The base of communications between devices is the physical layer of the OSI model. This is the domain of the actual connection between devices, whether by wire, fiber, or radio frequency waves. The physical layer separates the definitions and protocols required to transmit the signal physically between boxes from higher level protocols that deal with the details of the data itself. Four common methods are used to connect equipment at the physical layer:

  • Coaxial cable

  • Twisted-pair cable

  • Fiber-optics

  • Wireless

Coaxial Cable

Coaxial cable is familiar to many households as a method of connecting televisions to VCRs or to satellite or cable services. It is used because of its high bandwidth and shielding capabilities. Compared to standard twisted-pair lines such as telephone lines, "coax" is much less prone to outside interference. It is also much more expensive to run, both from a cost-per-foot measure and from a cable-dimension measure. Coax costs much more per foot than standard twisted pair and carries only a single circuit for a large wire diameter.

Twisted-pair wires have all but completely replaced coaxial cables in Ethernet networks. Twisted-pair wires use the same technology used by the phone company for the movement of electrical signals. Single pairs of twisted wires reduce electrical crosstalk and electromagnetic interference. Multiple groups of twisted pairs can then be bundled together in common groups and easily wired between devices.

Twisted pairs come in two types, shielded and unshielded. Shielded twisted-pair (STP) has a foil shield around the pairs to provide extra shielding from electromagnetic interference. Unshielded twisted-pair (UTP) relies on the twist to eliminate interference. UTP has a cost advantage over STP and is usually sufficient for connections, except in very noisy electrical areas.


Fiber-optic cable uses beams of laser light to connect devices over a thin glass wire. The biggest advantage to fiber is its bandwidth, with transmission capabilities into the terabits per second range. Fiber-optic cable is used to make high-speed connections between servers and is the backbone medium of the Internet and large networks. For all of its speed and bandwidth advantages, fiber have one major drawback-cost. The cost of using fiber is a two-

edged sword. It is cheaper when measured by bandwidth to use fiber than competing wired technologies. The length of runs of fiber can be much longer, and the data capacity of fiber is much higher. But connections to a fiber are difficult and expensive and fiber is impossible to splice. Making the precise connection on the end of a fiber-optic line is a highly skilled job and is done by specially trained professionals who maintain a level of proficiency. Once the connector is fitted on the end, several forms of connectors and blocks are used.

Unguided Media

Electromagnetic waves have been transmitted to convey signals literally since the inception of radio. Unguided media is a phrase used to cover all transmission media not guided by wire, fiber, or other constraints; it includes radio frequency (RF), infrared (IR), and microwave methods. Unguided media have one attribute in common: they are unguided and as such can travel to many machines simultaneously. Transmission patterns can be modulated by antennas, but the target machine can be one of many in a reception zone. As such, security principles are even more critical, as they must assume that unauthorized users have access to the signal.


Infrared (IR) is a band of electromagnetic energy just beyond the red end of the visible color spectrum. IR has been used in remote control devices for years, and it cannot penetrate walls but instead bounces off them. IR made its debut in computer networking as a wireless method to connect to printers. Now that wireless keyboards, wireless mice, and PDAs exchange data via IR, it seems to be everywhere. IR can also be used to connect devices in a network configuration, but it is slow compared to other wireless technologies. It also suffers from not being able to penetrate solid objects, so stack a few items in front of the transceiver and the signal is lost.


The use of radio frequency (RF) waves to carry communication signals goes back to the beginning of the twentieth century. RF waves are a common method of communicating in a wireless world. They use a variety of frequency bands, each with special characteristics. The term microwave is used to describe a specific portion of the RF spectrum that is used for communication as well as other tasks, such as cooking. Point-to-point microwave links have been installed by many network providers to carry communications over long distances and rough terrain. Microwave communications of telephone conversations were the basis for forming the telecommunication company MCI. Many different frequencies are used in the microwave bands for many different purposes. Today, home users can use wireless networking throughout their house and enable laptops to surf the Web while they move around the house. Corporate users are experiencing the same phenomenon, with wireless networking enabling corporate users to check e-mail on laptops while riding a shuttle bus on a business campus

Security Concerns for Transmission Media

The primary security concern for a system administrator has to be preventing physical access to a server by an unauthorized individual. Such access will almost always spell disaster, for with direct access and the correct tools, any system can be infiltrated. One of the administrator's next major concerns should be preventing unfettered access to a network connection. Access to switches and routers is almost as bad as direct access to a server, and access to network connections would rank third in terms of worst-case scenarios. Preventing such access is costly, yet the cost of replacing a server because of theft is also costly.

