When you purchase a mobile phone or a PC, the specifications list tells you the important characteristics of the device, just as specific characteristics of a network help describe its performance and structure. When you understand what each characteristic of a network means, you can better understand how the network is designed, how it performs, and which aspects you may need to adjust to meet user expectations.
當你購買手機或個人電腦時，規格表會告訴你該裝置的重要特性；同樣地，網路的特定特性也有助於描述其效能與結構。當你了解網路的每項特性代表什麼意義，就能更清楚網路是如何設計、如何運作，以及需要調整哪些面向才能符合使用者的期望。
You can describe the qualities and features of a network by considering these characteristics:
你可以從下列特性來描述網路的品質與功能：
- Topology: A network topology is the arrangement of its elements. Topologies give insight into physical connections and data flows among devices. In a carefully designed network, data flows are optimized, and the network performs as desired.拓樸（Topology）：網路拓樸是網路各元素的排列方式。拓樸讓我們了解裝置之間的實體連接與資料流向。在精心設計的網路中，資料流會被最佳化，網路也能依預期運作。
- Bitrate or Bandwidth: Bitrate measures the data rate in bits per second (bps) of a given link in the network. This measure is often referred to as bandwidth or speed in device configurations, which is sometimes thought of as speed. However, it is not about how fast 1 bit is transmitted over a link—which is determined by the physical properties of the medium that propagates the signal—it is about the number of bits transmitted in a second. Link bitrates commonly encountered today are 1 and 10 gigabits per second (1 or 10 billion bits per second). 100-Gbps links are also not uncommon.位元率（Bitrate）或頻寬（Bandwidth）：位元率衡量網路中某條鏈路的資料傳輸速率，單位為每秒位元數（bps）。在裝置設定中，這個量測值常被稱為頻寬或速度，有時也被視為速度。然而，它並不是指 1 個位元在鏈路上傳送得多快（那是由傳遞訊號的媒介之實體特性所決定），而是指一秒內傳送的位元數。現今常見的鏈路位元率為每秒 1 和 10 gigabit（每秒 10 億或 100 億位元）。100 Gbps 的鏈路也並不罕見。
- Availability: Availability indicates how much time a network is accessible and operational. Availability is expressed in terms of the percentage of time the network is operational. This percentage is calculated as a ratio of the time in minutes that the network is available and the total number of minutes over an agreed period, multiplied by 100. In other words, availability is the ratio of uptime and total time, expressed in percentage. To help ensure high availability, networks should be designed to limit the impact of failures and to allow quick recovery when a failure does occur. High availability design usually incorporates redundancy. The redundant design includes extra elements, which serve as backups to the primary elements and take over the functionality if the primary element fails. Examples include redundant links, components, and devices.可用性（Availability）：可用性表示網路可存取且正常運作的時間有多少。可用性以網路正常運作時間的百分比表示。這個百分比的算法是：網路可用的分鐘數除以約定期間內的總分鐘數，再乘以 100。換句話說，可用性就是正常運行時間與總時間的比率，以百分比表示。為了確保高可用性，網路設計應限制故障的影響範圍，並在故障確實發生時能快速復原。高可用性設計通常會納入備援。備援設計包含額外的元素，作為主要元素的備份，並在主要元素故障時接手其功能。例如備援鏈路、元件與裝置。
- Reliability: Reliability indicates how well the network operates. It considers the ability of a network to operate without failures and with the intended performance for a specified time period. In other words, it tells you how much you can count on the network to operate as you expect it to. For a network to be reliable, the reliability of all its components should be considered. Highly reliable networks are highly available, but a highly available network might not be highly reliable—its components might operate at lower performance levels. A standard measure of reliability is the mean time between failures (MTBF), which is calculated as the ratio between the total time in service and the number of failures. Not meeting the required performance level is considered a failure. Choosing highly reliable redundant components in the network design increases both availability and reliability.可靠性（Reliability）：可靠性表示網路運作得有多好。它考量的是網路在指定期間內無故障、且以預期效能運作的能力。換句話說，它告訴你能有多大程度地信賴網路會如你所期望地運作。要讓網路可靠，就必須考量其所有元件的可靠性。高可靠性的網路必然具有高可用性，但高可用性的網路未必高可靠，因為其元件可能以較低的效能水準運作。可靠性的標準量測值是平均故障間隔時間（MTBF），其計算方式為總服務時間與故障次數的比率。未達到要求的效能水準也視為故障。在網路設計中選用高可靠性的備援元件，可同時提高可用性與可靠性。
For instance, let’s consider a networking device that reboots every hour. The reboot takes 5 minutes, after which the device works as expected. The figure shows the calculations of availability and reliability.
舉例來說，假設有一台網路裝置每小時重新開機一次。重新開機需要 5 分鐘，之後裝置即可正常運作。下圖顯示可用性與可靠性的計算方式。
The availability percentage for one day can be calculated as follows:
一天的可用性百分比可依下列方式計算：
- Scalability: Scalability indicates how easily the network can accommodate more users and data transmission requirements without affecting current network performance. If you design and optimize a network only for the current conditions, it can be costly and difficult to meet new needs when the network grows.可擴充性（Scalability）：可擴充性表示網路能多容易地容納更多使用者與資料傳輸需求，而不影響目前的網路效能。如果你只針對目前的狀況設計與最佳化網路，當網路成長時，要滿足新需求可能既昂貴又困難。
- Security: Security tells you how well the network is defended from potential threats. Both network infrastructure and the information that is transmitted over the network should be secured. The subject of security is important, and defense techniques and practices are constantly evolving. You should consider security whenever you take actions that affect the network.安全性（Security）：安全性告訴你網路對潛在威脅的防禦程度如何。網路基礎架構以及在網路上傳送的資訊都應受到保護。安全性是重要的課題，防禦技術與實務也不斷演進。凡是採取會影響網路的行動時，都應該考量安全性。
- Quality of Service (QoS): QoS includes tools, mechanisms, and architectures, which allow you to control how and when applications use network resources. QoS is essential for prioritizing traffic when the network is congested.服務品質（QoS）：QoS 包含各種工具、機制與架構，讓你能控制應用程式如何以及何時使用網路資源。當網路壅塞時，QoS 對於流量的優先順序處理至關重要。
- Cost: Cost indicates the general expense for the initial purchase of the network components and any costs associated with installing and maintaining these components.成本（Cost）：成本表示初次購買網路元件的整體費用，以及安裝與維護這些元件的相關費用。
- Virtualization: Traditionally, network services and functions have only been provided via hardware. Network virtualization creates a software solution that emulates network services and functions. Virtualization solves many of the networking challenges in today’s networks, helping organizations centrally automate and provision the network from a central management point.虛擬化（Virtualization）：傳統上，網路服務與功能只能透過硬體提供。網路虛擬化建立一種軟體解決方案，用來模擬網路服務與功能。虛擬化解決了現今網路中的許多挑戰，協助組織從單一的集中管理點自動化並佈建網路。
These characteristics and attributes provide a means to compare various networking solutions.
這些特性與屬性提供了比較各種網路解決方案的依據。
Which three statements about network characteristics are true? (Choose three.)關於網路特性，下列哪三項敘述正確？（選擇三項。）