Typical fiber-based backbone networks support the transport of a tremendous amount of bandwidth. Free space optical technology is quickly deployed and is based on standardized protocols, compatible with fiber-based networks. Modern wireless technology and protocols allow for multiservices, end-user flexibility and for mobility in the access regime. In this paper we describe the seamless protocol integration of these three technologies and their protocols.

In this paper, we propose an integrated Mobile WiMAX and free space optics (FSO) broadband access network where the FSO network is used to provide high capacity backhaul for the Mobile WiMAX front end. The integrated network extends the coverage of Mobile WiMAX networks to larger areas in applications where the fiber network is expensive and difficult to deploy. We present a unified EAP-based security framework, and we evaluate and compare the performance of EAP-TLS and EAP-TTLS for this integrated FSO-WiMAX access network. Furthermore, we evaluate the impact of the point-to-point FSO link in the integrated access network. Our measurement shows that compared to EAP-TLS, EAP-TTLS provides a more flexible, efficient, and secure way to protect the integrated FSO-WiMAX access network. Our experiment also demonstrates that the point-to-point FSO link does not degrade the performance of EAP authentication in the integrated network.

Abstract Passive optical networks (PONs) offer a popular broadband access solution allowing for high bandwidth and long transmission range to meet user fast evolving needs. However, in certain applications, PON deployment cannot reach the end user, because geographic restrictions make fiber installation non‐cost effective. Recently, hybrid fiber–wireless (FiWi) access networks have been proposed to combine the advantages of optical and wireless technologies and integrate them seamlessly. However, this integrated wired and wireless access network raises new security issues that need to be addressed. This paper reviews the architectures of currently proposed integrated access networks, proposes a new Worldwide Interoperability for Microwave Access (WiMAX) over Ethernet PON (EPON) architecture which achieves simplified and efficient system management. We present a unified security framework for the proposed architecture using Public Key Infrastructure (PKI). Through our analysis, we show that this security framework enhances the system security and realizes unified key management. Copyright © 2010 John Wiley & Sons, Ltd.

Mesh FSO networks combined with WDM technology, carry an aggregate data rate of many Gbps. However, a large percentage of the enterprise data in the traffic is sensitive; thus, FSO links become a formidable target for eavesdropping. In order to maintain the proper level of data integrity and confidentiality, the vulnerabilities of mesh FSO communication links need to be understood and to develop an effective link security strategy and a security mechanism, which in addition to end-to-end cryptography assure intrusion-free operation and data security. In this paper we examine the security vulnerabilities of WDM mesh FSO links and describe how the link security using dynamically reconfigurable chaotic functions is enhanced.

