Delay and Delay VariationUsers of i

Delay and Delay Variation
Users of interactive applications expect minimal delay in receiving feedback from the network. Voice and video applications also require minimal delay. In addition, voice and
video applications require a minimal variation in the amount of delay that packets
Chapter 2: Analyzing Technical Goals and Tradeoffs 41
experience. Variations in delay, called jitter, cause disruptions in voice quality and
jumpiness in video streams.
Applications that use the Telnet protocol are also sensitive to delay because the user
expects quick feedback when typing characters. Telnet is becoming obsolete, but it hasn’t
disappeared yet. With the Telnet remote echo option, the character typed by a user doesn’t appear on the screen until it has been acknowledged and echoed by the far end, and
the near end has sent an acknowledgment for the echo. To help you recognize the need to
design a network with low delay, you should determine if your customer plans to run any
delay-sensitive applications, such as voice or video, or applications based on delay-sensitive
protocols such as Telnet.
Causes of Delay
Any goals regarding delay must take into account fundamental physics. Despite science
fiction stories that say differently, any signal experiences a propagation delayresulting
from the finite speed of light, which is about 300,000 kilometers per second (186,000
miles per second). Network designers can also remember 1 nanosecond per foot. These
values are for light traveling in a vacuum. A signal in a cable or optical fiber travels
approximately two-thirds the speed of light in a vacuum.
Delay is relevant for all data transmission technologies but especially for satellite links
and long terrestrial cables. Geostationary satellites are in orbit above the earth at a height
of about 36,000 kilometers, or 24,000 miles. This long distance leads to a propagation
delay of about 270 milliseconds (ms) for an intercontinental satellite hop. In the case of
terrestrial cable connections, propagation delay is about 1 ms for every 200 kilometers
(120 miles).
Another fundamental cause for delay is serialization delay, the time to put digital data
onto a transmission line, which depends on the data volume and the speed of the line. For
example, to transmit a 1024-byte packet on a 1.544-Mbps T1 line takes about 5 ms.
An additional fundamental delay is packet-switching delay. Packet-switching delayrefers
to the latency accrued when switches and routers forward data. The latency depends on
the speed of the internal circuitry and CPU, and the switching architecture of the internetworking device. Latency also depends on the type of RAM that the device uses.
Dynamic RAM (DRAM) needs to be refreshed thousands of times per second. Static
RAM (SRAM) doesn’t need to be refreshed, which makes it faster, but it is also more
expensive than DRAM. Low-end internetworking devices often use DRAM to keep the
cost low.
Packet-switching delay can be quite small on high-end switches, in the 5- to 20-microsecond range for 64-byte Ethernet frames. Routers tend to introduce more latency than
switches. The amount of latency that a router causes for packet switching depends on
many variables, including the router architecture, configuration, and software features
that optimize the forwarding of packets. Despite marketing claims by switch salespeople,
you should not assume that a router has higher latency than a switch. A high-end router

Kết quả (Việt) 1:

Delay and Delay VariationUsers of interactive applications expect minimal delay in receiving feedback from the network. Voice and video applications also require minimal delay. In addition, voice andvideo applications require a minimal variation in the amount of delay that packetsChapter 2: Analyzing Technical Goals and Tradeoffs 41experience. Variations in delay, called jitter, cause disruptions in voice quality andjumpiness in video streams.Applications that use the Telnet protocol are also sensitive to delay because the userexpects quick feedback when typing characters. Telnet is becoming obsolete, but it hasn’tdisappeared yet. With the Telnet remote echo option, the character typed by a user doesn’t appear on the screen until it has been acknowledged and echoed by the far end, andthe near end has sent an acknowledgment for the echo. To help you recognize the need todesign a network with low delay, you should determine if your customer plans to run anydelay-sensitive applications, such as voice or video, or applications based on delay-sensitiveprotocols such as Telnet.Causes of DelayAny goals regarding delay must take into account fundamental physics. Despite sciencefiction stories that say differently, any signal experiences a propagation delayresultingfrom the finite speed of light, which is about 300,000 kilometers per second (186,000miles per second). Network designers can also remember 1 nanosecond per foot. Thesegiá trị cho ánh sáng đi du lịch trong chân không. Một tín hiệu trong một cáp hoặc sợi quang đikhoảng hai phần ba tốc độ ánh sáng trong chân không.Sự chậm trễ là có liên quan cho các công nghệ truyền tải dữ liệu tất cả nhưng đặc biệt là cho các vệ tinh liên kếtvà dây cáp dài trên mặt đất. Vệ tinh địa tĩnh trên quỹ đạo trên trái đất ở độ caokhoảng 36.000 km, hoặc 24.000 dặm. Điều này dài đường dẫn đến một tuyên truyềnsự chậm trễ của khoảng 270 mili giây (ms) cho một vệ tinh liên lục địa hop. Trong trường hợp củakết nối cáp trên đất liền, trễ truyền là khoảng 1 ms cho mỗi 200 km(120 dặm).Một nguyên nhân cơ bản của sự chậm trễ là đăng trên sự chậm trễ, thời gian để đưa dữ liệu kỹ thuật sốlên một đường truyền, mà phụ thuộc vào khối lượng dữ liệu và tốc độ của dòng. ChoVí dụ, để truyền tải một gói tin 1024 byte trên một dòng 1.544 Mbps T1 mất khoảng 5 ms.Một sự chậm trễ thêm cơ bản là gói chuyển mạch chậm trễ. Chuyển đổi gói delayrefersvới độ trễ tích luỹ khi thiết bị chuyển mạch và định tuyến chuyển dữ liệu. Độ trễ phụ thuộc vàotốc độ của các mạch nội bộ và CPU, và kiến trúc chuyển thiết bị liên mạng. Độ trễ cũng phụ thuộc vào loại RAM có các thiết bị sử dụng.Bộ nhớ RAM động (DRAM) cần phải được làm mới hàng nghìn lần mỗi giây. Tĩnh điệnRAM (SRAM) không phải là làm mới, mà làm cho nó nhanh hơn, nhưng nó cũng là nhiều hơn nữađắt hơn DRAM. Low-End liên mạng thiết bị thường xuyên sử dụng DRAM để giữ cho cáccost low.Packet-switching delay can be quite small on high-end switches, in the 5- to 20-microsecond range for 64-byte Ethernet frames. Routers tend to introduce more latency thanswitches. The amount of latency that a router causes for packet switching depends onmany variables, including the router architecture, configuration, and software featuresthat optimize the forwarding of packets. Despite marketing claims by switch salespeople,you should not assume that a router has higher latency than a switch. A high-end router