CCNA学习：10BaseT与100BaseT详解

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"这是一份CCNA学习指南的词汇表，包含了网络技术中的一些关键术语。这份词汇表有助于读者理解10BaseT、100BaseT等 Ethernet 标准，以及A&B位信号等通信协议的基础知识。" 在学习CCNA的过程中，掌握相关的网络术语是非常重要的。本词汇表提供了对这些术语的详细解释，帮助读者深化理解。例如： 1. **10BaseT**：这是IEEE 802.3标准的一部分，是10Mbps基带以太网规范，使用两对双绞线（Category 3, 4, 或 5）进行通信，一对用于发送数据，另一对用于接收。每段的最大距离约为100米。10BaseT与以太网和IEEE 802.3标准紧密相关。 2. **100BaseT**：基于IEEE 802.3u标准，是100Mbps基带快速以太网规范，使用UTP电缆。与10BaseT相比，100BaseT在无数据传输时发送包含更多信息的链路脉冲。它也包括100BaseTX，这是一种使用两对UTP或STP线缆的100Mbps快速以太网规格，其中一对用于接收数据，另一对用于发送数据。100BaseTX的单段长度不能超过100米。 3. **100BaseTX**：与100BaseT相似，但更具体地定义了使用两对线缆来确保正确的信号定时。每个100BaseTX段的最大长度限制为100米，以保证数据传输的稳定性。此外，词汇表还提到了**A&B位信号**，这种信号在T1传输设施中使用，有时被称为“第24通道信号”。在这种方法中，24个T1子信道中的每一个都会利用每六个帧中的一个比特来发送监控信号信息，用于网络管理和控制。通过深入学习这些术语，读者可以更好地理解以太网的不同速度等级和通信协议，这对于理解和配置网络设备、解决网络问题以及规划网络架构至关重要。在CCNA的学习过程中，这份词汇表将作为强大的参考资料，帮助学生巩固基础，提升技能。

Glossary

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cost Also known as path cost, an arbitrary value, based on hop count, bandwidth, or

another calculation, that is typically assigned by a network administrator and used by the

routing protocol to compare different routes through an internetwork. Routing protocols use

cost values to select the best path to a certain destination: the lowest cost identifies the best

path. Also known as “path cost.” See also: routing metric.

count to infinity A problem occurring in routing algorithms that are slow to converge

where routers keep increasing the hop count to particular networks. To avoid this problem,

various solutions have been implemented into each of the different routing protocols. Some

of those solutions include defining a maximum hop count (defining infinity), route poising,

poison reverse, and split horizon.

CPCS Common Part Convergence Sublayer: One of two AAL sublayers that is service

dependent, it is further segmented into the CS and SAR sublayers. The CPCS prepares data

for transmission across the ATM network; it creates the 48-byte payload cells that are sent

to the ATM layer. See also: AAL and ATM layer.

CPE Customer premises equipment: Items such as telephones, modems, and terminals

installed at customer locations and connected to the service provider network.

crankback In ATM, a correction technique used when a node somewhere on a chosen path

cannot accept a connection setup request, blocking the request. The path is rolled back to an inter-

mediate node, which then uses GCAC to attempt to find an alternate path to the final destination.

CRC Cyclic redundancy check: A methodology that detects errors, whereby the frame recip-

ient makes a calculation by dividing frame contents with a prime binary divisor and compares

the remainder to a value stored in the frame by the sending node. Contrast with: checksum.

crossover cable Type of Ethernet cable that connects a switch to switch, host to host, hub

to hub, or switch to hub.

CSMA/CD Carrier Sense Multiple Access with Collision Detection: A technology defined by

the Ethernet IEEE 802.3 committee. Each device senses the cable for a digital signal before

transmitting. Also, CSMA/CD allows all devices on the network to share the same cable, but

one at a time. If two devices transmit at the same time, a frame collision will occur and a jam-

ming pattern will be sent; the devices will stop transmitting, wait a predetermined as well as

a self-imposed random amount of time, and then try to transmit again.

CSU Channel service unit: A digital mechanism that connects end-user equipment to the local

digital telephone loop. Frequently referred to along with the data service unit as CSU/DSU. See

also: DSU.

CSU/DSU Channel service unit/data service unit: Physical layer device used in wide area net-

works to convert the CPE digital signals to what is understood by the provider’s switch. A

CSU/DSU is typically one device that plugs into a RJ-45 (8-pin modular) jack, known as the

demarcation point.