Security has become a primary concern in order to provide protected communication in mobile networks. Unlike the wired networks, the unique characteristics of mobile networks pose a number of nontrivial challenges to security design, such as open peer-to-peer network architecture, shared wireless medium, stringent resource constraints, highly dynamic network topology and absence of a trusted infrastructure. Ubiquitous roaming impacts on a radio access system by requiring that it supports handover between neighbouring cells and different networks. Also, mobile networks are more exposed to interferences than wired networks. There are several components that contribute to this: adjacent channels, co channels, Doppler shifts, multipath and fading. This SI aims to identify and explore the different issues and challenges related to security aspects in mobile networks. It is dedicated to present the state-of-the-art research results and findings on Security in Mobile and Wireless networks. We accepted 12 papers out of a total of 31 submissions from all over the world. Each paper was reviewed by at least three reviewers and has undergone through two cycles of revisions. The first paper introduces the notion of malnets: networks of adversary-controlled wireless routers targeted to a physical geography. Similar to Internet worms such as Slammer and Code-Red, malnets are created by the recursive compromise of targeted devices. However, unlike their traditionally wired counterparts, malnet worms exploit only other routers that are within their transmission range. The malnet creates a parallel wireless infrastructure that is (a) completely under control of the adversary, and (b) spans a targeted physical area, creating a valuable infrastructure for a variety of virtual and physical attacks. A study of the propagation characteristics of commercial routers and model inter-router connectivity using publicly available war-driving data is given. The resulting characterization is applied to well-known epidemiological models to explore the success rates and speeds of malnet creation across cities such as New York, Atlanta and Los Angles. Finally, a sampling of available exploits is used to demonstrate the construction of multi-vector, multi-platform worms capable of targeting wireless routers. Wireless vehicular communications pose significant challenges for the deployment of next generation roadside services. Some important issues that must be tackled are security, billing and reliability while guarantying a scalable service delivery. The second paper addresses the assignation of secure service session parameters upon the reception of on-demand service requests by an incumbent services district domain, and studies and analyses the performance of the underlying mechanisms. Three types of service request protocols are introduced: single-hop, extended connectivity and multi-hop service requests. A detailed analytical model and cost study for the access protocols are presented. The analysis study covers the estimation of total cost in terms of latency for each access protocol with different mobility characteristics and vehicle densities within the service coverage area and across different serving district domains. The third paper noted that providing secure routing in mobile ad hoc networks (MANETs) is far more difficult than establishing secure routing in wired networks or static wireless networks, because node mobility and the relative scarcity of bandwidth render prior solutions ineffective. Further solutions based on securing link or path information do not work well in MANETs because the dynamic nature of links requires extensive use of flooding to establish effective countermeasures. The authors argue in their paper that secure routing in MANETs must be based on the end-to-end verification of physical-path characteristics aided by the exploitation of path diversity to increase the probability of finding secure paths. They hence applied this approach to the design of the Secure Routing through Diversity and Verification (SRDV) protocol and proved that the countermeasures used in SRDV can defend against a variety of known attacks to routing protocols, including attacks involving collusion, and the fabrication and modification of routing packets. The fourth paper proposes a new architecture based on an efficient trust model and Secure Distributed Clustering Algorithm (SDCA) in order to distribute a certification authority (CA) for ensuring the distribution of certificates in each cluster. They use a combination of a fully self-organized security for trust models like PGP adapted to adhoc technology and the clustering algorithm with respect to trust and mobility metrics. Furthermore, they present a new approach: the Dynamic Demilitarized Zone (DDMZ) to protect the CA in each cluster. The principal idea of DDMZ consists in selecting the dispensable nodes, also called registration authorities (RA); these nodes must be confident and located at one-hope from the CA. Their roles are to receive, filter and treat the requests from any unknown node to the CA. This approach allows to avoid the single point of failure in each cluster. Moreover, they propose a probabilistic model to define the direct connectivity between confident nodes in order to study the resistance degree of the DDMZ against different attacks. The performance evaluation of the proposed SDCA shows the robustness and the availability of DDMZ. The effects of direct connectivity and transmission range on the stability and security of the network are analysed. In the fifth paper, authors observe that, in MANETs, there is an increasing interest in providing anonymity for witnesses, i.e. those users who share their knowledge in detecting either malicious or selfish users. As well, it is a challenging problem to prevent the misuse of anonymous sources. They proposed the PlainClothesMan (PCM) protocol to provide anonymity for the witness who helps identify malicious or selfish users. Once there are more than a certain number of claims from distinct users against the same user, she is identified as a malicious or selfish user. Moreover, in PCM, the misuse of the witness anonymity is prevented in such a way that malicious users who broadcast the same invalid claims repeatedly can be identified. Two exemplary scenarios are designed and simulated to model the necessities of witness anonymity in MANETs. Simulation results show that, witness anonymity is very important for ensuring proper and efficient executions of fundamental functionalities of MANETs, e.g. certificate revocation and fairness, and PCM is both effective and efficient in providing such a type of anonymity The sixth paper provides an overview of Internet security to meet the IP Multimedia Subsystem (IMS). The IMS, a platform designed for