Physical Security

A balanced approach is the most sensible approach when addressing physical security, and this applies to transmission media as well. Keeping network switch rooms secure and cable runs secure seems obvious, but cases of using janitorial closets for this vital business purpose abound. One of the keys to mounting a successful attack on a network is information. Usernames, passwords, server locations-all of these can be obtained if someone has the ability to observe network traffic in a process called sniffing. A sniffer can record all the network traffic and this data can be mined for accounts, passwords, and traffic content, all of which can be useful to an unauthorized user. Many common scenarios exist when unauthorized entry to a network occurs, including these:

  • Inserting a node and functionality that is not authorized on the network, such as a sniffer device or unauthorized wireless access point

  • Modifying firewall security policies

  • Modifying ACLs for firewalls, switches, or routers

  • Modifying network devices to echo traffic to an external node

One starting point for many intrusions is the insertion of an unauthorized sniffer into the network, with the fruits of its labors driving the remaining unauthorized activities. The best first effort is to secure the actual network equipment to prevent this type of intrusion.

Wireless networks make the intruder's task even easier, as they take the network to the users, authorized or not. A technique called war-driving involves using a laptop and software to find wireless networks from outside the premises. A typical use of war driving is to locate a wireless network with poor (or no) security and obtain free Internet access, but other uses can be more devastating. Methods for securing even the relatively weak Wired Equivalent Privacy (WEP) protocol are not difficult; they are just typically not followed. A simple solution is to place a firewall between the wireless access point and the rest of the network and authenticate users before allowing entry.

Home users can do the same thing to prevent neighbors from "sharing" their Internet connections. To ensure that unauthorized traffic does not enter your network through a wireless access point, you must either use a firewall with an authentication system or establish a VPN.

Removable Media

One concept common to all computer users is data storage. Sometimes storage occurs on a file server and sometimes on movable media, allowing it to be transported between machines. Moving storage media represents a security risk from a couple of angles, the first being the potential loss of control over the data on the moving media.

Second is the risk of introducing unwanted items, such as a virus or a worm, when the media are attached back to a network. Both of these issues can be remedied through policies and software. The key is to ensure that they are occurring. To describe media-specific issues, the media can be divided into three categories: magnetic, optical, and electronic.

Magnetic Media

Magnetic media store data through the rearrangement of magnetic particles on a nonmagnetic substrate. Common forms include hard drives, floppy disks, zip disks, and magnetic tape. Although the specific format can differ, the basic concept is the same. All these devices share some common characteristics: Each has sensitivity to external magnetic fields. Attach a floppy disk to the refrigerator door with a magnet if you want to test the sensitivity. They are also affected by high temperatures as in fires and by exposure to water.

Hard Drives

Hard drives used to require large machines in mainframes. Now they are small enough to attach to PDAs and handheld devices. The concepts remain the same among all of them: a spinning platter rotates the magnetic media beneath heads that read the patterns in the oxide coating. As drives have gotten smaller and rotation speeds increased, the capacities have also grown. Today gigabytes can be stored in a device slightly larger than a bottle cap. Portable hard drives in the 120 to 320GB range are now available and affordable.

One of the latest advances is full drive

encryption built into the drive hardware. Using a key that is controlled, through a Trusted Platform Module (TPM) interface for instance, this technology protects the data if the drive itself is lost or stolen. This may not be important if a thief takes the whole PC, but in larger storage environments, drives are placed in separate boxes and remotely accessed. In the specific case of notebook machines, this layer can be tied to smart card interfaces to provide more security. As this is built into the controller, encryption protocols such as Advanced Encryption Standard (AES) and Triple Data Encryption Standard (3DES) can be performed at full drive speed.


