Английский язык для пользователей ПК и программистов - Гольцова Е.В.
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238 Урук 16
destination. The protocols that, operate at the internet layer include IP (Internet Protocol), and RIP (Route Information Protocol) among others.
Host Host Host
jade alto Routers tenor
R6 R5
Figure 20.3.
Network Access Layer
The network access layer is where media access and transmission mechanisms take place. At this layer, both the hardware and the software drivers are implemented. The protocols at this la,yer provide the means for the system to deliver data to other devices on a directly attached network. This is the only layer that is aware of the physical characteristics of the underlying network, including rules of access, data frame (name of a unit of data at this layer) structure, and addressing.
While the network access layer i.s equipped with the means for delivering data to devices on a directly attached network, it does so based on directions from IP at the internet layer. To understand the implications of this statement, look at the internetwork of Figure 20.3. Hosts jade and alto are said to belong to the same network since they are directly attached to the same physical wire. In contrast, host tenor belongs to a different network.
When a requirement arises to deliver data out of host alto, the internet layer (in particular the IP protocol) has to determine whether- the destined host is directly attached to the same network. If so, IP passes Английский для пользователей ПК_
the data packet to the network access layer and instructs it to deliver the data to the designated host. So, should, for example, the packet be destined to host jade, IP instructs the network access layer to take the necessary steps to deliver it to that host.
However, if IP on host alto is required to deliver the data packet to a host on a different network (for instance, host tenor), IP has to determine to which network the host belongs and how to get the packet there. As can be seen from the diagram, to deliver packets to host tenor, IP in host alto has to send the packet first to router Rl, then Rl in turn has to forward it to R2 (or R3), and so on, as explained in the previous subsection. Consequently, IP passes the packet on to the network access layer and instructs it to deliver the packet to router Rl. Notice how in both cases, the case of a host directly attached to same network (host jade) and the case of a host on different network (host tenor), the network access layer followed the addressing instructions imposed by IP at the internet layer. In other words, the network access layer relies on IP at the layer above it to know where to send the data.
Упражнение 14. Look through the Texts of lesson 15 and 16 and memorise the terms to the theme TCP/IP Protocol Architecture.
Просмотрите Тексты уроков 15 и 16 и повторите терминологию TCP/IP Protocol Architecture, подчеркивая термины в текстах. Грамматика
Настоящее время
§ 1. Повторение настоящего времени заставляет нас еще раз вспомнить разницу между длительным действием и кратким или постоянным, то есть разговор опять пойдет об употреблении глагольных конструкций в сравнении друг с другом.
I. Длительное действие (Present Progressive) — то, что происходит в (близко от) момент речи.
II. Краткое или постоянное действие (Present Simple) — то, что происходит вообще или часто.
Переведите примеры:
Present Progressive Present Simple
The proeram is running and we have some minutes to decide. This program runs when a computer has not less than 128 Mb of RAM.
What are they doing? (We see them now.) What й> they usually da to run the procedure? What do you da? (What's your job?)
I am leamine English. (I dm reading this self-study book now.) People usually learn foreign languages when they 0L? children.
The device, isn't working. And we don't know the reason yet. The device doesn't work. It hasn't worked for a week.
Fm livins in St Petersburg until I finish тії course of studying. I live in Moscow. I live in Russia.
Особенности употребления конструкций Progressive и Simple сохраняются и в прошедшем, и в будущем времени!
241 Английский для пользователей ПК
Теперь посмотрим список глаголов, которые «не любят» форму Progressive:
• want_
need_
prefer_
belong_
have _
like_
love _
hate _
know_
realise _
believe_
think_
suppose_
understand_
seem_
mean_
remember_
forget_
see_
hear_
Переведите глаголы, вспомните, какие из них неправильные, напомните себе три формы неправильных глаголов.
Обратите внимание, все эти глаголы на рисунке-схеме (если бы мы ее нарисовали) вполне могли бы расположиться в голове человека: мыслительные процессы, чувства, восприятие. Действительно, трудно вообразить процесс «знаю или думаю» длительным, кратким или еще каким-то. Это просто данность. Исключения: have (как действие), think (то, что происходит в голове). Эти два глагола с такими значениями могут иметь форму Progressive.
Упражнение 1. Translate: Переведите:
