3.2 CIRCUITS3.2.1 Circuit Configura

3.2 CIRCUITS
3.2.1 Circuit Configuration
Circuit configuration is the basic physical layout of the circuit. There are two fundamental circuit configurations: point-to-point and multipoint. In practice, most complex
computer networks have many circuits, some of which are point-to-point and some of
which are multipoint.
Figure 3.1 illustrates a point-to-point circuit, which is so named because it goes
from one point to another (e.g., one computer to another computer). These circuits sometimes are called dedicated circuits because they are dedicated to the use of these two
computers. This type of configuration is used when the computers generate enough data
to fill the capacity of the communication circuit. When an organization builds a network
using point-to-point circuits, each computer has its own circuit running from itself to the other computers. This can get very expensive, particularly if there is some distance
between the computers.
Figure 3.2 shows a multipoint circuit (also called a shared circuit). In this configuration, many computers are connected on the same circuit. This means that each must
share the circuit with the others. The disadvantage is that only one computer can use the
circuit at a time. When one computer is sending or receiving data, all others must wait.
The advantage of multipoint circuits is that they reduce the amount of cable required and
typically use the available communication circuit more efficiently. Imagine the number
of circuits that would be required if the network in Figure 3.2 was designed with separate point-to-point circuits. For this reason, multipoint configurations are cheaper than
point-to-point circuits. Thus, multipoint circuits typically are used when each computer
does not need to continuously use the entire capacity of the circuit or when building
point-to-point circuits is too expensive. Wireless circuits are almost always multipoint
circuits because multiple computers use the same radio frequencies and must take turns
transmitting.
3.2.2 Data Flow
Circuits can be designed to permit data to flow in one direction or in both directions. Actually, there are three ways to transmit: simplex, half-duplex, and full-duplex (Figure 3.3).
Simplex transmission is one-way transmission, such as that with radios and TVs.
Half-duplex transmission is two-way transmission, but you can transmit in only
one direction at a time. A half-duplex communication link is similar to a walkie-talkie
link; only one computer can transmit at a time. Computers use control signals to negotiate which will send and which will receive data. The amount of time half-duplex communication takes to switch between sending and receiving is called turnaround time (also
called retrain time or reclocking time). The turnaround time for a specific circuit can
be obtained from its technical specifications (often between 20 and 50 milliseconds).
Europeans sometimes use the term simplex circuit to mean a half-duplex circuit.
With full-duplex transmission, you can transmit in both directions simultaneously,
with no turnaround time.
How do you choose which data flow method to use? Obviously, one factor is the
application. If data always need to flow only in one direction (e.g., from a remote sensor
to a host computer), then simplex is probably the best choice. In most cases, however,
data must flow in both directions.
The initial temptation is to presume that a full-duplex channel is best; however,
each circuit has only so much capacity to carry data. Creating a full-duplex circuit means
that the available capacity in the circuit is divided—half in one direction and half in the
other. In some cases, it makes more sense to build a set of simplex circuits in the same
way a set of one-way streets can increase the speed of traffic. In other cases, a half-duplex
circuit may work best. For example, terminals connected to mainframes often transmit
data to the host, wait for a reply, transmit more data, and so on, in a turn-taking process;
usually, traffic does not need to flow in both directions simultaneously. Such a traffic
pattern is ideally suited to half-duplex circuits.
3.2.3 Multiplexing
Multiplexing means to break one high-speed physical communication circuit into several
lower-speed logical circuits so that many different devices can simultaneously use it but
still “think” that they have their own separate circuits (the multiplexer is “transparent”). It
is multiplexing (specifically, wavelength division multiplexing [WDM], discussed later
in this section) that has enabled the almost unbelievable growth in network capacity
discussed in Chapter 1; without WDM, the Internet would have collapsed in the 1990s.
Multiplexing often is done in multiples of 4 (e.g., 8, 16). Figure 3.4 shows a
four-level multiplexed circuit. Note that two multiplexers are needed for each circuit:one to combine the four original circuits into the one multiplexed circuit and one to
separate them back into the four separate circuits.
The primary benefit of multiplexing is to save money by reducing the amount of
cable or the number of network circuits that must be installed. For example, if we did not
use multiplexers in Figure 3.4, we would need to run four separate circuits from the clients
to the server. If the clients were located close to the server, this would be inexpensive.
However, if they were located several miles away, the extra costs could be substantial.
There are four types of multiplexing: frequency division multiplexing (FDM), time
division multiplexing (TDM), statistical time division multiplexing (STDM), and wavelength division multiplexing WDM.
