment and operation of these types of WANs is presented. The main focus will be on addressing and routing, which are also of importance to the overlying Internet layer. On the basis of various technological examples, a closer look is taken at how WANs function.

“He shall have dominion from sea to sea, and from the river to the ends of the earth.”.

- Psalm 72:8.

Glossary

Access network

Generally-speaking, most end users do not have their own direct access to a wide area network (WAN). Due to the high costs involved, this remains the privilege of large companies or scientific and military organizations. The end user gains access to the WAN, most of the time, via an already existing access network, e.g., the analog telephone network.

ARPANET

The packet-switched WAN was developed at the end of the 1960’s by the Advanced Research Projects Agency (ARPA) for the U.S. Department of Defense (DoD). The ARPANET is considered a precursor of the global Internet.

Asynchronous Transfer Mode (ATM)

From the ITU for wide area networks with high bandwidth standardized data transmission procedure. It is based on the principle of asynchronous time multiplexing. ATM is also used in LANs. The basic unit for information transmission is the so-called ATM cell. This is a data packet with a fixed length (53 bytes). Virtual connections can be permanently switched between two linked ATM end systems or established, as needed. ATM defines multiple service classes for the support of multimedia applications, among these are an isochronous mode, which emulates the behavior of a line-switched connection and is used for the transmission of video and audio data streams.

Bandwidth

The bandwidth of a connection route in a network is a physical size, which is given in hertz (1 Hz = 1/s). In the analog range the bandwidth indicates the frequency area in which electrical signals are transmitted with an amplitude decrease of up to 3 dB. The bigger the bandwidth, the more information can theoretically be transmitted in a time unit. The term “bandwidth” is also often used in connection with the transmission of digital signals, while here the transmission rate is meant. Nevertheless, there is a direct connection between bandwidth and transmission rate. The transmission speed reached in data transmission depends directly on the bandwidth of the network. The maximum bandwidth utilization for binary signals is 2 bits per hertz bandwidth.

Bit stuffing

In synchronous data transmission, the clock of transmitter and receiver are synchronized with each other by means of a so-called synchronization bit at the beginning of a data packet. The synchronization bits form a fixed bit pattern and are ordered in such a way that they can never be mixed up with the payload information. If e.g., 01111110 is used as a synchronization bit sequence, care must be taken that no sequence of six consecutive ones could be identified in the payload as a synchronization bit sequence. Because of this, after five ones a zero is added by the transmitter and then removed again by the receiver.

Broadcasting

In the broadcasting procedure a transmission is made to all connected systems in the network at the same time. If, in contrast, there is a 1:1 connection with only one communication partner, the procedure is designated as an unicast transmission.

Default routing

To avoid the occurrence of multiple identical entries in routing tables, every router is assigned a default route, and the corresponding entry in the routing table is simply deleted. If an entry does not exist in the routing table for a particular destination, the default route is used to forward the relevant data packet.

Digital Subscriber Line (DSL)

Digital access procedure for wide area networks. DSL enables high bandwidths for the connected end users it is at the same time subject to strict length and quality restrictions regarding the implemented network infrastructure.

Dijkstra algorithm

Named after Edsger W. Dijkstra, it is the algorithm for determining the shortest path from a specific start node in a graph. The Dijkstra algorithm is employed in the routing procedure. It is used to attempt to find the shortest connection path from transmitter to receiver.

Distance vector routing

A routing procedure in which all routers in the network receive their routing information through exchange with the directly adjacent router. It is assumed that every router is capable of establishing the distance to its direct neighbor itself. The routing information determined this way is subsequently exchanged with all direct neighbors. This continues, in a multi-stage process, until, finally, all routers in the network have received the complete routing information. Distance vector routing, a very simple method, was employed as the first routing procedure in the ARPANET.

Distributed Queue Dual Bus (DQDB)

DQDB is a technology concept for MANs, based on two parallel, counter-powered buses. End systems wishing to transmit receive the access right to a bus by means of a divided queue. They can then transmit cells of a constant length. DQDB is efficient when there is a heavy load, but not always fair. DQDB networks can extend over several hundred kilometers.

