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Introduction 1Basic Ideas in Message Switching 1

Message Switching Characteristics 2Message-Switched Networks 2

Network Elements 2Store-and-Forward Switches 3Message-based Protocols and the OSI Model 4

Message-Switched Networks andtheir Applications 5Store-and-Forward Networks 6

Message-Switched Protocols 7Performance of Message-Switched Networks 9

Delay Performance 9Node Delay 9Comparison of Message, Packet, and

Circuit Switching 11Conclusion 12Glossary 12References 12Further Reading 13

Message SwitchingMessage SwitchingDr. Farid Farahmand, Ph.D, Central Connecticut State University

INTRODUCTIONA communication network consists of a collection ofdevices (or nodes) that wish to communicate and inter-connect together. The primary objective in any commu-nication network is simply moving information from onesource to one or more destination nodes. Based on thetechniques used to transfer data, communication networkscan be categorized into broadcast and switched net-works. In broadcast networks, data transmitted by one nodeis received by many, sometimes all, of the other nodes.In switched-communication networks, however, the datatransferred from source to destination is routed through

the switch nodes. The way in which the nodes switch datafrom one link to another as it is transmitted from sourceto destination node is referred to as aswitching technique.Three common switching techniques are circuit switch-ing, packet switching, and message switching.

In circuit switching, the end-users are interconnectedusing dedicated paths. The most common example of acircuit-switched communications network is the plainold telephone service (POTS) network. One major issue

with circuit switching is that it can be rather inefficient,particularly in data communications (Stallings, 2004).This is because in a circuit-switched network the channelcapacity is dedicated for the entire duration of a connec-tion, even if no information is being transferred. Further-more, in a circuit-switched network, the actual time tosetup and tear down the path may in fact be longer than

the data transfer time between end-users. Refer to Chap-ter 73 for a complete discussion on circuit switching.In packet switching, data is divided into smaller units,

called packets, and then transmitted through the network.The minimum length of the packet is determined by thenetwork and varies from network to network. Packets aretransported over the network between nodes. Each inter-mediate node queues the packet for a brief time while itdetermines the route the packet should take to reach thenext node. For this reason, a packet-switched networkis sometimes called hold-and-forward network (Tomasi,2004). Please refer to Chapter 72 for a complete discus-sion on packet-switched networks.

Prior to advances in packet switching, message switch-ing was introduced as an effective alternative to circuitswitching. In message switching, end-users communi-cate by sending each other a message, which contains theentire data being delivered from the source to destina-tion node. As a message is routed from its source to itsdestination, each intermediate switch within the net-work stores the entire message, providing a very reliableservice. In fact, when congestion occurs or all networkresources are occupied, rather than discarding the traffic,the message-switched network will store and delay thetraffic until sufficient resources are available for success-

ful delivery of the message (Davis, 1973). The messagestoring capability can also lead to reducing the cost oftransmission; for example, messages can be delivered atnight when transmission costs are typically lower.

Message switching techniques were originally used in

data communications. Early examples of message switch-ing applications are paper tape relay systems and telexnetworks. Electronic mail (e-mail) and voice mail are alsoexamples of message switching systems. Today, messageswitching is used in many networks, including ad hocsensor networks, satellite communications networks, andmilitary networks.

Basic Ideas in Message SwitchingMessage-switched data networks are hop-by-hop sys-tems that support two distinct characteristics: store-and-forward and message delivery.

In a message-switched network, there is no direct con-nection between the source and destination nodes. Insuch networks, the intermediary nodes (switches) havethe responsibility of conveying the received messagefrom one node to another in the network. Therefore, eachintermediary node within the network must store all mes-sages before retransmitting them one at a time as properresources become available. This characteristic is often

referred to asstore-and-forward (Stallings, 2004). In mes-sage switching systems (also called store-and-forwardsystems), the responsibility of the message delivery ison the next hop, as the message travels through the path

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toward its destination. Hence, to ensure proper delivery,each intermediate switch may maintain a copy of themessage until its delivery to the next hop is guaranteed.In case of message broadcasting, multiple copies may bestored for each individual destination node.

The store-and-forward property of message-switchednetworks is different from queuing, in which messagesare simply stored until their preceding messages areprocessed. With store-and-forward capability, a messagewill only be delivered if the next hop and the link con-necting to it are both available. Otherwise, the messageis stored indefinitely. For example, consider a mail serverthat is disconnected from the network and cannot receivethe messages directed to it. In this case, the intermediaryserver must store all messages until the mail server is con-nected and receives the e-mails. The store-and-forward

technology is also different from admission control tech-niques implemented in packet-switched or circuit-switched networks. Using admission control, the datatransmission can temporarily be delayed to avoid over-provisioning the resources. Hence, a message-switchednetwork can also implement an admission control mech-anism to reduce networks peak load.

The message delivery in message-switched networksincludes wrapping the entire information in a singlemessage and transferring it from the source to the des-tination node. The message size has no upper bound;

although some messages can be as small as a simple da-tabase query, others can be very large. For example, mes-sages obtained from a meteorological database centercan contain several million bytes of binary data. Practicallimitations in storage devices and switches, however, canenforce limits on message length.

Each message must be delivered with a header. Theheader often contains the message routing information,including the source and destination, priority level, ex-piration time. Figure 1 shows an example of a messagedatagram with possible information embedded in themessage header.

It is worth mentioning that while a message is beingstored at the source or any other intermediary node in thenetwork, it can be bundled or aggregated with other mes-sages going to the next node. This is known as messageinterleaving. One important advantage of message inter-leaving is that it can reduce the amount of overhead gen-erated in the network, resulting in higher link utilization.

Message Switching CharacteristicsMessage switching offers a number of attractive benefits,

including efficient usage of network resources, traffic

regulation, and message storage when messages cannotbe delivered. However, there are several drawbacks asso-ciated with message switching techniques. For example,since messages are entirely processed and stored indefi-nitely at each intermediate node, switches require largestorage capacity (Davis, 1973).

In general, message-switched networks are relatively

slow. This is because all messages must be entirely stored,processed, and retransmitted. This slow switching schememay lead to poor performance, particularly in wide areanetworks (WAN). Although, using various schedulingtechniques, time critical messages can be given highertransmission priority, it is always possible that a shortmessage falls in line behind a very long one. Consequently,in message switching, messages initiated by different us-ers can expect large delay variance.

Large overall delay and high delay variations makemessage-switched networks less suitable for real-time andinteractive applications, such as voice communications ormultimedia games. Instead, message-switched networksare very attractive for supporting applications which cantolerate high delays, yet very little loss. Furthermore, mes-sage switching can be an alternative solution for networkswhere continuous end-to-end connectivity cannot beguaranteed, such as ad hoc sensor networks. A detaileddiscussion on advantages and disadvantages of mes-sage switching compared to circuit switching and packet

switching can be found in Subsection Comparison ofMessage, Packet, and Circuit Switching in this chapter.

MESSAGE-SWITCHED NETWORKSIn this section we first describe different elements of amessage-switched network and examine their basic func-tionalities. Then we focus on store-and-forward switchnode architecture and explain its building blocks.

Network ElementsFigure 2 shows a generic message-switched networkallowing message transport from one end-user to another.This network consists of transmission links (channels),store-and-forward switch nodes, and end stations (Tomasi,2004). The transmission links differ depending on thetransmission technology and the physical transmissionmedia over which the communications take place. Thestore-and-forward switch nodes provide end stations

with access to transmission links and other switch nodes.They allow data storage and transport of messages from

one node to another until they reach their destination

Figure 1: An example of a messagedatagram structure

2 MESSAGE SWITCHING

User Message(M bytes)

Header(H bytes)

Message Header Possible ContentsMessage Source

Message DestinationIntermediary Nodes VisitedMessage Priority

Message Datagram Size (in bytes) = M + H

Message Expiration Time

Message Arrival TimeMessage IdError Control (e.g., Checksum or CRC)

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nodes. The end stations can be individual computer ter-minals, servers, and mainframes. Many networks may beequipped with message gateways, which can convert onemessage protocol to another.

Store-and-Forward SwitchesIn general, a switch is a connectivity device that makesforwarding decisions. A store-and-forward switch per-forms forwarding functionalities similar to a switch in apacket-switched network. However, a store-and-forwardswitch forwards a message only if sufficient resources

are available and the next hop is accepting new data.Hence, the switching strategy in store-and-forward

switches is different from the common switching strategyused in packet-switched networks in which the incom-ing data is forwarded as soon as its destination address isdetermined.

Figure 3 shows the basic functional blocks of a genericstore-and-forward switch node (Shafritz, 1964). In thefollowing paragraphs we briefly outline the basic func-tionalities of each block.

The message received by a store-and-forward switchis first decoded and queued. When the input processingcontroller becomes available, it receives the message andanalyzes its header for its destination, associated id, andpriority level.

The input processing controller can also check for anyerrors in the message. Upon detection of any errors, itrequests message retransmission. Examples of messageerrors include transmission errors and invalid address-ing. Accepted messages with valid headers are storedin storage devices. Any stored message can be accessedor requested for retransmission using a specific id orserial number. After delivery, a copy of the message willbe stored for future retransmission. These messages maybe retained for many hours, days, or even weeks. Thelookup table maintains the forwarding information andkeeps track of which messages are waiting for delivery orhave already been delivered.

When the output phase begins, the output processingcontroller retrieves the proper message from the storagedevice and after assigning a header, passes it to the out-put buffer queue and line coder block for appropriate for-matting and transmission. The output buffer queue canbe used to control the message transmission rate.

The output processing controller is also responsible fordeciding how to forward the message to the next hop. Ingeneral, forwarding decisions used in message-switchednetworks are similar to routing schemes implemented inpacket-switched networks. For example, forwarding deci-sions can be based on the least congested link, or shortestend-to-end path (Tanenbaum, 2002). A number of routingtechniques in packet-switched networks are discussed inChapter 72.

Figure 3: A generic store-and-forwardswitch node architecture

MESSAGE-SWITCHED NETWORKS 3

Output BufferQueue

Primary/Secondary

Lookup Table

Statistics

OutputProcessingController

Input

ProcessingController

StorageDevices

Message in

LineCoder

TX

Line

DecoderRX

Message out

Input BufferQueue

Figure 2: A message-switched network

MainframeEmail Server

Terminal (2)Terminal (1)

SW A

SW C

SW B

SW D

Store-&-ForwardSwitch

Message

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The exact characteristics of message-switched networksvary depending on features supported by the store-and-forward switch nodes in the network. In the followingparagraphs, we elaborate on storage requirements andother features commonly provided by store-and-forwardswitch nodes.

The storage devices used in a store-and-forward switch

node have large storage capacities. This is because mes-sages delivered to each switch are often long and may bestored in the switch indefinitely until sufficient resourcesare available in order to forward them to the next node.In cases where a message has to be delivered to multi-ple destination nodes, multiple copies of the messagesmay be stored for a potentially long duration. A copyof the transmitted message may be stored for possibleretransmission until message delivery to the next hop is

confirmed. Moreover, the storage capacity must be largeenough to avoid any types of data overflow and there-fore data loss. The actual storage capacity of the switch isbased on the desired performance and can be estimatedthrough statistical analyses. Clearly, as the messagearrival rate increases, the amount of data stored in theswitch becomes greater, leading to larger storage require-ment. Larger storage capacity, on the other hand, leadsto requiring faster processors and more efficient searchalgorithms.

Each store-and-forward switch node is typically pro-

grammed to detect, report, and record any fault conditions,including presence of excessive noise, system tampering,and link failure. Switch nodes must also become awareof each others status. For example, in order to notify itsneighboring nodes that the node isalive, each node can pe-riodically send each neighboring node the serial number

of the last message it received from that node.A common capability of store-and-forward switches is

supporting message priority. Based on parameters suchas delay (or loss) tolerance or message length, every mes-sage can have a particular priority level. Messages withhigher priority must be treated first and forwarded inminimum time. Supporting message priority can addconsiderable complexity to message storage strategies,requiring separate listing for each priority group. Fur-thermore, the system must allow high-urgency messagesto interrupt others during their transmission. The inter-rupted messages will be automatically retransmitted assoon as possible, with no further action by the sender.A system that does not support message priority can

simply handle incoming messages on a first-in-first-outbasis.

A store-and-forward switch can also perform variousstatistical analysis, billing operations, and status reports.For example, it can examine each message for its numberof characters in order to charge the sending party. It mayalso analyze message headers to gather statistical infor-mation on various network links. The status report caninclude the status of facilities, error statistics, and mes-sage counts.