Frequency Division Multiplexing Frequency division multiplexing (FDM) can be
described as dividing the circuit “horizontally” so that many signals can travel a single
communication circuit simultaneously. The circuit is divided into a series of separate
channels, each transmitting on a different frequency, much like series of different radio
or TV stations. All signals exist in the media at the same time, but because they are on
different frequencies, they do not interfere with each other.
Figure 3.5 illustrates the use of FDM to divide one circuit into four channels. Each
channel is a separate logical circuit, and the devices connected to them are unaware that
their circuit is multiplexed. In the same way that radio stations must be assigned separate
frequencies to prevent interference, so must the signals in a FDM circuit. The guardbands
in Figure 3.5 are the unused portions of the circuit that separate these frequencies from
each other.
With FDM, the total capacity of the physical circuit is simply divided among the
multiplexed circuits. For example, suppose we had a physical circuit with a data rate of
64 Kbps that we wanted to divide into four circuits. We would simply divide the 64 Kbps
among the four circuits and assign each circuit 16 Kbps. However, because FDM needs
guardbands, we also have to allocate some of the capacity to the guardbands, so we might
actually end up with four circuits, each providing 15 Kbps, with the remaining 4 Kbps
allocated to the guardbands. There is no requirement that all circuits be the same size,
as you will see in a later section. FDM was commonly used in older telephone systems,
which is why the bandwidth on older phone systems was only 3,000 Hz, not the 4,000 Hz
actually available—1,000 Hz were used as guardbands, with the voice signals traveling
between two guardbands on the outside of the channel.
Time Division Multiplexing Time division multiplexing (TDM) shares a communication circuit among two or more computers by having them take turns, dividing the
circuit vertically, so to speak. Figure 3.6 shows the same four computers connected using
TDM. In this case, one character is taken from each computer in turn, transmitted down
the circuit, and delivered to the appropriate device at the far end (e.g., one character from
computer A, then one from B, one from C, one from D, another from A, another from
B, and so on). Time on the circuit is allocated even when data are not being transmitted,
so that some capacity is wasted when terminals are idle. TDM generally is more efficient
than FDM because it does not need guardbands. Guardbands use “space” on the circuit
that otherwise could be used to transmit data. Therefore, if one divides a 64-Kbps circuit
into four circuits, the result would be four 16-Kbps circuits.
Statistical Time Division Multiplexing Statistical time division multiplexing
(STDM) is the exception to the rule that the capacity of the multiplexed circuit must
equal the sum of the circuits it combines. STDM allows more terminals or computers to be connected to a circuit than does FDM or TDM. If you have four computers
connected to a multiplexer and each can transmit at 64 Kbps, then you should have a
circuit capable of transmitting 256 Kbps (4 × 64 Kbps). However, not all computers
will be transmitting continuously at their maximum transmission speed. Users typically
pause to read their screens or spend time typing at lower speeds. Therefore, you do not
need to provide a speed of 256 Kbps on this multiplexed circuit. If you assume that
only two computers will ever transmit at the same time, 128 Kbps would be enough.
STDM is called statistical because selection of transmission speed for the multiplexed
circuit is based on a statistical analysis of the usage requirements of the circuits to be
multiplexed.
The key benefit of STDM is that it provides more efficient use of the circuit
and saves money. You can buy a lower-speed, less-expensive circuit than you could
using FDM or TDM. STDM introduces two additional complexities. First, STDM can
cause time delays. If all devices start transmitting or receiving at the same time (or
just more than at the statistical assumptions), the multiplexed circuit cannot transmit
all the data it receives because it does not have sufficient capacity. Therefore, STDM
must have internal memor

Kết quả (Việt) 1:

3.2 MẠCH3.2.1 mạch cấu hìnhCấu hình mạch là bố trí vật lý cơ bản của các mạch. Có hai cơ bản mạch cấu hình: điểm-điểm và đa điểm. Trong thực tế, phức tạp nhấtmạng máy tính có nhiều mạch, một số trong đó là điểm và một sốđó là đa.3.1 hình minh hoạ một mạch điểm-điểm, được đặt tên như vậy bởi vì nó đitừ một điểm khác (ví dụ, một máy tính với một máy tính). Các mạch đôi khi được gọi là mạch chuyên dụng bởi vì họ dành riêng cho việc sử dụng cả haiMáy vi tính. Loại cấu hình được sử dụng khi các máy tính tạo ra đủ dữ liệuđể điền vào năng lực của các mạch giao tiếp. Khi một tổ chức xây dựng một mạng lướisử dụng điểm mạch, mỗi máy tính có riêng của mình mạch chạy từ chính nó với các máy tính khác. Điều này có thể nhận được rất tốn kém, đặc biệt là nếu có một số khoảng cáchgiữa các máy tính.3.2 con số cho thấy một mạch đa (tiếng Anh thường gọi là một mạch được chia sẻ). Trong cấu hình này, nhiều máy tính được kết nối trên các mạch tương tự. Điều này có nghĩa rằng mỗi phảichia sẻ mạch với những người khác. Những bất lợi là rằng chỉ có một máy tính có thể sử dụng cácMạch tại một thời điểm. Khi một máy tính việc gửi hoặc nhận dữ liệu, tất cả những người khác phải chờ đợi.Lợi dụng đa mạch là họ làm giảm lượng cáp cần thiết vàthường sử dụng các vi mạch sẵn giao tiếp hiệu quả hơn. Hãy tưởng tượng sốmạch đó sẽ được yêu cầu nếu mạng trong hình 3.2 được thiết kế với điểm tới điểm riêng biệt mạch. Vì lý do này, cấu hình đa rẻ hơnđiểm mạch. Vì vậy, đa mạch thường được sử dụng khi máy tính mỗikhông cần phải liên tục sử dụng toàn bộ năng lực của các mạch hoặc khi xây dựngđiểm mạch là quá đắt. Không dây mạch được hầu như luôn luôn đamạch vì nhiều máy tính sử dụng cùng một tần số vô tuyến và phải quaytruyền.3.2.2 data FlowMạch có thể được thiết kế để cho phép các dữ liệu lưu trong một hướng hoặc trong cả hai hướng. Trên thực tế, có ba cách để truyền tải: simplex, half-duplex và đầy đủ-song (con số 3.3).Simplex truyền là một chiều truyền, chẳng hạn như rằng với Radio và TV.Half-duplex truyền là hai chiều truyền, nhưng bạn có thể truyền tải trong chỉmột hướng tại một thời điểm. Một nửa-song thông tin liên lạc liên kết là tương tự như bộ đàmliên kết; chỉ có một máy tính có thể truyền tải một lúc. Máy tính sử dụng tín hiệu điều khiển để thương lượng đó sẽ gửi và đó sẽ nhận được dữ liệu. Số lượng thời gian nửa-song thông tin liên lạc cần để chuyển đổi giữa việc gửi và nhận (cũng là thời gian gọi là quay vòngđào tạo lại được gọi là thời gian hay thời gian reclocking). Thời gian quay vòng cho một mạch cụ thể có thểđược lấy từ các thông số kỹ thuật (thường giữa 20 và 50 mili giây).Người châu Âu đôi khi sử dụng các thuật ngữ simplex mạch để có nghĩa là một nửa-song mạch.Với đầy đủ-đôi truyền, bạn có thể truyền trong cả hai hướng cùng một lúc,không có thời gian quay vòng.Làm thế nào để bạn chọn dữ liệu dòng chảy phương pháp để sử dụng? Rõ ràng, một yếu tố là cácứng dụng. Nếu dữ liệu luôn luôn cần phải chảy chỉ theo một hướng (ví dụ như, từ một bộ cảm biến từ xađến một máy chủ), simplex sau đó có lẽ là lựa chọn tốt nhất. Trong nhiều trường hợp, Tuy nhiên,dữ liệu phải chảy trong cả hai hướng.Cám dỗ ban đầu là để đoán rằng một kênh đầy đủ-kép là tốt nhất; Tuy nhiên,mỗi mạch có chỉ rất nhiều năng lực để thực hiện dữ liệu. Tạo một mạch song công toàn có nghĩa làcó khả năng trong các mạch được chia — một nửa trong một hướng và một nửa trong cáckhác. Trong một số trường hợp, nó làm cho ý nghĩa hơn để xây dựng một bộ simplex mạch trong cùng mộtcách một tập hợp các đường một chiều có thể tăng tốc độ của lưu lượng truy cập. Trong trường hợp khác, một nửa-songmạch có thể làm việc tốt nhất. Ví dụ, thiết bị đầu cuối kết nối với máy tính lớn thường truyềncác dữ liệu đến máy chủ, chờ đợi cho một thư trả lời, truyền tải thêm dữ liệu, và như vậy, trong một quá trình biến-uống;thông thường, lưu lượng truy cập không phải chảy trong cả hai chiều đồng thời. Lưu lượng truy cậpMô hình lý tưởng nhất là phù hợp với một nửa-song mạch.3.2.3 ghép kênhGhép kênh có nghĩa là để phá vỡ một tốc độ cao giao tiếp vật lý mạch thành nhiềuthấp hơn tốc độ hợp lý mạch thiết bị khác nhau vì vậy mà nhiều người có thể đồng thời sử dụng nó nhưngstill “think” that they have their own separate circuits (the multiplexer is “transparent”). Itis multiplexing (specifically, wavelength division multiplexing [WDM], discussed laterin this section) that has enabled the almost unbelievable growth in network capacitydiscussed in Chapter 1; without WDM, the Internet would have collapsed in the 1990s.Multiplexing often is done in multiples of 4 (e.g., 8, 16). Figure 3.4 shows afour-level multiplexed circuit. Note that two multiplexers are needed for each circuit:one to combine the four original circuits into the one multiplexed circuit and one toseparate them back into the four separate circuits.The primary benefit of multiplexing is to save money by reducing the amount ofcable or the number of network circuits that must be installed. For example, if we did notuse multiplexers in Figure 3.4, we would need to run four separate