providing various multimedia services, requires high availability and trustability. However, the IMS specifications do not explicitly introduce the protection mechanisms. By focusing on the security issues of IMS, the seventh paper discusses and surveys the potential security threats on IMS. Those security threats include: (a) routing attacks and DNS attacks that result from vulnerabilities of IP technology, and (b) Denial of Service (DoS) attacks and the privacy control threats that are IMS-specific. (c) Spam threats such as Spam over Internet Telephony (SPIT), Spam over Instant Messaging (SPIM) and presence spam. On the other hand, this paper also reviews the existing work, available countermeasures and tools for preventing these threats from harm. The seventh article addresses schemes to provide key recovery capability for mobile communications. In recent year, wireless communication network has been the most convenient and powerful way for all computer users to access the resources from the Internet. However, the lack of suitable key recovery mechanism that meets the special security and performance requirements for wireless communication networks is still an open issue. By addressing this issue, the eighth paper proposes a novel security mechanism with key recovery capability satisfying the requirement of privacy protection and monitoring while maintaining low computational complexity. The scheme provides a ‘backdoor’ for an authorized agency to monitor suspected communications while protecting legal users from unauthorized disclosure of their data privacy. With the security and performance analysis of the proposed protocol, the author shows that the scheme is suitable for deploying in wireless communication network. The eighth paper introduces the analysis of performance overheads for securing the signalling plane in beyond 3G networks. Although the countermeasures are urgent for mobile network such as IMS, the cost should also be considered. For signalling plane, since no built-in protection mechanism was defined for SIP, 3GPP technical specifications introduce IPSec to protect SIP messages. However, the additional protection can lead to significant computational and spatial overheads both in the core network and in the terminals, which might affect their performance. By focusing on the possible performance overhead from protecting SIP by additional IPSec methods, the ninth paper presents and validates the analysis of potential performance overheads brought by security mechanisms developed to protect signalling messages in beyond-3G core networks and mobile terminals. According to the analysis result, the overheads for both cases can significantly impact the performance of the entire architecture. The ninth article addresses the impact of key assignment on VANET privacy. The privacy issue is also an important topic for VANET. To know the best way of key assignment for maintaining user privacy from (1) the CA or (2) non-CA entities, the tenth paper gives the analysis of the privacy provided by three key assignment approaches for VANETs: (1) all vehicles share (a) a copy of the same key and (b) a copy of multiple same keys, (2) each unique key is held by only one vehicle and (3) each key is held by several vehicles. According to the mathematical and logical analysis result, the authors found that ‘keys should be unique to vehicles and vehicles should be given multiple keys.’ Besides, the author also verified that ‘it is impossible to provide good privacy and fast revocation when keys are shared among vehicles.’ The tenth paper proposes a Dirichlet reputation system in reliable routing of wireless ad hoc network. In peer-based network such as wireless ad hoc networks, the reliable routing is most important for maintaining the normal operation and good throughput of the whole network. However, the misbehaviour of some nodes will impact the routing reliability. To predict and select a reliable path for each nodes of wireless ad hoc network, the 11th paper proposes Dirichlet reputation model based on Bayesian inference theory that evaluates reliability of each node in terms of packet delivery. To adjust the responsiveness of the reputation system, the moving window mechanism is applied to change the speed of updating nodes' reputation: the larger size implies the fast misbehaviour detection and the smaller size slows the speed of redeeming bad reputation. According to the simulation result, the negative impacts of misbehaving nodes can be reduced and the throughput can be improved by integrating the proposed reputation system into the routing protocol of wireless ad hoc network. The 11th paper suggests an efficient reputation based hybrid key management architecture for ad hoc networks. In fact, it is difficult to deploy the common key management approaches into the wireless ad hoc network. To address this issue, the twelfth paper proposes adaptable hybrid public key management architecture for wireless ad hoc networks based on a clustering algorithm—RECA. The reputation system is applied to compute the trust levels information. All trustworthy cluster-heads that managed clusters are elected based on the trust levels. In addition, each cluster-head is a CA for the members of the cluster and the key management among all the cluster-heads is done by applying totally distributed CA. According to the simulation result, the enhancement of scalability and certification services availability can be achieved. The last article offers a new cluster based algorithm for Black Hole Intrusion Detection System (BHIDS). This paper focused on intrusion detection on wireless ad hoc network. Only applying the traditional firewall or cryptography is not enough to protect the wireless ad hoc network from active attack. This paper addresses this issue and proposes a 2-layered cluster-based BHIDS for mobile, ad hoc networks. By splitting the nodes in the network into 2-layered clusters, the processing and communication overhead between the cluster heads at layer 1 and 2 are reduced the cluster heads of each layer can cooperate to detect the potential attacks. The features of monitoring include: mobility of nodes, variation in number of attacking nodes, packet delivery rate and density of the network. The simulation result shows marked improvement on packet dropping compared with the other work. Furthermore, the authors found that the stability of routes in sparse network is more than dense network. We would like to thank the contributors as well the reviewers towards the success of this special issue. We specially thank Professor Hsiao-Hwa Chen, Editor-in-Chief, Wiley's Security and Communication Networks Journal, for his advice and help during the process of putting together this Special Issue. We hope that you will enjoy reading this Special Issue.

