Sockets

• developed for Berkeley UNIX : recall early Berkeley TCP/IP implementation, first delivered with BSD 2.1
• central features: central abstraction – the socket – an end-point like an electrical connector, not TCP/IP specific (e.g. UNIX named pipes), uses normal read/write system calls, sockets associated with UNIX file descriptors but some not for normal I/O, some extra system calls
• sits more comfortably with TCP than with UDP because of byte-stream nature of UNIX I/O
• special UDP functions e.g., recv(…) – accepts a UDP datagram
• additional non-socket functions e.g., gethostbyname(…) – domain name server

Establishing a TCP Connection
Initial State

• TCP is connection based … establishing it is a complex multistage process
• initially all machines are the same
• no special ‘server’ machines
• the difference is all in the software

Passive Open

• server process does a ‘passive’ open on a port

Fiber Optic Cable Assembly Capabilities

• Harsh environment multi-channel lanyard connector shell with metal M29504/4 (pins)
• Custom ceramic ferrule single channel connectors.
• Typical loss < 0.5dB.
• Termini and connectors are Cooper products
• Custom connector backshell with two D38999 Series IV connectors
• Ceramic ferrule M29504/4 (pin) and /5 (sockets) termini
• Typical optical loss < 0.4dB.
• Termini, connector and backshell are Cooper products
• D38999 series III with ceramic ferrule M29504/5 (sockets) to ST fiber optic connectors
• Typical loss < 0.3dB
• Clear convoluted tubing provides crush and bend protection while allowing a visual fault finder to locate problems through the protective conduit
• Convoluted tubing is attached to the connector backshell to provide strain relief without clamping or crushing the fiber optic cable
• The connector, backshell, convoluted tubing and ST connectors are not Cooper products

The Design of Fiber Core and Cladding

An optical fiber consists of two different types of highly pure, solid glass, composed to form the core and cladding. A protective acrylate coating (see Figure 1) then surrounds the cladding. In most cases, the protective coating is a dual layer composition.

A protective coating is applied to the glass fiber as the final step in the manufacturing process. This coating protects the glass from dust and scratches that can affect fiber strength. This protective coating can be comprised of two layers: a soft inner layer that cushions the fiber and allows the coating to be stripped from the glass mechanically and a harder outer layer that protects the fiber during handling, particularly the cabling, installation, and termination processes.

Single-Mode and Multimode Fibers
There are two general categories of optical fiber: single-mode and multimode (see Figure 2).

Single Mode cable is a single stand of glass fiber with a diameter of 8.3 to 10 microns that has one mode of transmission. Single Mode Fiber with a relatively narrow diameter, through which only one mode will propagate typically 1310 or 1550nm. Carries higher bandwidth than multimode fiber, but requires a light source with a narrow spectral width. Synonyms mono-mode optical fiber, single-mode fiber, single-mode optical waveguide, unimode fiber.

Single-mode fiber gives you a higher transmission rate and up to 50 times more distance than multimode, but it also costs more. Single-mode fiber has a much smaller core than multimode. The small core and single light-wave virtually eliminate any distortion that could result from overlapping light pulses, providing the least signal attenuation and the highest transmission speeds of any fiber cable type.

Single-mode optical fiber is an optical fiber in which only the lowest order bound mode can propagate at the wavelength of interest typically 1300 to 1320nm.

In most fiber optic systems, optical fiber and waveguides are coupled to and from bulk optical devices, creating hybrid optical devices. In this tutorial article, the authors explain in detail how to successfully collimate, focus, and align hybrid optics, including polarizing and polarization maintaining optics.

Glossary
Mode field diameter (MFD): A measure of the intensity profile of light traveling within a fiber. In Gaussian optics, this diameter is measured at the point where the light intensity drops to 13.5% (1/e2) of the peak intensity. In single-mode fibers, it is typically about 15% larger than the actual core size.

Numerical Aperture (NA): A measure of the maximum angle at which a given optical system can receive light. If a system can receive light within a cone having a half angle q, then the NA of that system is equal to sin q.

Understanding VPN
A truly private network is a network where a single entity (e.g., a company) owns all the wires from point A to point B. In a Virtual Private Network (VPN), some part of the path from A to B is a public network (e.g., the Internet or the public telephone system). VPN software technology creates a private “tunnel” through the public network system for your sensitive traffic. Using encryption and authentication methods, a VPN provides security over unsecured media.

VPN Benefits
VPNs provide a very cost-effective means of private communication by using inexpensive local call ISDN or telephone connections (with the Internet as the backbone).

VPN Limitations
Obviously, when a technology incorporates portions of the network that are physically not in its control, there are Quality of Service (QoS) limitations. With a true private network, users can demand a guaranteed QoS from the telephone company or provider. However, this is not as clear-cut with VPNs.