A protocol is a set of rules that governs the communications between computers on a network. These rules include guidelines that regulate the following characteristics of a network: access method, allowed physical topologies, types of cabling, and speed of data transfer.
Types of Network Protocols
The most common network protocols are:
The following is some common-used network symbols to draw different kinds of network protocols.
In computer networking, encapsulation is a method of designing modular communication protocols in which logically separate functions in the network are abstracted from their underlying structures by inclusion or information hiding within higher level objects.
The physical layer is responsible for physical transmission of the data. Link encapsulation allows local area networking and IP provides global addressing of individual computers; UDP adds application or process selection, i.e., the port specifies the service such as a Web or TFTP server.
In 16-bit mode, such as provided by the Pentium processor when operating as a Virtual 8086 (this is the mode used when Windows 95 displays a DOS prompt), the processor provides the programmer with 14 internal registers, each 16 bits wide. They are grouped into several categories as follows:
Four general-purpose registers, AX, BX, CX, and DX. Each of these is a combination of two 8-bit registers which are separately accessible as AL, BL, CL, DL (the “low” bytes) and AH, BH, CH, and DH (the “high” bytes). For example, if AX contains the 16-bit number 1234h, then AL contains 34h and AH contains 12h.
Four special-purpose registers, SP, BP, SI, and DI.
Four segment registers, CS, DS, ES, and SS.
The instruction pointer, IP (sometimes referred to as the program counter).
The status flag register, FLAGS.
Although I refer to the first four registers as “general-purpose”, each of them is designed to play a particular role in common use:
AX is the “accumulator”; some of the operations, such as MUL and DIV, require that one of the operands be in the accumulator. Some other operations, such as ADD and SUB, may be applied to any of the registers (that is, any of the eight general- and special-purpose registers) but are more efficient when working with the accumulator.
BX is the “base” register; it is the only general-purpose register which may be used for indirect addressing. For example, the instruction MOV [BX], AX causes the contents of AX to be stored in the memory location whose address is given in BX.
CX is the “count” register. The looping instructions (LOOP, LOOPE, and LOOPNE), the shift and rotate instructions (RCL, RCR, ROL, ROR, SHL, SHR, and SAR), and the string instructions (with the prefixes REP, REPE, and REPNE) all use the count register to determine how many times they will repeat.
DX is the “data” register; it is used together with AX for the word-size MUL and DIV operations, and it can also hold the port number for the IN and OUTinstructions, but it is mostly available as a convenient place to store data, as are all of the other general-purpose registers.
Here are brief descriptions of the four special-purpose registers:
SP is the stack pointer, indicating the current position of the top of the stack. You should generally never modify this directly, since the subroutine and interrupt call-and-return mechanisms depend on the contents of the stack.
BP is the base pointer, which can be used for indirect addressing similar to BX.
SI is the source index, used as a pointer to the current character being read in a string instruction (LODS, MOVS, or CMPS). It is also available as an offset to add to BX or BP when doing indirect addressing; for example, the instruction MOV [BX+SI], AX copies the contents of AX into the memory location whose address is the sum of the contents of BX and SI.
DI is the destination index, used as a pointer to the current character being written or compared in a string instruction (MOVS, STOS, CMPS, or SCAS). It is also available as an offset, just like SI.
Since all of these registers are 16 bits wide, they can only contain addresses for memory within a range of 64K (=2^16) bytes. To support machines with more than 64K of physical memory, Intel implemented the concept of segmented memory. At any given time, a 16-bit address will be interpreted as an offset within a 64K segment determined by one of the four segment registers (CS, DS, ES, and SS).
As an example, in the instruction MOV [BX], AX mentioned above, the BX register really provides the offset of a location in the current data segment; to find the true physical address into which the contents of the accumulator will be stored, you have to add the value in BX to the address of the start of the data segment. This segment start address is determined by taking the 16-bit number in DS and multiplying by 16. Therefore, if DS contains 1234h and BX contains 0017h, then the physical address will be 1234h TIMES 16+0017h=12340h+0017h=12357h. (This computation illustrates one reason why hexadecimal is so useful; multiplication by 16 corresponds to shifting the hex digits left one place and appending a zero.) We refer to this combined address as 1234:0017 or, more generally, as DS:BX.