An important function of a store-and-forward switchis that it can intercept a message for various reasons. Forexample, if the destination node is temporarily unavaila-ble, the switch can store the message until the destination

is working again. The switch can also reroute the messageto other alternative recipients acquiring the same mes-sage. Message interception can also be considered forcases where the destination node has moved and it is nolonger in its previous location. In such cases, assumingthe system can locate the user, the message will be redi-rected to the new location.

A store-and-forward switch that can also act as amessage gateway is often called a transactional switch(Tomasi, 2004). In addition to storing messages and per-forming message routing, a transactional switch canchange the message format and bit rate and then convertit to an entirely different form. Hence, through messagereformatting, message switching can effectively allow fortranslating messages between dissimilar high-level appli-cations and protocols.

Message-based Protocols and theOSI ModelIn order to better understand functionalities of differentmessage-based protocols, in this section we conform theseprotocols to the seven-layer Open Systems Interconnec-tions/Reference Model (OSI/RM). Different layers of theOSI model are depicted in Figure 4. As we mentioned be-fore, message transmission and message switching over

the physical layer are performed one hop at a time. Eachintermediate store-and-forward switch node receivesthe entire message, stores it, and forwards it to the nexthop. A network using this technique is called a store-and-forward network.

Data-link layer protocols in message-switched net-works deal with the algorithms (e.g., error detection

and message flow regulation) for achieving reliable andefficient message passing between two neighboring store-and-forward switch nodes. Another important functionof such protocols is providing proper services to the net-work layer (Tanenbaum, 2002). Such services include thefollowing: (a) unacknowledged connection-less service(the sender sends the message randomly and expectsno response back from the receiver); (b) acknowledgedconnection-less service (the sender sends the messagerandomly and expects some kind of response back fromthe receiver); (c) connection-oriented service (prior tosending the message, a direct circuit must be establishedbetween the two adjacent nodes).

Depending on the data-link protocol, different retrans-mission mechanisms, if any, may be implemented. Asimple approach is to send an acknowledgement frameback to the sender. In this case, the sender can assumeproper reception of the message by the downstream nodeand remove its copy of the message. In some networksthe retransmission occurs when a negative acknowledg-ment or simply nothing is received.

The main functionality of the network layer in mes-sage switching is to provide routing algorithms, such assource routing and shortest-path routing. Based on therouting procedure, the intermediate store-and-forwardnode must determine the next hop and forward the mes-sage there. The network layer is also responsible for sup-porting message broadcasting and message multicasting.

4 MESSAGE SWITCHING

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In general, message-switched networks do not requirean official transport protocol. In such networks, each pairof interconnected store-and-forward switch node cannegotiate the use of any desired transport protocol (ornone at all), as long as the underlying communication isreliable.

Because in message switching the entire message is

delivered and stored at the next hop, message-based pro-

tocols can offer features beyond the transport layer. Theseprotocols are typically application-aware and hence, theycan operate in various transmission environments. Forexample, they are capable of tolerating temporary nodeunavailability or converting message format. Two com-mon examples of message-based protocols are X.400 andSMTP, which will be discussed in Store-and-ForwardElectronic Mail later in this chapter.

The store-and-forward technology can also be imple-mented in middleware. Middleware is a software thatprovides a translation mechanism, interconnectingapplication software across a network, such as a server/client network, and creating a single-system image. Onecategory of middleware that utilizes store-and-forwardtechnology and provides program-to-program commu-nications by message passing is called Message-OrientedMiddleware (MOM). We discuss MOM in Message-BasedClient/Server Systems later in this chapter.

Figure 4 shows that an incoming message into the

store-and-forward switch node can be entirely reformat-ted prior to its retransmission. In many systems a copyof each message will be stored until its reception by thenext hop is acknowledged. Message storage can be per-formed at different layers, including the network layeror application layer. Data-link and possibly other higherlayer protocols are responsible to provide message statusinformation. Note that middleware mediates between toplayers of the OSI reference model.

MESSAGE-SWITCHED NETWORKS ANDTHEIR APPLICATIONSThe basic concept of store-and-forward implemented in

message switching has been around since the early use ofsmoke signals and beacons. The postal delivery system inthe eighteenth century, which was not significantly differ-ent from the current postal system, also used store-and-

forward techniques. In this system a piece of mail (e.g.,a parcel) is dropped off at the local post office. The mailwill be picked up and taken to the next post office clos-est to the destination when the delivery service becomesavailable. Eventually, the mail is delivered to the recipientor stored in the destination post office, waiting for pickupby the recipient. We should note that in the postal deliv-ery system example, proper delivery to the next station istypically assumed.

In the late eighteenth century, semaphore networksbecame popular. These networks used visual signals(flags, lamps, heliographs) to convey messages betweenoperators in towers. Each tower, in turn, would relay themessage to the next until the message was delivered.

The store-and-forward method was also implemented

in telegraph message switching centers, commonlyknown as torn-tape centers (Davis, 1973). In these cent-ers operators tore message tapes from tape punches onincoming circuits and stored them in output basketsaccording to their outgoing address. Then, when the dis-patcher clerk became available, the messages were keyedin an appropriate circuit to the next center. Telegraphymessages were also called telegrams or cable grams.

The development of mechanical teleprinters, alsoknown as teletypewriters or teletypes, resulted in anincrease in the message carrying capacity of telegraphlines. Furthermore, by using the teleprinter terminalsthe operators were no longer required to know special

MESSAGE-SWITCHED NETWORKSANDTHEIR APPLICATIONS 5

Store & Forward Switch NodeConversion

MessageMessage

Source Destination

Physical Layer(e.g., Message Transmission and Message Switching)

Data Link Layer(e.g., Error Detection, Message Flow Regulation)

Network Layer(e.g., Message Routing)

Transport Layer

Session Layer

Presentation Layer

Application Layer(e.g., SMTP, X.400)

Middle

ware(e.g., MOM)

1

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1

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3

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Figure 4: Store-and-forward technology can be implemented at different layers of the OSI model

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codes (e.g., Morse code). The telex, short for teleprinterexchange, was introduced as a public service using tel-eprinters connected through automatic exchanges (telexswitching facilities). Using the telex network a subscribercan type out the telex number or code for the called party,wait for an acknowledgment from the called machine,and then transmit. The printed message is often stored at

the other end even if the called teleprinter is not staffed.Advancements in digital computers and digital stor-

age devices made radical improvements in messageswitching systems. During the 60s and 70s, the store-and-forward technology was largely promoted by WesternUnion, which traditionally was a message carrying or-ganization. Due to long distances between cities in theUnited States, Western Union primarily was focusing onstore-and-forward techniques to maximize line utilization

in order to offer telex and telegraph, as well as variety ofcomputer-based services.Today, although many major networks and systems

are packet-switched or circuit switched networks, theirdelivery processes can be based on message switching.For example, in most electronic mail systems the deliveryprocess is based on message switching, while the networkis in fact either circuit-switched or packet-switched.