Flooding

Isolated routing method in which an incoming data packet is forwarded by a router to all connections, except for the one by which the data packet reached the router. To avoid overburdening the network, the life-time of a forwarded data packet can be limited through different procedures. Although this method produces a high redundancy, it always finds the shortest route.

Frame Relay

A connection-oriented, packet-switched network technology that was developed in the mid-80 s especially for wide area networks. Frame Relay is based on an asynchronous time multiplex procedure. While transmission errors are detected, they are not automatically corrected. In a Frame Relay network, virtual connections are set up between the communicating end systems.

Global System for Mobile Communication (GSM)

Mobile radio system of the second generation with currently the largest distribution worldwide. GSM is based on individual cells, whose expansion varies depending on subscriber density. GSM operates in the time multiplex procedure and offers different carrier services for data communication. These are based on line or packet-switched methods on individual channels, each with data rates of up to 9, 6 kbps.

Hop

Designates the leg of a route from one end system to the nearest switching computer, or between two adjacent switching computers, or from the switching computer to a connected end system. A route through a network, from a transmitter-end system to a receiver-end system, is made up of multiple hops.

Hot-Potato routing

An isolated routing method in which the router tries to forward an incoming data packet on the quickest path. The forwarding connection of the router is chosen with the shortest queue. The forwarded data packet is often forced to accept detours with this procedure.

ISDN

The WAN technology, Integrated Services Digital Network, integrates various services such as voice, data, and image communication in a single network. ISDN provides channels with 64 kbps each. ISDN was developed as a substitute for the old analog telephone network and has been expanded with worldwide, extensive coverage since the mid-80’s.

Isolated routing

Designation for a routing procedure that works locally and does not exchange information over the network topology with its neighbors. Examples for this method are flooding or hot potato routing.

Link-state routing

Decentralized, adaptive routing procedure (also known as SPF routing/Shortest Path First routing), in which every router sends the routing information it can determine about its direct neighbors to all other routers in the network via a broadcast With the help of this information they are constantly kept informed about the current topology of the network. Routes are calculated in link-state routing with the help of the Dijkstra algorithm.

Local Area Network (LAN)

Spatially restricted computer network that can accept only a limited number of end devices (computers). A LAN enables an efficient and fair communication of all end systems connected to it. As a rule, all of the connected computers additionally share a transmission medium.

Local Loop

This is a designation for the subscriber line from the local exchange to the end user. The distance to be bridged to the local exchange is also known as the last mile, and it is implemented over a dedicated transmission medium (in contrast to a shared LAN medium).

Mobile routing

A special form of routing for networks with mobile network nodes. A mobile network node can move freely through different wireless networks (WLANs). This is why a router must first determine its location before a data packet can be forwarded to the mobile network node. The procedure used (mobile IP) assumes that the mobile node always has a fixed assigned address in a home network. If the mobile network node enters a foreign network, it must register there at a special router (foreign agent). This establishes contact to a router in the home network of the mobile node (home agent). Together they organize the correct forwarding of the data packet addressed to the mobile node.

Modem

For data transmission over an analog telephone network, a modem must be employed to facilitate the transformation of digital signals into analog signals and back again. The frequency spectrum of the original digital signal is moved into the frequency range of the analog telephone network with so-called modulation. This shift is undone with demodulation.

Multicasting

In a multicast transmission, a source transmits to a group of receivers simultaneously. This involves a 1: n communication. Multicasting is often used for the transmission of multimedia data.

Next hop forwarding

This is a switching variation in a packet-switched network. Here, the relevant switching computers do not store information about the entire route that a data packet must cover to another end system, but only for the next leg to be undertaken (hop).