circuits from the clientsto the server. If the clients were located close to the server, this would be inexpensive.However, if they were located several miles away, the extra costs could be substantial.There are four types of multiplexing: frequency division multiplexing (FDM), timedivision multiplexing (TDM), statistical time division multiplexing (STDM), and wavelength division multiplexing WDM.Frequency Division Multiplexing Frequency division multiplexing (FDM) can bedescribed as dividing the circuit “horizontally” so that many signals can travel a singlecommunication circuit simultaneously. The circuit is divided into a series of separatechannels, each transmitting on a different frequency, much like series of different radioor TV stations. All signals exist in the media at the same time, but because they are ondifferent frequencies, they do not interfere with each other.Figure 3.5 illustrates the use of FDM to divide one circuit into four channels. Eachchannel is a separate logical circuit, and the devices connected to them are unaware thattheir circuit is multiplexed. In the same way that radio stations must be assigned separatefrequencies to prevent interference, so must the signals in a FDM circuit. The guardbandsin Figure 3.5 are the unused portions of the circuit that separate these frequencies fromeach other.With FDM, the total capacity of the physical circuit is simply divided among themultiplexed circuits. For example, suppose we had a physical circuit with a data rate of64 Kbps that we wanted to divide into four circuits. We would simply divide the 64 Kbpsamong the four circuits and assign each circuit 16 Kbps. However, because FDM needsguardbands, we also have to allocate some of the capacity to the guardbands, so we mightactually end up with four circuits, each providing 15 Kbps, with the remaining 4 Kbpsallocated to the guardbands. There is no requirement that all circuits be the same size,as you will see in a later section. FDM was commonly used in older telephone systems,which is why the bandwidth on older phone systems was only 3,000 Hz, not the 4,000 Hzactually available—1,000 Hz were used as guardbands, with the voice signals travelingbetween two guardbands on the outside of the channel.Time Division Multiplexing Time division multiplexing (TDM) shares a communication circuit among two or more computers by having them take turns, dividing thecircuit vertically, so to speak. Figure 3.6 shows the same four computers connected usingTDM. In this case, one character is taken from each computer in turn, transmitted downthe circuit, and delivered to the appropriate device at the far end (e.g., one character fromcomputer A, then one from B, one from C, one from D, another from A, another fromB, and so on). Time on the circuit is allocated even when data are not being transmitted,so that some capacity is wasted when terminals are idle. TDM generally is more efficientthan FDM because it does not need guardbands. Guardbands use “space” on the circuitthat otherwise could be used to transmit data. Therefore, if one divides a 64-Kbps circuitinto four circuits, the result would be four 16-Kbps circuits.Statistical Time Division Multiplexing Statistical time division multiplexing(STDM) is the exception to the rule that the capacity of the multiplexed circuit mustequal the sum of the circuits it combines. STDM allows more terminals or computers to be connected to a circuit than does FDM or TDM. If you have four computersconnected to a multiplexer and each can transmit at 64 Kbps, then you should have acircuit capable of transmitting 256 Kbps (4 × 64 Kbps). However, not all computerswill be transmitting continuously at their maximum transmission speed. Users typicallypause to read their screens or spend time typing at lower speeds. Therefore, you do notneed to provide a speed of 256 Kbps on this multiplexed circuit. If you assume thatonly two computers will ever transmit at the same time, 128 Kbps would be enough.STDM is called statistical because selection of transmission speed for the multiplexedcircuit is based on a statistical analysis of the usage requirements of the circuits to bemultiplexed.The key benefit of STDM is that it provides more efficient use of the circuitand saves money. You can buy a lower-speed, less-expensive circuit than you couldusing FDM or TDM. STDM introduces two additional complexities. First, STDM cancause time delays. If all devices start transmitting or receiving at the same time (orjust more than at the statistical assumptions), the multiplexed circuit cannot transmitall the data it receives because it does not have sufficient capacity. Therefore, STDMmust have internal memor