Store-and-Forward NetworksMessage switching technology has been the forerunner oftodays advanced data communications. It set the trendsfor packet switching technology and more sophisticatednetworks of the twenty-first century. In fact, many of theearly data networks and communications services wereactually built upon the message-switched concept and

store-and-forward technology. In the remainder of thissection, we first discuss a few of such networks, whichare considered as major breakthroughs in developmentof current complex data communications networks.

Then, we examine some of the common communicationsprotocols that are designed based on store-and-forwardtechnology.

AUTODIN NetworkThe Automatic Digital Network (AUTODIN), developedin the mid-1960s, is one of the largest and most complexmessage switching systems. Using store-and-forwardswitching technology, AUTODIN provided a message-

based worldwide communication network for the U.S.military, handling the data communication needs for theDepartment of Defense and other Federal organizations(the alternative system to handle voice communicationswas AUTOVON) (FAS, 2003). AUTODIN has also beenused by other major agencies including the NSA (NationalSecurity Agency), the DIA (Defense Intelligence Agency),and NATO (North Atlantic Treaty Organization).

AUTODIN was originally considered as a dedicatedbackbone network serving primarily for secret data trans-mission. All switches and routers in the network were con-trolled by a system called the AUTODIN Switching Center.The system was designed to run at 2400 bits-per-second(bps); however, speeds up to 9600 bps were possible.

The header of each message sent by AUTODIN in-cluded source, destination, security classification, priority

statement, and a subject line. A very important aspect ofAUTODIN was providing highly secure communications.Hence, all external connections to the network fromtelephone lines were subject to strong encryption tech-niques.

For the past several decades, AUTODIN has beenregarded as a highly sophisticated message-switchednetwork. It also had major contributions in advancing

data communications. For example, in 1966 AUTODINwas one of the first systems that allowed electronic textmessages to be transferred between users on differentcomputers.

In 1982 an enhanced program, known as AUTODIN II,aiming to improve the system performance and security,was terminated in favor of using packet-based ARPANETtechnology. As of 2003, many AUTODIN switching sites

have been shut down. The intention is to replace the oldsystem with the Defense Message System (DMS). DMSis based on implementations of the OSI X.400 e-mail,which provides message services, through supportingmultimedia messaging, directory services, and differentgrades of services, to all Department of Defense users.

SITA and AFTN NetworksSocit Internationale de Tlcommunications Aro-nautiques (SITA) was originally established in 1949 by a

group of airlines. The purpose of the project was to pro-vide computer-based seat reservation service to facilitatethe sale of seats on airplanes and exchange of operationalinformation (Martin, 1985). SITA organized a commonglobal service supported by a low-speed message switch-ing network that was based on teletype message formatgenerated by teleprinters.

Aeronautical Fixed Telecommunication Network(AFTN) is also another example of a worldwide messageswitching network. After World War II, following SITAexperience, AFTN was introduced to support exchangeof messages between aeronautical fixed stations and air-crafts. AFTN is composed of aviation entities, includingaviation service providers, airport authorities, and govern-ment agencies. It exchanges vital information for aircraftoperations such as urgency messages, flight safety mes-sages, meteorological messages, flight regularity messages,and aeronautical administrative messages (Bush, 2003).

Today, the airline industry continues to use teletypemessages over SITA and AFTN as a medium for commu-

nicating via messages. These teletype messages aremachine-generated by automatic processes. The Inter-national Air Transport Association (IATA) is responsiblefor standardizing the message format throughout the air-line industry. In the last several decades, however, as thepacket switching has become the standard means of tele-communications, both SITA and AFTN networks have un-dergone major reengineering and their nodes have beenreplaced to support packet switching.

USENET NetworkUSENET (USEr NETwork) began in 1979 as a bulletinboard between two universities in North Carolina. It isbasically a public access network on the Internet andother TCP/IP-based networks that provides user newsand group e-mail.

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USENET provides a set of protocols for generating,storing, retrieving, and exchanging news articles amonga widely distributed user group. The format of news mes-sages and e-mail messages are structurally the same.Hence, USENET permits news messages to be carried thesame way as e-mail messages. The only difference isthe additional header fields indicating which newsgroups

(a repository of articles posted by the users) the messagebelongs to.

USENET is considered as one of the first peer-to-peerapplications. In this case thepeers are the servers and canbe accessed by the users. A major advantage of USENETis that it reduces network traffic and eliminates the needto store a copy of each article on every subscribers sys-tem. This is done by putting all articles in a central data-base and allowing users to access the database to get the

articles they need (Tanenbaum, 2002).Originally, USENET relied on a message exchangesystem called UUCP (UNIX-to-UNIX Copy Program).UUCP is aflood broadcast mechanism. Hosts send newsarticles they receive to other hosts, which in turn forwardthe news on to other hosts that they feed. Usually, a hostreceives duplicates of articles and must discard thoseduplicates. This can be a time-consuming process, result-ing in waste of bandwidth.

Today, message delivery in USENET is primarily han-dled by an Internet protocol called NNTP (network news

transport protocol). NNTP uses an interactive commandand response mechanism that lets hosts determine whicharticles are to be transmitted. A host acting as a clientcontacts a server host using NNTP, and then inquires ifany new newsgroups have been created on any of theserving host systems.

Performance-Challenged NetworksPerformance-challenged networks (or simply challengednetworks) are interconnected networks in which end-to-end latency, bandwidth asymmetry, and/or path stabilityare substantially worse compared to typical Internet en-

vironments. Such network environments are particularlycommon in space or ocean (acoustic underwater) com-munications, sensor networks, and military tacticalcommunications, all of which lack an infrastructure.

In challenged networks the signal propagation timeis comparatively long as a result of either long physi-cal separation of end nodes, as in space, or because of

slow transmission speed, as in water. Consequently, thetotal transmission time in such networks can take liter-ally hours or perhaps days. Protocols such as TCP/IP thatexpect acknowledgments may not be appropriate for suchenvironments. This is mainly because too many timeoutscan occur because of heavy congestions in the low capac-ity links. Another issue with challenged networks is pathinstability, which is generally the result of short node life-time or mobility. For example, in sensor networks manyof the sensors may run out of battery power after someperiod of time (Fall, 2003).

One approach to support interconnecting challengednetworks and their interoperability is called delay toler-ance networking (DTN), which is based on message switch-ing. The use of message switching for these networksallows all the small request/responses to be aggregated,

hence reducing the number of packets being routedthrough the network (Fall, 2002). Aggregated messagesare often stored, scheduled, and eventually forwardedtoward the destination node. The other benefit of imple-menting message switching is that it does not requirepre-established end-to-end state of the network. Usingstore-and-forward technology, each node discovers itsavailable neighboring nodes and forwards them the mes-

sages. Research in challenged networks is a relativelynew area and is still being investigated.

Message-Switched ProtocolsIn this subsection we look at some of the most commonprotocols that are based on store-and-forward technol-ogy. As a result of their vast applications, we only focus on

three specific systems: electronic mail systems, message-based client/server systems, and wireless messagingsystems.

Store-and-Forward Electronic MailWith the advancement in digital technology and digitaldata communications, sending electronic data, text mes-sages, documents, as well as voice and images, collectivelyknown as electronic mail (e-mail), became very popular.

The first e-mail systems simply consisted of file trans-fer protocols. Over the years, many new protocols weredeveloped for more ambitious e-mail systems. The major-ity of these protocols have been designed based on store-and-forward technology. In the following paragraphs, webriefly discuss two e-mail delivery protocols: X.400 andSMTP.

X.400. The main motivation in development of X.400was to create a common standard that would allow dif-ferent electronic mail systems to communicate together.X.400 (also referred to as message-oriented text inter-change systems [MOTIS]) is a set of standards developedby ITU-T (the International Telecommunications Union-Telecommunications, formerly known as CCITT), whichincludes protocols dealing with message handling sys-tems (MHS). These standards are designed to exchangemessages (e-mails) between different store-and-forwardservers and networks. In its original form, X.400 out-lines a set of basic message-handling characteristics thatestablish functionalities including the following:

Store-and-forward delivery of messages to multiplerecipients

Conversion of message content to allow message trans-fer between dissimilar sending and receiving devices(fax, telex, PC)

Delivery-time control

X.400 was first published in 1984 (Red Book) and a sub-stantially revised version, known as X.400/88, was pub-lished in 1988 (Blue Book). In 1992, new features andupdates were added (White Book) (Betanov, 1992). Al-though X.400 was originally designed to accommodateOSI model transport services, today it is typically runover TCP/IP.

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A network model defined by X.400 consists of anumber of basic elements including user agent (UA),message transfer agent (MTA), and message store (MS),as shown in Figure 5. The UA is a program that allowsthe user to compose, send, and receive mail. The MTAis the electronic post office, accepting the e-mail from useragents and making sure it is delivered. MS, which often

co-resides with MTA, maintains electronic mailboxesfor each user. When a message is delivered, it may passthrough multiple MTAs, each of which reads the messageaddress and passes it to another message transfer agent,until the message reaches its destination. As indicated byFigure 5, messages can be transmitted directly by MTA tothe UA recipient or, alternatively, they can be passed onto MS (the equivalent of a mailbox) which makes it possibleto store the message. Incoming messages can be stored

in these mailboxes until the user logs in. A collection ofall message transfer agents cooperating to relay the mes-sages until their destinations is called the message transfersystem (MTS).

Figure 5 also shows some of main protocols of X.400.These are P1, defining the communication between MTAs;P3, standardizing the connection between the user agentand MTA; P2 standardizing the (virtual) protocol betweenthe UAs; and P7, describing the interaction between theuser agent and message store (Tanenbaum, 2002).

At its early development, X.400 was widely implemen-

ted, especially in Europe. However, as e-mails becamemore popular, many of X.400 features, such as structuredaddressing and central control, made it more complexfor everyday usage. On the other hand, advances in theInternet-based e-mail protocols, in particular simple mailtransfer protocol (SMTP), made X.400 less popular. Iron-

ically, the same features which made X.400 less popular,made it attractive for special networks such as militaryand aviation. In fact, extended versions of X.400, suchas military message handling systems (MMHS) and avia-tion message handling systems (AMHS), are still underresearch and development. These protocols employ inte-grated security capabilities, including message routing,password management, and provisioning of public keyinfrastructure (PKI).

SMTP. Simple mail transfer protocol (SMTP) is con-sidered an application-level protocol, which runs overpacket-based TCP/IP. SMTP was developed by IETF(Internet Engineering Task Force) and has become thede facto standard for mail transfer on the Internet. Vir-tually all e-mail systems that send mail via the Internetuse SMTP to send their messages. SMTP is used for for-warding messages from one host to the other and writesthem to a message store (e.g., mbox or Maildir). How-ever, SMTP does not provide the functionality of allowingusers to retrieve mail. The post office protocol (POP) hasbeen developed for retrieving messages from the massagestore.

Although SMTP is the most prevalent of the e-mail

protocols, it lacks some of the rich features of X.400.A primary weakness of SMTP is the lack of support for

non-text messages. Multipurpose Internet mail exten-sions (MIME) has been introduced to supplement SMTPallowing the encapsulation of multimedia (non-text) mes-sages inside of a standard SMTP message (Parker, 2002).

X.400 and SMTP have similar features but also uniquefeatures in themselves. Generally speaking, X.400 is amore complex protocol and it makes it harder for usersto fake e-mail addresses and contents, compared with thesituation in SMTP. For a complete discussion on SMTPprotocol, please refer to Chapter 88.