Packet switching

The dominant communication method in digital networks. The message is segmented into single data packets of a fixed size and the packets are sent individually and independent of each other by the transmitter to the receiver. A distinction is made between connection-oriented and connectionless (datagram network) packet switching networks. In connection-oriented packet-switching networks, before the beginning of actual data transmission a connection is set up over fixed, selected local exchanges. In connectionless packet switching, in contrast, no fixed connection path is established.

Plesiochronous Digital Hierarchy (PDH)

The PDH defines a synchronous time multiplexing-based WAN technology whereby signals from different sources are sent over a shared channel. The bandwidth of the different channels are, however, in reality not exactly the same, rather nearly the same (thus the designation from plesios = [Greek] close). Because this is a synchronous procedure, differences must be balanced out. This is possible with so-called bit stuffing.

Router

A switching computer that is able to connect two or more subnetworks with each other. Routers operate on the transport layer (IP layer) of the network and are able to forward incoming data packets on the shortest route through the network based on their destination address.

Routing

In a WAN there are often multiple switching elements along the path from the transmitter to the receiver. These take over the transfer of the data sent to the corresponding receiver. The determination of the correct path from transmitter to receiver is called routing. Dedicated local exchanges (routers) receive the transmitted data packet, evaluate its address information and forward it accordingly to the designated receiver.

Routing table

Principle data structure of routers containing partial route information regarding forwarding of the incoming data packets. Every target address in the routing table is assigned to a specific exit (port) of the router, via which the next leg - hop – of the data packet proceeds to its receiver.

Routing algorithm

A calculation procedure for the creation of the routing table. There is a distinction made between static and dynamic routing algorithms, which can adjust dynamically (adaptive) to changes in the network topology. Routing algorithms can be executed from a central place in the network or from every router itself.

Signaling

Designation for the exchange of all information that is required for the establishment, monitoring and dismantling of connections in the telecommunications network. This task is also known as call control or connection control. A fundamental distinction is made between in-band signaling, in which the signal information is transmitted in the same logical channel as the payload, and out-of-band signaling, whereby a separate logical channel is used for the transmission of command and control information.

Spanning tree

The spanning tree of a graph designates a subgraph containing all nodes and edges of the original graph necessary to maintain original accessibility relations without looping. Those parts of the network connections are thereby designated that link all routers and prevent an occurrence of looping.

Store-and-forward switching

Switching procedure in a network, whereby a fixed connection between the transmitter and receiver is not required to be switched. The message received from the transmitter is sent to exchanges in the network where, in each case, it is buffered before being forwarded. A difference is made between message switching, in which the transmitter’s complete message is buffered before being forwarding to the exchange, and packet switching, in which the message from the transmitter is segmented into individual data packets. These are sent over the network independent of each other.

Synchronous Digital Hierarchy

The data transmission technology known as synchronous digital hierarchy presents a synchronous multiplexing based transmission technique. It was especially conceived for data transmission via fiber-optic cable and radio. SDH currently represents the primary standard for networks in the WAN field, and will continue to gain importance in the future.

Universal Mobile Telecommunication System (UMTS)

Mobile radio system of the third generation, whose application from 2003 allows data rates of up to 2 Mbps (in virtually steady-state operation) in data communication.

Wide Area Network (WAN)

Freely scalable computer network that is not subject to spatial or capacity-related limitations. Individual partial networks are connected by routers, which coordinate data transfer in the WAN. The WAN technology supplies the foundation for internetworking.

WiMAX

Under the name Worldwide Interoperability for Microwave Access, and standardized as IEEE 802.16 wireless broadband wide area network technology, it presents a broadband radio transmission system technology for mobile and stationary end devices. In contrast to the other IEEE 802 standards for LANs and PANs, WiMAX encompasses a much larger range of up to 50 km (wireless MAN), with data transmission rates of 70 Mbps and more.

X.25

A packet switching technology for wide area networks that was standardized at the beginning of the 70 s. It enables the construction of multiple, virtual connections between two end systems (within this norm, designated as DTE - Data Terminal Equipment). Considered the first generation of the public data transmission technology, X.25 is still supported today by many network operators, particularly in Europe