CTD Cell Transfer Delay: For a given connection in ATM, the time period between a cell exit

event at the source user-network interface (UNI) and the corresponding cell entry event at the

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Glossary

destination. The CTD between these points is the sum of the total inter-ATM transmission

delay and the total ATM processing delay.

cumulative interface delay This is a Cisco term for delay of the line. The composite metric

in IGRP and EIGRP is calculated by using the bandwidth and delay of the line by default.

cut-through frame switching A frame-switching technique that flows data through a

switch so that the leading edge exits the switch at the output port before the packet finishes

entering the input port. Frames will be read, processed, and forwarded by devices that use

cut-through switching as soon as the destination address of the frame is confirmed and the

outgoing port is identified.

data circuit-terminating equipment DCE is used to provide clocking to DTE equipment.

data compression See: compression.

data direct VCC A bidirectional point-to-point virtual control connection (VCC) set up

between two LECs in ATM and one of three data connections defined by Phase 1 LAN emu-

lation. Because data direct VCCs do not guarantee QoS, they are generally reserved for UBR

and ABR connections. Compare with: control distribute VCC and control direct VCC.

data encapsulation The process in which the information in a protocol is wrapped, or con-

tained, in the data section of another protocol. In the OSI reference model, each layer encap-

sulates the layer immediately above it as the data flows down the protocol stack.

data frame Protocol Data Unit encapsulation at the Data Link layer of the OSI reference

model. Encapsulates packets from the Network layer and prepares the data for transmission

on a network medium.

datagram A logical collection of information transmitted as a Network layer unit over a

medium without a previously established virtual circuit. IP datagrams have become the primary

information unit of the Internet. At various layers of the OSI reference model, the terms cell,

frame, message, packet, and segment also define these logical information groupings.

Data Link Control layer Layer 2 of the SNA architectural model, it is responsible for the

transmission of data over a given physical link and compares somewhat to the Data Link layer

of the OSI model.

Data Link layer Layer 2 of the OSI reference model, it ensures the trustworthy transmission

of data across a physical link and is primarily concerned with physical addressing, line disci-

pline, network topology, error notification, ordered delivery of frames, and flow control. The

IEEE has further segmented this layer into the MAC sublayer and the LLC sublayer. Also

known as the link layer. Can be compared somewhat to the data link control layer of the SNA

model. See also: Application layer, LLC, MAC, Network layer, Physical layer, Presentation

layer, Session layer, and Transport layer.

data terminal equipment See: DTE.

DCC Data Country Code: Developed by the ATM Forum, one of two ATM address formats

designed for use by private networks. Compare with: ICD.

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DCE Data communications equipment (as defined by the EIA) or data circuit-terminating

equipment (as defined by the ITU-T): The mechanisms and links of a communications net-

work that make up the network portion of the user-to-network interface, such as modems.

The DCE supplies the physical connection to the network, forwards traffic, and provides a

clocking signal to synchronize data transmission between DTE and DCE devices. Compare

with: DTE.

D channel (1) Data channel: A full-duplex, 16Kbps (BRI) or 64Kbps (PRI) ISDN channel.

Compare with: B channel, E channel, and H channel. (2) In SNA, anything that provides a

connection between the processor and main storage with any peripherals.

DDP Datagram Delivery Protocol: Used in the AppleTalk suite of protocols as a connection-

less protocol that is responsible for sending datagrams through an internetwork.

DDR Dial-on-demand routing: A technique that allows a router to automatically initiate and

end a circuit-switched session per the requirements of the sending station. By mimicking keep-

alives, the router fools the end station into treating the session as active. DDR permits routing

over ISDN or telephone lines via a modem or external ISDN terminal adapter.

DE Discard Eligibility: Used in Frame Relay networks to tell a switch that a frame can be pref-

erentially discarded if the switch is too busy. The DE is a field in the frame that is turned on by

transmitting routers if the committed information rate (CIR) is oversubscribed or set to 0.

dedicated line Point-to-point connection that does not share any bandwidth.

de-encapsulation The technique used by layered protocols in which a layer removes header

information from the Protocol Data Unit (PDU) from the layer below. See: encapsulation.

default route The static routing table entry used to direct frames whose next hop is not oth-

erwise spelled out in the routing table.

delay The time elapsed between a sender’s initiation of a transaction and the first response

they receive. Also, the time needed to move a packet from its source to its destination over a

path. See also: latency.

demarc The demarcation point between the customer premises equipment (CPE) and the

telco’s carrier equipment.

demodulation A series of steps that return a modulated signal to its original form. When

receiving, a modem demodulates an analog signal to its original digital form (and, conversely,

modulates the digital data it sends into an analog signal). See also: modulation.

demultiplexing The process of converting a multiplexed signal comprising more than one

input stream back into separate output streams. See also: multiplexing.

designated bridge In the process of forwarding a frame from a segment to the root bridge,

the bridge with the lowest root path cost.

designated port Used with the Spanning Tree Protocol (STP) to designate forwarding ports.