As the number of communication security violations and the sophistication of intruders and bad actors increases, complex processes such as chaos and quantum theory become applicable to cryptographic systems and to network security. In this paper we demystify chaos and chaotic processes with simple examples; we provide examples of using chaos in cryptographic systems, and we identify synchronization issues in modern communication networks. We then describe a pragmatic method that resolves the transmitter-receiver synchronization issue and also enhances chaotic cryptography.

The increased demand of data services within the last few years has prompted a commensurate explosion in transportable bandwidth, most of which contain sensitive information, personal data, bank accounts, credit card numbers, proprietary documents, and more. As a result, there are two types of security in communications, securing user data and securing the physical network. The first is addressed with encryption algorithms, the unbreakability of which does not depends so much on the sophistication of the algorithm but on the difficulty to break it using a sequential computer; that is, it capitalizes on the inability of the attacker to use a fast supercomputer. The second is not a well addressed topic, particularly in modern fiber-optic networks. One attempt to create an unbreakable cipher-text is quantum cryptography that emanated from quantum mechanics; this method capitalizes on the no-cloning principle of particular characteristics of photons, such as polarization. In this paper, we examine the security of advanced optical networks, fiber-optic and free space optical (FSO). We describe quantum cryptography and the quantum key distribution, including the principle of teleportation, we identify vulnerabilities and we discuss detection mechanisms and countermeasure strategies against attacks on the physical network.

Abstract This paper presents three new algorithms over a dual quantum channel to enhance the convergence speed of the key distribution process. The proposed algorithms effectively use a dual quantum channel between the communicating entities and, when combined with the proposed selection of the basis, improve the efficiency of the key distribution process as well as deter Eve. With the use of an initialization vector (IV), the proposed algorithm does not necessitate information exchange about the bases between Alice and Bob, thus removing the requirement of the availability of a two‐way conventional channel. Simultaneously, it allows the detection of the presence of Eve in real time. Copyright © 2008 John Wiley & Sons, Ltd.

Abstract Modern optical networks transport aggregate traffic that exceeds 1 Tbps per single fiber. Thus, the next generation backbone and access optical network will be able to transport to/from residential and enterprise customers multi‐protocol diverse payloads, such as voice, video, music, text, interactive games, surveillance, business transactions, and more at broadband rates (several Mbps) thus solving the first/last mile. Because of the diverse payload to the enterprise and to the residential customers, an opportunity is presented for malicious attacks to the network in order to copy, destroy or alter data, mimic a source, and in general to engage in unauthorized activities that jeopardize the optical communications security at the information, the node, and the physical layer. To combat this, strong cipher methods and advanced key distribution methods have been developed, in addition to intruder detection and localization, source authentication, channel signature identification, and also countermeasure strategies. Because some cryptographic methods have been found to have vulnerabilities, new methods based on quantum mechanical principles have been deployed promising the [holy grail] of cryptography and of network security. The first protocol in quantum cryptography was the BB84, which however did not take advantage of the full potential of multiple superposition states. In this paper, we review quantum cryptography, we review the BB84 protocol, and we present an m‐ary generalized protocol, the K08, which is based on multiple random polarization states. Copyright © 2009 John Wiley & Sons, Ltd.

Quantum key distribution has been a relatively recent addition to data security and integrity. It is based on advanced ideas that emanate from Quantum theory. Additionally, the optical network and the quantum photon provide a formidable basis for the application of quantum key distribution. We propose a new way of distributing encryption keys over quantum channels. The proposed method takes advantage of dual quantum channels between the communicating entities and complementary measuring bases in order to improve the efficiency with which the keys can be exchanged.

Quantum cryptographic systems use quantum mechanical concepts that are based on qubit superposition of states, and on the no cloning or no copying theorem to establish unbreakable cipher keys. Using optical communications the most commonly quantum mechanical property used is the polarization state of photon. However, in most quantum cryptographic algorithms a random polarization state is required. In this case, the random number generator should not be publically known, and it should be preferably reproducible. A naturally reproducible RNG can be produced using chaos functions. In this paper we review a quantum key distribution algorithm and chaos functions as RNGs. We then describe how chaos functions can be used in quantum key establishment and also a method that requires substantially fewer bits in the stream to establish the key and also a faster method.

Data security, since antiquity, relies on cryptography. Cryptography is as good as the inability of a person or a computing machine to "break" the generated cipher code. It has been demonstrated that this inability is temporary because some strong cryptographic methods that were believed to be unbreakable have been broken. Thus, as cryptography is seeking new and more difficult cryptographic venues, cryptoanalysts become increasingly sophisticated in breaking codes. However, both ciphertext and secret keys are transported over the communications network from where, with proper means, they can be harvested for analysis, and furthermore to impersonate a source, or cause service denial. Thus, in addition to hard ciphers for data encryption and sophisticated distribution methods, the network itself needs to be attack-hard and attack-smart. That is, the network should have appropriate mechanisms that monitor and detect attacks, intelligence that discriminates between degradations/failures and attacks, and also protocols for robust countermeasure strategies to outsmart the attacker. Thus, although "data security" deals with ciphering client-originating payload, "network security" deals with the secure transport of data. Because the latter is a relatively new topic, it requires more insight and particularly a thorough understanding of fault/degradation mechanisms and attack scenarios and how they distinguished. In this paper we outline data security and differentiate from network security. Additionally, we describe a method for detecting the channel signature and how it is used to discriminate between degradation/failure and attacks and thus enhance network security. Moreover, we describe a method for attack detection and countermeasure strategies.

Passive optical networks (PONs) solve the bandwidth bottleneck issue as they extend optical networks to homes and businesses. Security concerns in time division multiplexed PONs that are traditionally addressed through filtering and encryption techniques are replaced by an enhanced physical security technique that is presented herein. This security enhancement is accomplished through the inclusion of tunable lasers in the optical line terminal of a PON architecture. Simulation tools verify the viability of the physical security scheme. Presented results verify that implementation of this enhanced security technique can be successfully employed in a PON.

Advanced optical networks deploy wavelength division multiplexing technology to transport an unprecedented amount of information per fiber. Each optical channel transports information data rates, typically 2.5, 10 or 40 Gbps. As a consequence, the monitoring frequency and speed of detecting a failure or a severe degradation in conjunction with a fast and intelligent protocol will determine the speed of channel protection and restoration, as well as the speed an intruder is detected. In this paper we present a novel channel self-restoring strategy applicable to optical WDM networks. The performance parameters of each channel such as BER, SNR, Q-factor, NF and signal power in the WDM signal are continuously and un-intrusively monitored. The rate of performance of a channel and the correlation of degradation among the channels in the WDM signal distinguishes between natural degradation or intrusion in an attempt to compromise the security of a channel. When one or more optical channels performance is compromised according to a specific profile, a link-layer proactive protocol triggers a fast switch to channel protection counteracting the compromise. In this paper, we present a real-time and in-service performance estimation mechanism that also detects intrusions and a protocol to remedy failures, degradations, and to countermeasure intrusions.

Dense wavelength division multiplexing optical networks transport a huge aggregate traffic exceeding Tbps per single fiber, and passive optical network technology has been extended to include the residential and enterprise network layer with fiber-to-the premises. Because this network allows for multimedia and extremely high data rates to end-users, optical communications security is important not only on the client and the node management layers, but also on the physical layer. Thus, cyber-security becomes also important in optical networks, including unbreakable cipher keys, cipher key distribution, identification of malicious actors, source authentication, physical link signature, and countermeasure strategies. In this paper we overview the security of the WDM optical network. We discuss advanced cryptographic methods that stem from quantum mechanics, known as quantum cryptography, and we identify specific vulnerabilities. We also look into the security of the next-generation optical network.

This paper identifies the security vulnerabilities in ethernet passive optical networks (EPONs), and proposes a RC4-based encryption and a key distribution scheme based on the Diffie-Hellman key exchange algorithm. The proposed method is expected to provide the benefits of a streaming cipher while at the same time using securely shared secret keys among the OLT and each ONU.

Biometrics is a technology for authenticating a person's unique identity beyond doubt. Existing popular biometric techniques for ID authentication have relied on a person's photograph and/or fingerprints. However, masters of disguise have repeatedly defeated this type of authentication by temporarily or permanently changing face, voice, appearance and other features of a person in order to accomplish an outlawed objective. Recently, as a result of an increased ID fraud, biometric authentication methods have been advanced to include feature extracted from fingertip(s), palm, iris, face, voice, writing, walking, typing and DNA. However, biometric data authentication requires a robust and encrypted mechanism the data of which is transmitted over a communications network. In general, biometric devices may be considered end devices of an access communications network. Such access network must be capable to handle bandwidth with relatively good security assurance; the most popular access networks today is the wireless GSM and the optical fiber to the premises (FTTP). In this paper, we discuss the biometric processes for identity verification, we describe the key authentication processes in the GSM and in the FTTP optical access networks, cryptographic processes that assure data secrecy between the data source and destination, and the ramifications to biometric data authentication.