Message-Based Client/Server SystemsMany client/server systems use middleware to exchangemessages. Middleware is used to simplify the complex-ity of dealing with communication protocols and lay-ers of software. Broadly speaking, middleware softwareis defined as the glue between software components orbetween software and the network or it is the slash inclient/server (MRC, 2004). One category of middlewarethat provides program-to-program communication bymessage passing is called message-oriented middleware(MOM). MOM is based on message queuing and store-and-forward technology. MOM is a non-blocking asyn-chronous form of communication. Hence, every node inthe system can randomly send message to the queue or

Figure 5: A general network model defined byX.400

8 MESSAGE SWITCHING

UA

MS

MTA

Sender Receiver

P7

P3

P1 P1

P2

MTA

UA

MS

MTA

P7

P3

Presentation Layer

Session Layer

Transport Layer

Network Layer

Data Link Layer

Physical Layer

MTS

User Agent (UA)

Message Transfer

Agent (MTA)

Message Store (MS)

Message Transfer

System (MTS)

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taking message from queues. Note that the queue can bea part of the client or a dedicated server. Message storageand queuing can be persistent (logged on disk) or non-persistent (in memory). Persistent messages are slowerbut they can be recovered in case of power failures.

The message flow in MOM messaging products canhave different types, including send-and-pray (no res-

ponse about the message is required), send-and-nay (aresponse is required if there is something wrong with themessage), and send-and-say (response is mandatory).

Store-and-Forward Wireless MessagingToday, many wireless applications and services are basedon store-and-forward technology in order to reliablytransfer messages between end-users. A popular example

is short message service (SMS). Although, SMS is beingsupported by many digital-based mobile communicationssystems, it was introduced as a datagram service in theGSM (global system for mobile communications) system.SMS is a text message service that enables short text mes-sages (up to 160 characters in length) to be transmittedand received by a cell phone. Similar to e-mail, short mes-sages are stored and forwarded at SMS centers, whichmeans messages can be retrieved later if the user is notimmediately available (MWR, 2004). SMS messages travelto the cell phone over out-of-band control channels, sepa-rated from voice channels. SMS has been broadly used

in Europe and Asia for many years. In North America,SMS was made available initially on digital wireless net-works built by early pioneers such as BellSouth Mobility,PrimeCo, and Nextel, among others.

Paging systems capable of storing and delivering mes-

sages (or pages) are also examples of store-and-forwardwireless systems. A wireless pager receiver can be acti-

vated and alert its user via a tone or a vibrator. This istypically called a one-way paging system. Pager activa-tion can be done using a telephone or a PC. In store-and-forward based paging systems, if the network cannotreach the user to deliver the message, it stores the mes-sage until the user can be reached (Taylor, 1996). Somepaging systems have SMS capability and allow displayingsmall alphanumeric messages. Two-way paging systemsinclude an acknowledgement feature that allows the userto acknowledge the receipt of the message.

Wireless sensor networks (WSN) also use store-and-forward technology. WSN are mesh networks of smallsensor nodes communicating among themselves using

RF communication. Sensor nodes can be deployed inlarge scale (from tens to thousands) to sense the physicalworld, for example, monitoring, tracking, and control-ling. The data messages gathered by each sensor nodewill be periodically exchanged between nodes or passedon to intermediate nodes. Intermediate nodes can aggre-gate small data messages together prior to retransmittingthem to the next node (Zhao, 2005).

Store-and-forward technology has also been consid-ered to be implemented in satellite communications sys-tems in order to ensure higher reliability and networkrobustness. For example, message switching is recentlybeing investigated for inter-satellite networks, wheremessages are transmitted between two communication

satellites, as opposed to communications between a sin-gle satellite and Earth stations (Charles, 1999). Messageswitching is also being considered for interplanetary net-works, a high-bandwidth infrastructure designed to linkEarths Internet to other parts of our solar system.

Research suggests that store-and-forward technol-ogy can also enhance the wireless range of low-costshort-range technologies, such as WiFi (IEEE 802.11).

One approach is simply relaying the data betweenmultiple wireless devices over long distance. A more cost-effective approach is to transmit data over short point-to-point links between mobile storage devices calledmobile access points (MAP). Through the use of low-cost WiFi radio transceivers, the data carried by theMAP is automatically and wirelessly transferred at high-bandwidth for each point-to-point connection (Pent-

land, 2002). The motivation in utilizing this approach isproviding a very low-cost and affordable solution toresolve the last mile connectivity problem, particularly inpoor rural areas.

PERFORMANCE OFMESSAGE-SWITCHED NETWORKSIn this section we examine the performance of message-switched networks in terms of delay and peak traffic load.

Delay PerformanceFigure 6 depicts the transmission of a message acrossfour nodes. Three types of delays can be identified on thediagram: (a) propagation delaythe time it takes a sig-nal to propagate from one node to the next; (b) transmis-

sion delaythe time it takes for a transmitter to send outa message (e.g., it takes 0.1 sec. to transmit a 10,000 bitmessage onto a 100 kbps line); (c) node delaythe time ittakes for a node to perform the necessary processing as itswitches data plus the message storage time. Hence, thetotal delay will be the sum of all propagation and trans-

mission delays at each node plus the sum of node delaysat each switch node (Rosner, 1982).

As Figure 6 indicates, prior to retransmission, the en-tire message must be received at each switch node. Thisresults in a large end-to-end delay, particularly if the mes-sage size is very large. In fact, under heavy load, whenthe node delay is not negligible, the total delay using mes-sage switching can be significantly longer than for circuitswitching, which requires time to setup and tear-down aconnection.

Node DelayAn important factor impacting node delay is unavailabil-ity of the next hop or the destination node. The store-and-forward switches in a message-switched network avoidrepeated attempts to send messages to the next hop.Hence, if the next node is not available, the messageretransmission is simply delayed. Clearly, as the totalnumber of messages in the network increases, the averagenode delay through the network will become longeruntil no storage resources are available and message drop-ping occurs. This is illustrated in Figure 7a.

PERFORMANCEOF MESSAGE-SWITCHED NETWORKS 9

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Another important factor affecting the node delay ina store-and-forward switch is network congestion. Thatis the case where switches cannot deliver as fast as mes-sages are arriving. When congestion occurs, the systemcontinues to accept new messages, as long as its storagecapacity allows. In message switching new messages willbe stored until free resources are available and the deliv-ery is ensured; hence, no message blocking occurs as longas sufficient storage capacity is available. As the loadof the network increases, more messages will have towait for available processing resources and free links. The

result is an increase in the message delivery time throughthe network.

Another way of looking at a network with store-and-forward switches is to regard it as a large distributed setof storage facilities, which gradually fill as the load on thenetwork increases. Consequently, the peak load is shiftedin time by an amount depending on the available storage(Davis, 1973). Figure 7b illustrates this point by showinghow a peak load is reduced, but at the expense of beingdelayed. The area under each of the two curves repre-sents the total number of messages delivered. Clearly, thisarea will be the same for both curves.

Figure 6: Event timing diagram in message-switched networks (the example assumes all the link speeds and lengths are the same;node delays are assumed to be negligible)

10 MESSAGE SWITCHING

Given:

L (Link length) = 10Km

M (Message size) = 1M Byte

H (Header size) = 32Byte

C (Link rate) = 320kbps

S (Prop. speed) = 200Km/s

Dn(i) (Node (i) delay) = 0s

Calculated:

Dp (Prop. delay)=

L/S=

0.05

sD

t(Trans. delay=(M+H)/C=25.001s

DT

(Total delay)=

3(Dt+Dp)+iDn(i)=75.153s

Terminal (2)Terminal (1) SW A SW B

Message

Message

Message

Time

Dt Dp

Dn

(B)

DT

Dn

10Km;

320kbps

An Example:

(A)

Figure 7: (a) Delay in message-switched networks as the load increases; (b) traffic characteristic as a result of store-and-forwardswitches

Network Load

MessageDelay Throughthe Network

Total availablenetwork resources

NetworkLoad

Time

StorageDelay

(A) (B)

Peak

Load

Peak Load Spread

due to Storage

Total available

network resources

1 2

1- Traffic load into the network2- Traffic load delivered through the network

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Comparison of Message, Packet, andCircuit SwitchingMessage-switched networks provide a number of advan-tages over circuit-switched networks. For example,message-switched networks provide more efficient use ofnetwork resources. This is a result of their inherent store-and-forward capability. As new information enters thenetwork for future delivery, the message storage proc-ess creates a reservoir of traffic. This reservoir tends toremove the gaps between new message arrivals ontothe network. In addition, it smoothes the peak network

load, leading to some degree of traffic regulation over thenetwork.

When traffic becomes heavy on a circuit-switchednetwork, some connection requests are blocked; that is,the network refuses to accept any additional connection

requests until the load on the network decreases. In amessage-switched network, messages are still acceptedas long as the storage capacity is available but deliverydelay increases. As a result, message-switched networkstranslate potential message blocking into potential de-lay. As long as the total end-to-end message delay meetsthe user delay tolerance, message delaying can be farmore convenient than message retransmission due toblocking.

Another attractive feature of message-switched net-works is that in these networks each intermediary nodecan perform protocol translation. Since each intermedi-ary node processes the entire message, it can perform

any required line code, transmission speed, or protocolconversions between incompatible nodes. Therefore,message switching can be a potential solution for less ro-bust communication environments. In circuit switching,protocol conversion is only possible through protocolgateways.

In message-switched networks, unlike circuit switch-

ing, simultaneous availability of sender and receiveris not required and the network can store the messagepending the availability of the receiver. Moreover, featuressuch as broadcasting, data auditing, and data reroutingcan be supported by message-switched networks muchmore easily and typically require no major architecturalmodifications.

The difference between message switching and packetswitching begins with the characteristics of messages and

packets themselves. Messages are units of informationwith unrestricted lengths, whereas packets have maximumlength size. Furthermore, in a message-switched networkthe network is responsible for the message and typicallyno responses or feedbacks exist. Consequently, in messageswitching, rather than minimizing transit time of the mes-sage, its guaranteed delivery is emphasized. On the otherhand, packet-switched networks aim to deliver packetswith minimum delay and do not hold packets for delayeddelivery. Table 1 compares the main characteristics ofmessage, packet, and circuit-switching technologies.

Figure 8 illustrates the transmission of data across fournodes using circuit-switched and packet-switched net-works. An important characteristic of circuit switching

Table 1: Comparison of Message, Packet, and Circuit Switching (Stallings, 2004; Martin, 1985)

Circuit-Switched Networks Message-Switched Networks Packet-Switched Networks

Communication is performed through No dedicated path exists. No dedicated path exists.a dedicated path.

Provides real-time or continuous Too slow for real-time or Provides near real-time data transmission.transmission of data. interactive data transmission.

No data storing is required. Messages are stored for later Packets are queued for delivery; they areretrieval. not stored.

The switch path is established for The route is established for The route is established for each packet.the entire connection time. each message.

For small messages, the data The message delivery time The packet delivery time is very short.transmission time is negligible can be substantially long.compared to the time required tosetup and tear-down the connection.

The connection is blocked if the No message blocking can Packet blocking can occur, however, theend-user is busy or not available. occur as long as the storage blocked packets will be retransmitted toOnce the connection starts, no capacity is sufficiently large. the end-user.blocking may occur.

As the network load increases, As the load increases, As the load increases, packets on averagemore blocking can occur. messages on average experience longer queuing delay, although

experience longer delivery still very short compared to messagedelay. switching.

The length of transmission Messages have no theoretical Packets have a maximum length.is unlimited. maximum length and

can be very long.

PERFORMANCEOF MESSAGE-SWITCHED NETWORKS 11

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is the need to setup an end-to-end path. During this timeinterval (setup time), depicted in Figure 8a, the system isensuring path availability.

In packet switching, shown in Figure 8b, no physicalpath is established in advance between sender and re-ceiver. Hence, unlike message switching (Figure 6), eachnode along the route may begin transmission of a packetas soon as it arrives.

CONCLUSIONIn this chapter we examined the basic concepts of message-switched networks. We surveyed their advantages and dis-advantages and demonstrated their applications. We alsodescribed different elements of a message-switched net-work and examined their basic functionalities. Althoughpacket switching is becoming the dominant switchingscheme, message switching is still considered for manydedicated networks.

GLOSSARYARPANET: The Advanced Research Project Agency

Network created by the U.S. Department of Defense.This organization is credited with creating the Inter-net in 1969.