If there are multiple links to the same network, STP will shut a port down to stop network loops.

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designated router (DR)

An OSPF router that creates LSAs for a multi-access network and is

required to perform other special tasks in OSPF operations. Multi-access OSPF networks that

maintain a minimum of two attached routers identify one router that is chosen by the OSPF

Hello protocol, which makes possible a decrease in the number of adjacencies necessary on a

multi-access network. This in turn reduces the quantity of routing protocol traffic and the

physical size of the database.

desktop layer The access layer is sometimes referred to as the desktop layer. The access layer

controls user and workgroup access to internetwork resources.

destination address The address for the network device(s) that will receive a packet.

DHCP Dynamic Host Configuration Protocol: DHCP is a superset of the BootP protocol.

This means that it uses the same protocol structure as BootP, but it has enhancements added.

Both of these protocols use servers that dynamically configure clients when requested. The two

major enhancements are address pools and lease times.

dial backup Dial backup connections are typically used to provide redundancy to Frame

Relay connections. The backup link is activated over an analog modem or ISDN.

directed broadcast A data frame or packet that is transmitted to a specific group of nodes

on a remote network segment. Directed broadcasts are known by their broadcast address,

which is a destination subnet address with all the host bits turned on.

discovery mode Also known as dynamic configuration, this technique is used by an Apple-

Talk interface to gain information from a working node about an attached network. The

information is subsequently used by the interface for self-configuration.

distance-vector protocols The distance-vector protocols find the best path to a remote net-

work by judging distance. Each time a packet goes through a router, that’s called a hop. The

route with the least number of hops to the network is determined to be the best route. How-

ever, Cisco’s IGRP is considered distance vector and uses a composite metric of bandwidth

and delay of the line to determine the best path to a remote network.

distance-vector routing algorithm In order to find the shortest path, this group of routing

algorithms reports on the number of hops in a given route, requiring each router to send its

complete routing table with each update, but only to its neighbors. Routing algorithms of this

type tend to generate loops, but they are fundamentally simpler than their link-state counter-

parts. See also: link-state routing algorithm and SPF.

distribution layer Middle layer of the Cisco three-layer hierarchical model, which helps you

design, install, and maintain Cisco hierarchical networks. The distribution layer is the point

where access layer devices connect. Routing is performed at this layer.

DLCI Data Link Connection Identifier: Used to identify virtual circuits in a Frame Relay network.

DLSw Data Link Switching: IBM developed Data Link Switching (DLSw) in 1992 to provide

support for SNA (Systems Network Architecture) and NetBIOS protocols in router-based net-

works. SNA and NetBIOS are nonroutable protocols that do not contain any logical layer 3

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network information. DLSw encapsulates these protocols into TCP/IP messages that can be

routed and is an alternative to Remote Source-Route Bridging (RSRB).

DLSw+ Cisco’s implementation of DLSw. In addition to support for the RFC standards,

Cisco added enhancements intended to increase scalability and to improve performance

and availability.

DNS Domain Name System: Used to resolve hostnames to IP addresses.

DSAP Destination Service Access Point: The service access point of a network node, specified

in the destination field of a packet. See also: SSAP and SAP.

DSR Data Set Ready: When a DCE is powered up and ready to run, this EIA/TIA-232 inter-

face circuit is also engaged.

DSU Data service unit: This device is used to adapt the physical interface on a data terminal

equipment (DTE) mechanism to a transmission facility such as T1 or E1 and is also responsible

for signal timing. It is commonly grouped with the channel service unit and referred to as the

CSU/DSU. See also: CSU.

DTE Data terminal equipment: Any device located at the user end of a user-network interface

serving as a destination, a source, or both. DTE includes devices such as multiplexers, routers,

protocol translators, and computers. The connection to a data network is made through data

communication equipment (DCE) such as a modem, using the clocking signals generated by

that device. See also: DCE.

DTR Data Terminal Ready: An activated EIA/TIA-232 circuit communicating to the DCE

the state of preparedness of the DTE to transmit or receive data.

DUAL Diffusing Update Algorithm: Used in Enhanced IGRP, this convergence algorithm

provides loop-free operation throughout an entire route’s computation. DUAL grants routers

involved in a topology revision the ability to synchronize simultaneously, while routers unaf-

fected by this change are not involved. See also: Enhanced IGRP.

DVMRP Distance Vector Multicast Routing Protocol: Based primarily on the Routing Infor-

mation Protocol (RIP), this Internet gateway protocol implements a common, condensed-

mode IP multicast scheme, using IGMP to transfer routing datagrams between its neighbors.

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