Optical communication networks transport an enormous aggregate client traffic, a good part of which is highly sensitive. As such, the optical signal in the fiber medium becomes attractive to bad actors in an attempt to tap the fiber or a component on the path in order to eavesdrop, source mimic, or attack to cause service denial. Therefore, it is important that the network has built-in security features to monitor the channel integrity and to also detect if and when unauthorized intervention has taken place. It is also important that the network is intelligent to countermeasure bad actors. Because the integrity of optical channels in are also affected by component degradation, photon-matter interactions and other non-linear phenomena, the network should be able to differentiate between malicious interventions and parametric degradations. In this paper we identify observable performance parameters and predictors, we describe a estimating method for in-line parameter monitoring, we conclusively distinguish between component degradations and intervention of a malicious eavesdropper and we describe countermeasure mechanisms.

The key parameters that determine channel performance in telecommunications are bit error rate, Q-factor, signal to noise ratio and noise factor. Channel performance depends on signal susceptibility to attenuation, noise, jitter and other interferences in the propagating medium. Typical non-service affecting methods that estimate the channel performance rely on error detecting and correcting codes that have limited error detection capability and require relatively long time. In this paper, we review the statistical estimation of signal performance parameters. Based on this, we describe the functional block of a circuit that continuously estimates BER, SNR, Q-factor and NF at each port without service interruption. Our method is based on statistical sampling and therefore estimates are made in very short intervals.

The next generation fiber network will transport an unprecedented aggregate bandwidth of several terabits/s per fiber. Currently, only 5-10% of fiber in the network is lit, due to traffic bottlenecks at the access or edge of the network and due to slow deployment of traffic-aggressive consumer communications appliances. However, new consumer applications are emerging that will potentially add to the traffic landscape of the network. We examine emerging consumer technologies, the next generation access network, and the impact of these applications on the overall network.

High performance computer network architectures with interactive multi-task programs require concurrent and real-time program execution, which is achieved via inter-processor ultra-high data links requiring massive data transfers over long links that only fiber can support. We present a new method for inter-computer communication, which is characterized by elastic ultrahigh bandwidth, long fiber links, and low cost. We examine network topologies and we study a cost-effective point-to-point link capable to transport Terabits per second by utilizing wavelength division multiplexing (WDM) technology. We model and simulate exemplary cases at different fiber lengths and data rates. Simulation results suggest exceptional performance over 100 km links without amplification and with data transfers at Tbit/s.

The content addressable memory (CAM) is a memory device that executes fast recognition and validation of binary patterns of a limited set, as opposed to the random access memory (RAM) that executes slow recognition of a large set of patterns. Here, we present a logic circuit which, combined with existing RAM's, constructs an associative RAM-based content addressable memory (AR-CAM) that executes fast pattern recognition of very large pattern sets. In addition, we describe the applicability of the AR-CAM to broad-band communications systems. Broadband communications systems with an aggregate bandwidth of tens or hundreds of Gbps require real-time recognition and translation of large pattern sets. Although CAM's are a good candidate for such applications, nevertheless we demonstrate that AR-CAM's can better and inexpensively accomplish this task.

Multi-satellite communication networks have been proposed to address the explosive growth in global multi-media and personal communications services. A satellite network, must provide continuous, error-free, and uninterrupted high-throughput transmission even if one or more satellites in the network fail. Consequently, fault tolerance, disaster avoidance and network survivability of a satellite network are important characteristics. Herein, a fully connected global multi-satellite network is presented and its fault tolerance, disaster avoidance, network survivability, and traffic capacity of the network are discussed.

The time compression multiplexing (TCM) technique offers a solution to the problem of achieving digital penetration of the two-wire loop plant. TCM is planned to provide the digital loop transmission in the new circuit switched digital capability of the Bell System, so that a full-duplex 56 khit/s digital connection is obtained between subscriber's premises and the serving central office. Herein, the concept of TCM and its parameters are described. Tradeoffs leading to determination of system parameters such as burst frequency, burst length, guard time, and delay as they affect the range of the system are discussed. Also, the performance of the digital loops in terms of eye diagrams, system range limitations due to crosstalk and impulse noise, and percentage of loops covered are summarized. In addition, the circuit implementation using both analog and digital technologies is described. Experiences with various digital architectures to implement TCM and tradeoffs leading to a low-power digital architecture suitable for VLSI are discussed.