Since segment starts are computed by multiplying a 16-bit number by 16=2^4, the effect is that physical addresses have a 20-bit range, so that a total of 1M (=2^20) of memory may be used. Intel considered that this would be enough for applications of the 8086 over its projected lifetime of about five years from its introduction in 1978; by the time microcomputers were needing more than a meg of main memory, the next Intel processor (the iAPX432) was due to be available, with a 32-bit address space (able to address 4G—-over 4 billion memory locations). However, the IBM PC’s debut in 1981 and subsequent popularity has forced Intel to continue the 80x86 family of backward-compatible processors to the present, including support for a mode in which only 1M of memory is accessible. Processors since the 80286 have also provided the “protected” mode of operation, which in the Pentium gives each process a flat 32-bit address space of up to 4G.
You might think that a segment register would only need to provide the uppermost 4 bits to extend an address out to 20 bits, but consider one of the implications of only having 16 different, non-overlapping segments: every segment would have to occupy a full 64K of memory, even if only a small fraction of this space were needed. By allowing a segment to start at any address divisible by 16, the memory may be allocated much more efficiently—-if one program only needs 4K for its code segment, then theoretically the operating system could load another program into a segment starting just 4K above the start of the first. Of course, MS-DOS is not really this sophisticated, but the Intel designers wanted it to be possible.
Each segment register has its own special uses:
CS determines the “code” segment; this is where the executable code of a program is located. It is not directly modifiable by the programmer, except by executing one of the branching instructions. One of the reasons for separating the code segment from other segments is that well-behaved programs never modify their code while executing; therefore, the code segment can be identified as “read-only”. This simplifies the work of a cache, since no effort is required to maintain consistency between the cache and main memory. It also permits several instances of a single program to run at once (in a multitasking operating system), all sharing the same code segment in memory; each instance has its own data and stack segments where the information specific to the instance is kept. Picture multiple windows, each running Word on a different document; each one needs its own data segment to store its document, but they can all execute the same loaded copy of Word.
DS determines the “data” segment; it is the default segment for most memory accesses.
ES determines the “extra” segment; it can be used instead of DS when data from two segments need to be accessed at once. In particular, the DI register gives an offset relative to ES when used in the string instructions; for example, the MOVSB instruction copies a byte from DS:SI to ES:DI (and also causes SI and DI to be incremented or decremented, ready to copy the next byte).
SS determines the “stack” segment; the stack pointer SP gives the offset of the current top-of-stack within the stack segment. The BP register also gives an offset relative to the stack segment by default, for convenient access to data further down in the stack without having to modify SP. Just as with SP, you should not modify SS unless you know exactly what you are doing.
The instruction pointer, IP, gives the address of the next instruction to be executed, relative to the code segment. The only way to modify this is with a branch instruction.
The status register, FLAGS, is a collection of 1-bit values which reflect the current state of the processor and the results of recent operations. Nine of the sixteen bits are used in the 8086:
Carry (bit 0): set if the last arithmetic operation ended with a leftover carry bit coming off the left end of the result. This signals an overflow on unsigned numbers.
Parity (bit 2): set if the low-order byte of the last data operation contained an even number of 1 bits (that is, it signals an even parity condition).
Auxiliary Carry (bit 4): used when working with binary coded decimal (BCD) numbers.
Zero (bit 6): set if the last computation had a zero result. After a comparison (CMP, CMPS, or SCAS), this indicates that the values compared were equal (since their difference was zero).
Sign (bit 7): set if the last computation had a negative result (a 1 in the leftmost bit).
Trace (bit 8): when set, this puts the CPU into single-step mode, as used by debuggers.
Interrupt (bit 9): when set, interrupts are enabled. This bit should be cleared while the processor is executing a critical section of code that should not be interrupted (for example, when processing another interrupt).
Direction (bit 10): when clear, the string operations move from low addresses to high (the SI and DI registers are incremented after each character). When set, the direction is reversed (SI and DI are decremented).
Overflow (bit 11): set if the last arithmetic operation caused a signed overflow (for example, after adding 0001h to 7FFFh, resulting in 8000h; read as two’s complement numbers, this corresponds to adding 1 to 32767 and ending up with -32768).
