ZCS ( Zero Code Suppression )

Zero code suppression (ZCS) is a technique that used to be implemented on T1 systems to ensure ones density.

Ones density is a requirement imposed by the used of AMI as the line code on a T1. In this line code, a long series of the zeroes (i.e., more than 15) will lead to loss of clock and loss of synchronization. This was not a problem for T1 used in the PSTN because the PAM/PCM voice digitizing scheme never generated the all-zero code (i.e., 00000000). When these same facilities were used to send data, however, a long series of zeroes became possible.

To prevent this, the network equipment would reserve the 8th bit in each time slot of each T1 frame and set it to one (1). This ensured the minimum requirement of 12.5% ones density. However, it had side consequences. Because the subscriber now only had seven bits (instead of 8) 8,000 times per second, the communication channel operated at 56 kbit/s rather than 64 kbit/s. This had to be resolved in order to use the T1 to deploy an ISDN PRI. As demands for bandwidth continued to increase, the need to provide an unchannelized T1 also increased, and ZCS was not conducive to the use of the facility in this manner. Finally, ZCS still could result in a ones density near the 12.5% minimum, which would lead to minor clock slippage on the span.

Modern T1 facilities are provisioned using B8ZS, a far more efficient and flexible scheme for ensuring ones density.

ZBTSI

Zero Byte Time Slot Interchange. A technique used with the T carrier extended superframe format (ESF) in which an area in the ESF frame carries information about the location of all-zero bytes (eight consecutive "O"s) within the data stream.

ZB ( Zettabyte )

A combination of the Greek "zeta," the second last letter of the Greek alphabet (omega is the last letter), and the English "bite," meaning "a small amount of food." A unit of measurement for physical data storage on some form of storage device — hard disk, optical disk, RAM memory etc. and equal to two raised to the 70th power, i.e. 1,180,591,620,717,400,000,000 bytes.

KB = Kilobyte (2 to the 10th power)

MB = Megabyte (2 to the 20th power)

GB = Gigabyte (2 to the 30th power)

TB = Terabyte (2 to the 40th power)

PB = Petabyte (2 to the 50th power)

EB = Exabyte (2 to the 60th power)

ZB = Zettabyte (2 to the 70th power)

YB = Yottabyte (2 to the 80th power)

One googolbyte equals 2 to the 100th power.

Zapper Message

Sent by the Supervisor to tear down a virtual circuit after the transmission is complete.

Zap

To eradicate all or part of a program or database, sometimes by lightning, sometimes intentionally.

Server Message Block (SMB)

In computer networking, Server Message Block (SMB) operates as an application-level network protocol mainly applied to shared access to files, printers, serial ports, and miscellaneous communications between nodes on a network. It also provides an authenticated Inter-process communication mechanism. Most usage of SMB involves computers running Microsoft Windows: in Microsoft environments users often know it simply as "Microsoft Windows Network".

When discussing SMB, one should distinguish:

• SMB the protocol
• the SMB services that run on the protocol
• NetBIOS
• the DCE/RPC services that use SMB as an authenticated Inter-process communication channel (over named pipes)
• the "Network Neighborhood" protocols which primarily (but not exclusively) run as datagram services directly on the NetBIOS transport

SAMBA

Samba is a free software re-implementation of SMB/CIFS networking protocol, originally developed by Australian Andrew Trigell. SAMBA is released under the GNU General Public License. The name Samba comes from inserting one vowel, twice, into the name of the standard protocol used by the Microsoft Windows network file system, "SMB" (Server Message Block).

As of version 3, Samba not only provides file and print services for various Microsoft Windows clients but can also integrate with a Windows Server domain, either as a Primary Domain Controller (PDC) or as a Domain Member. It can also be part of an Active Directory domain. Samba runs on most Unix and Unix-like systems, such as Linux, Solaris, and the BSD variants, including Apple's Mac OS X Server (which was added to the Mac OS X client in version 10.2). Samba is standard on nearly all distributions of Linux and is commonly included as a basic system service on other Unix-based operating systems as well.

HDLC Transparency

The flag sequence must never occur within the content of a frame otherwise it could be confused with an intentionally sent flag. A technique known as 0-bit insertion is used to prevent random data synthesising a flag. The technique is said to make HDLC transparent, since any stream of bits may be present between the open and closing flag of a frame. The transparency is achieved by encoding the data by inserting a 0-bit after any sequence of 5 consecutive 1's within the payload, as shown:

Insertion of a "zero-bit" into the content of a frame to ensure transparency

Finite State Machine (FSM)

A model of computation consisting of a set of states, a start state, an input alphabet, and a transition function that maps input symbols and current states to a next state. Computation begins in the start state with an input string. It changes to new states depending on the transition function. There are many variants, for instance, machines having actions (outputs) associated with transitions (Mealy machine) or states (Moore machine), multiple start states, transitions conditioned on no input symbol (a null) or more than one transition for a given symbol and state (nondeterministic finite state machine), one or more states designated as accepting states (recognizer), etc.

Local Talk Link Access Protocol

LocalTalk Link Access Protocol (LLAP) is a link-level protocol that manages node-to-node delivery of data on a LocalTalk/AppleTalk network. LLAP manages bus access, provides a node-addressing mechanism, and controls data transmission and reception, ensuring packet length and integrity.

HDLC Flags

HDLC is a data link protocol which uses a unique bit sequence to delimit the start and end of each PDU transported by the data link layer service. In HDLC, frames are delimited by a sequence of bits known as a "flag". The flag sequence is a unique 8-bit sequence of the form 0111 1110. The way in which this is performed is described in the diagram

(fig)The flags before and after an HDLC frame indicate the start and end of the frame

HDLC Hunt Mode

Normally a HDLC receiver starts in the idle state, waiting for the start of a frame. This is called "Hunt" mode, since the receiver is said to be hunting for a non-flag sequence. This may be achieved through a shift register and combinational logic as shown (a Finite State Machine (FSM) is usually used to implement this):









(fig) Detection of a start of frame

Abort & Idle Patterns (HDLC)

Valid frames are terminated by a closing flag. If the link layer protocol needs to transmit a higher priority frame before the current frame has been fully sent, it may "Abort" the frame. An "aborted" frame is terminated by an "abort sequence": 0111 1111 instead of the normal "flag sequence". Abort sequences may also be caused by bit errors which occur while the frame is travelling from the transmitter to the receiver. (For instance a flag may suffer a 1-bit inversion of the least significant bit, resulting in a bit sequence identical to an "abort").

A frame which is terminated by an abort is received by the receiver in the normal way, but marked as being "aborted". The frame is then discarded without further processing. This is shown below:

(fig)An HDLC frame terminated by an "abort" is discarded by the HDLC receiver

An abort sequence is often followed by a series of 1's. The sequence of all 1's may be used to fill the gaps between frames (or alternatively a continuous series of flags may be transmitted). The all 1's sequence is known as the "idle" sequence, since the line becomes idle (N.B. represented at the physical layer by 0 Volts). This is useful for applications requiring half duplex operation (also known as Two Way Alternate (TWA) operation).

Sound Absorption Coefficient

The sound absorption coefficient indicates how much of the sound is absorbed in the actual material. The absorption coefficient can be expressed as:

α = I_a / I_i (1)

where

I_a = sound intensity absorbed (W/m²)

I_i = incident sound intensity (W/m²)

Absorption coefficient - α - for some common materials can be found in the table below:

Material	Sound Absorption Coefficient - α
Plaster walls	0.01 - 0.03
Unpainted brickwork	0.02 - 0.05
Painted brickwork	0.01 - 0.02
3 mm plywood panel	0.01 - 0.02
6 mm cork sheet	0.1 - 0.2
6 mm porous rubber sheet	0.1 - 0.2
12 mm fiberboard on battens	0.3 - 0.4
25 mm wood wool cement on battens	0.6 - 0.07
50 mm slag wool or glass silk	0.8 - 0.9
12 mm acoustic belt	0.5 - 0.5
Hardwood	0.3
25 mm sprayed asbestos	0.6 - 0.7
Persons, each	2.0 - 5.0
Acoustic tiles	0.4 - 0.8

Note! The absorption coefficient varies with the frequency of sound.

Total Room Sound Absorption

The total sound absorption in a room can be expressed as:

A = S₁ α₁ + S₂ α₂ + .. + S_n α_n = ∑ S_i α_i (2)

where

A = the absorption of the room (m² sabine)

S_n = area of the actual surface (m²)

α_n = absorption coefficient of the actual surface

Mean Absorption Coefficient

The mean absorption coefficient for the room can be expressed as:

a_m = A / S (3)

where

a_m = mean absorption coefficient

A = the absorption of the room (m² sabine)

S = total surface in the room (m²)

A rooms acoustic characteristics can be calculated with the formulas above, or estimated for typical rooms.

Abstract Syntax

In open systems architecture, the specification of application- layer data or application-protocol control information by using notation rules that are independent of the encoding technique used to represent the information.

Absorption Modulation

Amplitude modulation of the output of a radio transmitter by means of a variable-impedance circuit that is caused to absorb carrier power in accordance with the modulating wave.

Absorption Loss

That part of the transmission loss caused by the dissipation or conversion of electrical, electromagnetic, or acoustic energy into other forms of energy as a result of its interaction with a material medium.

Absorption Index

A measure of the attenuation caused by absorption of energy per unit of distance that occurs in an electromagnetic wave of given wavelength propagating in a material medium of given refractive index.

where K is the absorption coefficient, is the wavelength in vacuum, and n is the refractive index of the absorptive material medium.