CCITT: Short for Comit Consultatif International Tl-phonique et Tlgraphique, an organization based inGeneva, Switzerland, that sets international commu-

nications standards. CCITT changed its name to Inter-national Telecommunications Union (ITU), the parentorganization, on March 1, 1993.

Datagram: A sequence of bytes that constitutes the unitof transmission in the network layer.

Exchange: A room or building equipped with manualor automatic switching equipments that can intercon-nect incoming communication lines as required.

IETF: Internet Engineering Task Force is a large, openinternational community of network designers, opera-tors, vendors, and researchers, which sets standardsfor the Internet. It is also responsible for much of thework done on TCP/IP.

Peer-to-Peer Architecture (P2P): It is a type of networkarchitecture in which each workstation has equivalentcapabilities and responsibilities.

Protocol: A formal and pre-agreed set of rules thatgovern the communications between two or more

entities. The protocol determines the meaning ofspecific values occurring in specific positions in thestream, the type of error checking to be used, the datacompression method, how the sender will indicatethat it has finished sending a message, and how thereceiver will indicate that it has received a message.

Teleprinter: A typewriter-like terminal with a keyboardand built-in printer. It was used to communicate typedmessages between terminals through a simple point-to-point electrical communication channel, often justa pair of wire. A teleprinter terminal is also calledteletype or TTY machine.

Telegram: A message transmitted by telegraph system.

REFERENCESStallings, William. 2004. Data and Computer Communi-

cations, 7th ed. Upper Saddle River, NJ: Prentice Hall.Tomasi, Wayne. 2004.Introduction to Data Communica-

tions and Networking, 1st ed.. Upper Saddle River, NJ:Prentice Hall.

Davis, Donald Watts and Derek Leslie Barber. 1973. Com-munication Networks for Computers. New York: JohnWiley & Sons.

Tanenbaum, Andrew S. 2002. Computer Networks, 4thed. New York: Prentice Hall.

Figure 8: Event timing diagram in (a) a circuit-switched network and (b) a packet-switched network; we assume no loss of dataoccurs

12 MESSAGE SWITCHING

Terminal (2) SW A SW B

Time

Dp

DT

Terminal (1) SW A SW B

Time

Dt

Dp

Terminal (2)Terminal (1)

Packet

Packet

Packet

Packet

Packet

Packet

Packet

Packet

Packet

Dn

Dn

DT

(A) (B)

Message

RequestSignal

AcceptSignal

Terminate

Signal

SetupTime

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Shafritz, Arnold B. 1964. The use of computers in mes-sage switching networks. Proceedings of the 1964 19th

ACM national conference pp. 142.301142.306.Federation of American Scientists. 2003. Automatic

Digital Network (AUTODIN). http://www.fas.org (ac-cessed March 1, 2006).

Martin, James. 1985. Telecommunications and the Com-puter. Englewood Cliffs, NJ: Prentice Hall.

Bush, David. 2003. The AFTN Message Switch CaseStudy. http://www.ascolto.co.uk/Resources/ (accessedFebruary 1, 2006).

Fall, Kevin. 2003. A delay tolerant networking architec-ture for challenged Internets. Proc. SIGCOMM 2003.August 2529, 2003. Pages: 2734.

Fall, Kevin. 2002. A message-switched architecture forchallenged Internets. http://www.intel-research.net/

Publications/Berkeley/120520021026_66.pdf (accessedDecember 1, 2006).Middleware Resource Center. 2004. http://www.

middleware.org/ (accessed December 1, 2006).Betanov, Cemil. 1992. Introduction to X.400. Norwood,

MA: Artech House.Parker, Tim, and Karanjit B. Siyan. 2002. TCP/IP Unleashed

(Unleashed), 3rd ed. Indianapolis, IN: Sams.Taylor, Mark, Mohsen Banan, and William Waung. 1996.

Internetwork Mobility: The CDPD Approach. Upper

Saddle River, NJ: Prentice Hall.Zhao, Feng, and Leonidas Guibas. 2005. Wireless Sensor

Networks: An Information Processing Approach. SanFrancisco, CA: Morgan Kaufmann.

Charles, John. 1999. Interplanetary Network Aims for theStars.IEEE Computer Society, 32(9):168, 21.

Pentland, Alex (Sandy), Richard Fletcher, and Amir A.

Hasson. 2002. A Road to Universal Broadband Con-nectivity. http://www.itu.int/council/wsis/080_Annex4.pdf (accessed February 1, 2006).

Rosner, Roy, D. 1981. Packet Switching. Belmont, CA:Lifetime Learning Pub.

MobileIN Web Resources. 2004. http://www.mobilein.com /sms.htm (accessed January 1, 2007).

FURTHER READINGMany of the original publications on message switch-ing and store-and-forward technology belong to early

1960s and 1970s. Several comprehensive discussions onthese topics are found in Davis (1973) and Martin (1985).A basic analytical model for a message-switched networkin terms of data loss and delay is provided by Rosner(1982). Stallings (2004), Tomasi (2004), and Martin (1985)offer a list of advantages and disadvantages of messageswitching and compare the performance between mes-

sage, packet, and circuit switching. One of the firstdetailed tutorials on store-and-forward switch node func-tionalities can be found in Shafritz (1964).

Routing techniques in message-switched and packet-switched networks are very similar. Various routing tech-niques are described in Tanenbaum (2002).

Examples of message-switched networks, includingSITA and AUTODIN, along with detailed discussions oftheir technical specifications and evolutions, are provided

in Martin (1985). FAS (2003) is specifically dedicated tothe history of AUTODIN. Bush (2003) offers backgroundinformation on AFTN. Introductory information aboutUSENET, including its commands and protocols, can befound in Tanenbaum (2002). Betanov (1992) and Parker(2002) offer a comprehensive discussion on X.400 andSMTP protocols and their differences. More informa-tion about middleware and MOM can be found in MRC(2004).

Recent investigation on implementing message switch-ing in challenged networks and satellite communications

can be found in Fall (2003), Fall (2002), and Charles(1999). More information regarding the paging systemsand wireless sensor networks can be found in Taylor(1996) and Zhao (2005), respectively. A number of booksabout short message services are listed in MWR (2004).Moreover, an interesting work demonstrating how store-

and-forward technology can be used to provide a low-cost solution to resolve the last mile continuity problemin rural areas can be found in Pentland (2002).Message-Switched Networks

FURTHER READING 13

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ch70 message swiching

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