There are numerous operations that will test and manipulate various of these flags, but to get the contents of the entire FLAGS register one has to push the flags onto the stack (with PUSHF or by calling an appropriate interrupt handler with INT) and then pop them off into another register. To set the entire FLAGS register, the sequence is reversed (with POPF or IRET). For example, one way to set the carry flag (there are much better ways, including the STC instruction) is the following:
OR AX, 1
Most of the time you will not have to deal with the FLAGS register explicitly; instead, you will execute one of the conditional branch instructions, Jcc, where cc is one of the following mnemonic condition codes:
NO, Not Overflow
B, Below; C, Carry; NAE, Not Above or Equal
NB, Not Below; NC, Not Carry; AE, Above or Equal
E, Equal; Z, Zero
NE, Not Equal; NZ, Not Zero
BE, Below or Equal; NA, Not Above (true if either Carry or Zero is set)
NBE, Not Below or Equal; A, Above
NS, Not Sign
P, Parity; PE, Parity Even
NP, Not Parity; PO, Parity Odd
L, Less; NGE, Not Greater or Equal (true if Sign and Overflow are different)
NL, Not Less; GE, Greater or Equal
LE, Less or Equal; NG, Not Greater (true if Sign and Overflow are different, or Zero is set)
NLE, Not Less or Equal; G, Greater
All of the conditions on the same line are synonyms. The Above and Below conditions refer to comparisons of unsigned numbers, and the Less and Greater conditions refer to comparisons of signed (two’s complement) numbers.
Special purpose registers (SPRs) hold program state; they usually include the program counter (aka instruction pointer) and status register(aka processor status word). The aforementioned stack pointer is sometimes also included in this group. Embedded microprocessors can also have registers corresponding to specialized hardware elements.computer
While LAN and WAN are by far the most popular network types mentioned, you may also commonly see references to these others:
Wireless Local Area Network - a LAN based on WiFi wireless network technology
Metropolitan Area Network - a network spanning a physical area larger than a LAN but smaller than a WAN, such as a city. A MAN is typically owned an operated by a single entity such as a government body or large corporation.
Campus Area Network - a network spanning multiple LANs but smaller than a MAN, such as on a university or local business campus.
Storage Area Network - connects servers to data storage devices through a technology like Fibre Channel.
System Area Network - links high-performance computers with high-speed connections in a cluster configuration. Also known as Cluster Area Network.
One way to categorize the different types of computer network designs is by their scope or scale. For historical reasons, the networking industry refers to nearly every type of design as some kind of area network. Common examples of area network types are:
LAN - Local Area Network
WLAN - Wireless Local Area Network
WAN - Wide Area Network
MAN - Metropolitan Area Network
SAN - Storage Area Network, System Area Network, Server Area Network, or sometimes Small Area Network
CAN - Campus Area Network, Controller Area Network, or sometimes Cluster Area Network
PAN - Personal Area Network
DAN - Desk Area Network
LAN and WAN were the original categories of area networks, while the others have gradually emerged over many years of technology evolution.
Note that these network types are a separate concept from network topologies such as bus, ring and star.
In general, an NE can generate two types of maintenance information:
Information related to the quality or health of the transmission signal, and
Information related to its own internal hardware/software integrity.
The functional components of surveillance are performance monitoring and alarm/status monitoring, also known as alarm surveillance. In the national and international standards area for telecommunications operations, performance monitoring and alarm surveillance are classified as subcategories of the more general system management functional categories of performance management and fault management, respectively. Maintenance consists of both preventive and corrective procedures that are designed to (a) prevent troubles and identify potential troubles before they affect service, and (b) detect a network failure that impacts performance and make the appropriate repair(s). A typical seven-step maintenance process consists of:
Trouble Detection — Detect trouble by continuous monitoring, periodic tests, per-call or other pre-action tests, or other automatic processes.
Trouble Notification — Send notification of a specific event or condition to a local display or Operations System (OS). Trouble notifications include output messages and visual and audible alarms.
Service Recovery — Minimize the degradation of service by automatic or manual protection actions.
Trouble Verification — Determine whether the reported condition still exists.
Trouble Isolation — Isolate the trouble to its source, preferably to a single field-repairable element, e.g., circuit pack.
Repair — Fix or replace the faulty element.
Repair Verification and Return to Service — Verify that the trouble has been fixed and return the element to service.
Telcordia GR-474 establishes trouble-detecting and reporting criteria for signal transmission failures and internal hardware and/or software anomalies. GR-474 provides proposed generic requirements that pertain to the Fault and Performance Management functions in transport and switching Network Elements (NEs) used for alarm surveillance and control.
A network element is usually defined as a manageable logical entity uniting one or more physical devices. This allows distributed devices to be managed in a unified way using one management system. According to Telecommunications Act of 1996, the term `network element’ means a facility or equipment used in the provision of a telecommunications service. Such term also includes features, functions, and capabilities that are provided by means of such facility or equipment, including subscriber numbers, databases, signaling systems, and information sufficient for billing and collection or used in the transmission, routing, or other provision of a telecommunications service.
Gaano nga ba kahalaga ang nag-iisang ikaw sa mundong ibabaw? Una, ‘pag wala ka, wala rin ako. Pangalawa, ikaw ang tanging laman ng aking buhay na puso. Pangatlo, una ka sa lahat ng tao. Pangapat, panglima, anim, pito at marami pang ibang bagay na sumasagot sa tanong ko. Pero may iisang tanong na naglalaro sa isipan ko, kelan ko kaya mararamdaman ang halaga na hinahanap ko sa iyo? Kelan nga ba? ‘Pag ba wala na ako? Kapag ba iniiyakan na ako ng lahat ng tao sa bahay? Kapag ba lulan na ako ng isang kahon na puti, kasabay ng paglatag ng mga bulaklak sa aking tabi? Kapag ba ipinagtitirik mo na ako ng kandila kung saan man ako ilalagak? Kelan? Kelan pa?
Sana, makaramdam ka ng saya habang ako’y buhay pa. Sana, marinig ko naman kahit minsan man lang, ang mga katagang noon pa’y di ko pa narinig. Sana, maipakita mo naman ang tunay kong halaga…
(lahat ng laman ng maikling storya ay tanging kathang isip at gawa-gawa lamang ng mapaglarong isipan…kung may mga pangyayaring kahawig ng mga pangyayaring naganap po sa inyo, hindi po sadya ng manunulat…salamat…)
Kaibigan ko si Lea, may kaliitan, maikli kung manamit, bilog ang mukha, morena, sa murang edad masasabi mo ng pang-dalaga ang kaniyang dating.
Isa siyang batikan na manunulat sa kanilang pamantasan, at paminsan-minsan umeextra sa mga gawaing pang-photojourn. siguro’y nais niyang matuto nito.pra siguro, bukod sa pagsusulat,ay may iba pa siyang alam na gawin.minsan, nilinis niya ang camera,walang ibang tao sa opisina, siya lang. at tanging siya lang…kaya minabuti niyang kuhanin ito sa maalikabok at maagiw na kahon na kinalalagyan ng camera. nilinis niya gamit ang isang pirasong tissue ng pinagkainan niyang restoran. hinipan niya ang salamin pra maaninag ang LCD, sa sobrang dumi, sa sobrang kapal ng alikabok na nakapalibot sa camera, halatang hindi ito inaalagaan ng maayos.
“Hay naku! Kung di kita nilinis, wala kang ganda sa mas makabagong camera ngayon. Ayan, pumogi kana”, sambit ni Lea sa camera,wari’y may tenga ang camera at makakasagot ito gamit ang shutter nito.
Sinindihan ni Lea, pinagkunan niya ito sa kaniyang sarili, at matapos ang limang imahe’y bumukas ang pinto. “Hoy! bakit hawak mo yan?”, pasigaw na tanong ng kanilang patnugot sa dyaryo. Di mka-kibo ang dalagita, at napa-iling lang sa gulat. Kasunod ng adviser, dumating din ang isa sa mga editor nila. “Ma’am, palagi niya pong hawak yan at puro non-sense things lang po kinukuhanan niya”.
Sinamsam ng editor, at pinakita ang camera sa adviser,tumambad sa mata ng guro ang mga larawan ni Lea na kuha niya.
Kinausap si Lea…
Makalipas ang limang oras na pulong…
“Naiiyak parin ako”,bulong ni Lea sa srili habang naglalakad sa madilim na pasilyo.
Hapon habang nagpapahinga ang lahat ng mga tao sa knilang sara-sariling bahay. ako’y nag- iisip. sadyang npakaimportante ng bagay na bglang pumasok sa aking isipan…
ang magpalit ng TITLE sa tumblr… so wala ako magawa, gusto ko lang tlga palitan ito… dati, “THE UNTITLED”, bakit? ahahaha wala lang… after 5mins na pagkatagal- tagal na pag- iisip, may nagsajest, “MR. INCREDIBLE”… DI NA IMPORTANTE SAKIN KUNG BAKIT YUN ANG NASABI NYA… PERO RAMDAM KO ANG TAGLAY NG KNIYANG PAGSASALITA, NA AKO ANG KNIYANG mr. incredible…