Absorption Coefficient

A measure of the attenuation caused by absorption of energy that results from its passage through a medium. Absorption coefficients are usually expressed in units of reciprocal distance.

The sum of the absorption coefficient and the scattering coefficient is the attenuation coefficient.
Absorption coefficient is sufficient for describing energy propagation through a homogeneous system only. Propagation through a heterogeneous system is affected by scattering. There is more general term attenuation that takes into account both absorption and scattering. It is widely used in acoustics as attenuation coefficient for characterizing particle size distribution.

The fraction of energy which is absorbed on striking any surface. It therefore takes values between 0 and 1, and is usually frequency dependent. In room ACOUSTICS, this coefficient, given the symbol a, is expressed as a fraction of the perfect ABSORPTION at an open window of equal area.
A table such as the following might be consulted for choosing suitable materials for SOUND INSULATION in a room or building.

Material	128 Hz	256 Hz	512 Hz	1,024 Hz	2,048 Hz	4,096 Hz
Draperies hung straight, in contact with wall, cotton fabric, 10 oz. per square yard	0.04	0.05	0.11	0.18	0.30	0.44
The same, velour, 18 oz. per square yard	0.05	0.12	0.35	0.45	0.40	0.44
Same as above, hung 4 inches from wall	0.09	0.33	0.45	0.52	0.50	0.44
Felt, all hair, contact with wall	0.13	0.41	0.56	0.69	0.65	0.49
Rock wool (1 inch)	0.35	0.49	0.63	0.80	0.83	------
Carpet on concrete (0.4 inch)	0.09	0.08	0.21	0.26	0.27	0.37
Carpet, on 1/8 inch felt, on concrete (0.4 inch)	0.11	0.14	0.37	0.43	0.27	0.27
Concrete, unpainted	0.010	0.012	0.016	0.019	0.023	0.035
Wood sheeting, pine (0.8 inch)	0.10	0.11	0.10	0.08	0.08	0.11
Brick wall, painted	0.012	0.013	0.017	0.020	0.023	0.025
Plaster, lime on wood studs, rough finish (1/2 inch)	0.039	0.056	0.061	0.089	0.054	0.070

Also See:
Sound Absorption Coefficient

Absorption Band

An absorption band is a range of wavelengths (or, equivalently, frequencies) in the electromagnetic spectrum which are able to excite a particular transition in a substance.

A spectral region in which the absorption coefficient reaches a relative maximum, by virtue of the physical properties of the matter in which the absorption process takes place.

Absorption bands of interest to the atmospheric physicist

In oxygen:

• the Hopfield bands, very strong, between about 67 and 100 nanometres in the ultraviolet (named after John J. Hopfield);
• a diffuse system between 101.9 and 130 nanometres;
• the Schumann-Runge continuum, very strong, between 135 and 176 nanometres;
• the Schumann-Runge bands between 176 and 192.6 nanometres (named for Victor Schumann and Carl Runge);
• the Herzberg bands between 240 and 260 nanometres (named after Gerhard Herzberg);
• the atmospheric bands between 538 and 771 nanometres in the visible spectrum; and
• a system in the infrared at about 1000 nanometres.

In ozone:

• the Hartley bands between 200 and 300 nanometres in the ultraviolet, with a very intense maximum absorption at 255 nanometres (named after Walter Noel Hartley);
• the Huggins bands, weak absorption between 320 and 360 nanometres (named after Sir William Huggins);
• the Chappuis bands (sometimes misspelled "Chappius"), a weak diffuse system between 375 and 650 nanometres in the visible spectrum (named after J. Chappuis); and
• the Wulf bands in the infrared beyond 700 nm, centered at 4,700, 9,600 and 14,100 nanometres, the latter being the most intense (named after Oliver R. Wulf).

In nitrogen:

• The Lyman-Birge-Hopfield bands, sometimes known as the Birge-Hopfield bands, in the far ultraviolet: 140– 170 nm (named after Theodore Lyman, Raymond T. Birge, and John J. Hopfield)

Types of absorption band

Electronic transitions

Electronic transitions mainly take place at energies corresponding to the UV and visible part of the spectrum. The main factors that cause broadening of the spectral line into an absorption band are the distributions of vibrational and rotational energies of the molecules in the sample (and also those of their excited states). In gas phase spectroscopy, the fine structure afforded by these factors can be discerned, but in solution-state spectroscopy, the differences in molecular microenvironments further broaden the structure to give smooth bands. Electronic transition bands of molecules may be from tens to several hundred nanometers in breadth.

Vibrational transitions

Vibrational transitions take place in the infrared part of the spectrum, at wavelengths of around 1-30 micrometres.

Rotational transitions

Rotational transitions also take place in the infrared, but a lower energies than vibrational transitions.

Other transitions

• Absorbance bands in the radiofrequency range are found in NMR spectroscopy.