●The network layer provides services to the transport layer.It can be based on either（请作答此空）.In both cases,its main job is（72）packets from the source to the destination.In network layer,subnets can e

The network layer provides services to the transport layer．It call be based on either virtual circuits or datagrams．In both cases，its main job is（ ）packets from the source to the destination． A．dealing with B．sending C．routing D．receiving

Networks can be interconnected by different devices in the physical layer networks can be connected by (71) or hubs．which just move the bits from one network to an identical network．One layer up we find bridges and switches which operate at data link layer．They can accept (72) ，examine the MAC address and forward the frames to a different network while doing minor protocol translation in the process in me network layer，we have routers that can connect two networks，If two networks have (73) network layer，the router may be able to translate betweenthe packer formats．In the transport layer we find transport gateway，which can interface between two transport connections Finally，in the application layer，application gateways translate message (74) ．As an example，gateways between Internet e-mail and X，400 e-mail must (75) thee-mail message and change various header fields．

(68)

A．reapers

B．relays

C．packages

D．modems

The network layer provides services to the transport layer．It can be based on either (1)．In both cases，its main job is (2) packets from the source to the destination．

In network layer，subnets can easily become congested，increasing the delay and (3).

for packets．Network designers attempt to avoid congestion by proper design．Techniques include (4) policy，caching，flowcontrol，and more．

The next step beyond just dealing with congestion is to actually try to achieve a promised quality of service．The methods that can be used for this include buffering at the client，traffic shaping，resource (5)，and admission control．Approaches that have been designed for good quality of service include integrated services(including RSVP) ，differentiated services， and MPLS．

(1)

A．virtual circuits or datagrams

B．TCP or UDP

C．TCP or IP

D．IP or ARP

Together with the network layer,the transport layer is the heart of the protocol（ ）The network layer provides end-to-end（ ） delivery using datagrams or virtual circuits.The transport layer builds on the network layer to provide data transport from a process on a（ ）machine to a process on a destination machine with a desired level of reliability that is independent of the physical （ ） currently in use.It provides the abstractions that applications nedd to use the network..Without the（ ） layer,the whole concept of layered protocols would make little sense.

A.transport B.network C.hierarchy D.service A.packet B.data C.command D.record A.connection B.terminal C.source D.destination A.traffic B.connection C.lines D.networks A.network B.transport C.link D.physical

● TCP/IP communication protocol contains four layers. From bottom to top, the four layers are __(71)__.

（71）A．network interface layer, internet layer, transport layer and application layer

B．internet layer, network interface layer, transport layer and application layer

C．network interface layer, transport layer, network interface layer and application layer

D．application layer, transport layer, internet layer and network interface layer

In the open systems interconnection (OSI) reference model, "layer" means one of seven conceptually complete,(71) arranged groups of services, functions, and protocols, that extend across all open systems.Application layer provides means for the application(72) to access the OSI environment. Presentation layer provides for the selection of a common syntax for representing data.(73) layer provides the means necessary for cooperating presentation entities to organize and synchronize their dialog and to manage their data exchange. Transport layer provides a reliable end-to-end data transfer service. Network layer provides for the entities in the transport layer the means for transferring blocks of data, by(74) and switching through the network between the open systems in which those entities reside. Data link layer provides services to transfer data between network layer entities. Physical layer provides the mechanical, electrical, functional, and procedural means to establish, maintain and(75) physical connections for transfer of bits over a transmission medium.

A．rankly

B．levelly

C．layered

D．hierarchically

Networks can be interconnected by different devices. In the physical layer, networks can be connected by(66)or hubs, which just move the bits from one network to an identical network. One layer up we find bridges and switches, which operate at data link layer. They can accept(67), examine the MAC address, and forward the frames to a different network while doing minor protocol translation in the process. In the network layer, we have routers that can connect two networks. If two networks have(68)network layer, the router may be able to translate between the packet formats. In the transport layer we fmd transport gateway, which can interface between two transport connections. Finally, in the application layer, application gateways translate message(69). As an example, gateways between Internet e-mail and X.400 e-mail must(70)the e-mail message and change various header fields.

A．repeaters

B．relays

C．connectors

D．modems

●Networks can be interconnected by different devices．In the physical layer，networks can be connected by (66) or Hubs，which just move the bits from one network to an identical network．One layer up we find bridgas and swiehes,which operate at data link layer．They can accept (67) ，examine the MAC address，and forward the frames to a different network while doing minor protocol translation in the process．In the network layer，we have routers that can connect two netwoks．If two networks have (68) network layer，the router may be able to translate between the packet formats．In the transport layer we find transport gateway，which can interface between the two transport connections．Finally，in the application layer，application gateways translate message (69) ．As an example，gateways between Internet e-mail and X．400 e-mail must (70) the e-mail message and change various header fields．

(66) A．reapers

B．relays

C．connects

D．modems

(67) A．frames

B．bytes

C．packages

D．cells

(68) A．special

B．dependent

C．similar

D．dissimilar

(69) A．syntax

B．semantics

C．languages

D．format

(70) A．analyze

B．pairs

C．delete

D．create

TCP/IP communication protocol contains four layers． From bottom to top, the four layers are __（71）__．

A.network interface layer, internet layer, transport layer and application layer

B.internet layer, network interface layer, transport layer and application layer

C.network interface layer, transport layer, network interface layer and application layer

D.application layer, transport layer, internet layer and network interface layer

A. Layer 3 MPLS VPNs can forward only IP packets

B. Layer 2 MPLS VPNs can forward any network protocol

C. MPLS label paths are automatically formed based on Layer 2 frames

D. Layer 3 MPLS VPNs can forward any network protocol based on Layer 2 frames

E. In Layer 2 MPLS VPNS, the service provider controls the customer Layer 3 policies