Floppy disks were the computer industry's first attempt at portable magnetic media. The movable medium was placed in a protective sleeve, and the drive remained in the machine. Capacities up to 1.4MB were achieved, but the fragility of the device as the size increased, as well as competing media, has rendered floppies almost obsolete. A better alternative, the Zip disk from Iomega Corporation, improved on the floppy with a stronger case and higher capacity (250MB); it has been a common backup and file transfer medium. But even the increased size of 250MB is not large enough for some multimedia files, and recordable optical (CD-R) drives have arrived to fill the gap; they will be discussed shortly.


Magnetic tape has held a place in computer centers since the beginning of computing. Their primary use has been bulk offline storage and backup. Tape functions well in this role because of its low cost. The disadvantage of tape is its nature as a serial access medium, making it slow to work with for large quantities of data. Several types of magnetic tape are in use today, ranging from quarter inch to digital linear tape (DLT) and digital audio tape (DAT). These cartridges can hold upward of 60GB of compressed data.

Tapes are still a major concern from a security perspective, as they are used to back up many types of computer systems. The physical protection afforded the tapes is of concern, because if a tape is stolen, an unauthorized user could establish a network and recover your data on his system, because it's all stored on the tape. Offsite storage is needed for proper disaster recovery protection, but secure offsite storage and transport is what is really needed. This important issue is frequently overlooked in many facilities. The simple solution to maintain control over the data even when you can't control the tape is through encryption.

Backup utilities can secure the backups with encryption, but this option is frequently not used for a variety of reasons. Regardless of the rationale for not encrypting data, once a tape is lost, not using the encryption option becomes a lamented decision.

Optical Media

Optical media involve the use of a laser to read data stored on a physical device. Rather than a magnetic head picking up magnetic marks on a disk, a laser picks up deformities embedded in the media that contain the information. As with magnetic media, optical media can be read-write, although the read-only version is still more common.


The compact disc (CD) took the music industry by storm, and then it took the computer industry by storm as well.

A standard CD holds more than 640MB of data, in some cases up to 800 MB. The digital video disc (DVD) can hold almost 4GB of data. These devices operate as optical storage, with little marks burned in them to represent 1's and 0's on a microscopic scale. The most common type of CD is the read-only version, in which the data is written to the disc once and only read afterward. This has become a popular method for distributing computer software, although higher capacity DVDs have begun to replace CDs for program distribution.

DVDs will eventually occupy the same role that CDs have in the recent past, except that they hold more than seven times the data of a CD. This makes full-length movie recording possible on a single disc. The increased capacity comes from finer tolerances and the fact that DVDs can hold data on both sides. A wide range of formats for DVDs include DVD+R, DVD-R, dual layer, and now HD formats, HD-DVD and Blu-ray. This variety is due to competing "standards" and can result in confusion. DVD+R and -R are distinguishable only when recording, and most devices since 2004 should read both. Dual layers add additional space but require appropriate dual-layer-enabled drives.

HD-DVD and Blue-ray are competing formats in the high-definition arena, with devices that currently hold 50GB and with research prototypes promising up to 1TB on a disk. In 2008, Toshiba, the leader of the HD-DVD format, announced it was ceasing production, casting doubts onto its future, although this format is also used in gaming systems such as the Xbox 360.

Electronic Media

The latest form of removable media is electronic memory. Electronic circuits of static memory, which can retain data even without power, fill a niche where high density and small size are needed. Originally used in audio devices and digital cameras, these electronic media come in a variety of vendor-specific types, such as smart cards, SmartMedia, flash cards, memory sticks, and CompactFlash devices. Several recent photo-quality color printers have been released with ports to accept the cards directly, meaning that a computer is not required for printing. Computer readers are also available to permit storing data from the card onto hard drives and other media in a computer. The size of storage on these devices ranges from 256MB to 32GB and higher.

The advent of large capacity USB sticks has enabled users to build entire systems, OSs, and tools onto them to ensure security and veracity of the OS and tools. With the expanding use of virtualization, a user could carry an entire system on a USB stick and boot it using virtually any hardware. The only downside to this form of mobile computing is the slower speed of the USB 2.0 interface, currently limited to 480 Mbps.