无线通信网络

1、1wireless network layerwireless communication networks无线通信网络2wireless network layerwireless network layer无线网络层3wireless network layer主要教材与参考文献（仅限本节）yi-bing lin and imrichchlamtac 著, 方旭明等译， “ 无线与移动网络结构” ，人民邮电出版社， 2002james d. solomon, mobile ip the internet unplugged, prentice hall ptr, 1998, 人民邮电出版社

2、， 2000 （影印本）4wireless network layer计算机或移动终端移动位置之后，按原来分配的 ip地址还能否寻址到该计算机？当计算机或移动终端漫游到不同的子网时，如何保证数据通信对移动的透明性，使应用进程感觉不到移动的影响？本节讨论动态寻址、移动性管理、移动切换（ handoff, handover）和移动 ip（mobile ip ）等一般原理。5wireless network layer动态寻址和移动透明性机制动态寻址可使移动台被别的用户呼通。移动性管理和切换可使移动台在移动过程中仍保持通信。严格来讲，目前的移动电话网与移动计算机网是有区别的，本课程不做严格区分。6w

3、ireless network layer动态寻址和移动透明性机制蜂窝移动电话的动态寻址和移动切换如下图所示。每一个 ms都有唯一的电话号码，每一个移动局都有一个归属位置寄存器（hlr），其中存储 ms的所有数据，包括 ms当前所在区域的信息。在移动局中还有一个访问位置寄存器（vlr），存储进入其覆盖区域的 ms的所有信息。当一台 ms从一个区域漫游到另一个区域时，其信息便记录在该区域的vlr中，并把其当前地址登记到hlr中。别的用户要和该ms通话时，不必关心该ms在什么地方，只需拨打该ms号码。移动交换机会去查询该 ms的hlr，获得其当前地址，然后再建立到当前地址的连接。7wireless

4、 network layer若ms在通话过程中进入另一个区域，它会获得新的当前地址，在该区域vlr中登记，并把新的当前地址登记到 hlr中。移动交换机会重新建立到ms的连接，拆除原来的连接，原区域vlr删除该 ms的有关信息。该操作对用户来说是透明的。8wireless network layerhlrmscmsccpstn基站归属位置寄存器公共电话交换网移动交换中心访问位置寄存器vlrvlr通信塔通信塔通信塔通信塔通信塔通信塔通信塔通信塔9wireless network layer移动通信网网络层协议移动通信网网络层协议：有线网络协议+无线网络协议，有线部分主要是涉及路由问题 (routi

5、ng protocol) ，详见计算机网络教材及参考文献。这里主要讨论在已有有线网络基础上需要添加的 “ 移动性管理功能 ” 和“ 移动切换管理功能 ” 。10wireless network layer移动性管理以下以蜂窝移动通信网络为例，说明有关概念和原理。两个概念：切换和漫游。切换：当一个移动通信网络中的用户正在通话时，此ms通过无线链路连接到一个bs上。如果移动用户移动到另一个bs覆盖的区域，则连到原bs的无线链路最终被断掉，且需要建立一条到新基站的链路，以保持继续通话。这一过程被不同地称为自动链路转换、切换。漫游： 当一个移动用户从一个pcs系统（如在 a 城的系统）移动到另一个系统

6、（如在b城的系统）时，必须告诉系统目前用户所在的位置。否则，就无法给移动用户发送业务。11wireless network layer切换检测切换需求的 3种策略：1.在移动台控制的切换（mcho）中， ms一直监测周围 bs的信号，当满足某些切换准则时，启动切换过程。 mcho用于dect和pacs系统。2.在网络控制的切换（ncho）中，周围的 bs测量来自 ms的信号，且当满足某些切换准则时，网络启动切换过程。ncho用于 ct-2 plus和amps系统。3.在移动台辅助的切换（maho）中，网络要求 ms去测量来自周围bs的信号。网络基于ms的报告作出切换决定。 maho用于 gsm

7、和is-95 cdma 系统。12wireless network layer切换 基站 (bs) 间切换在bs间切换中，新的和原来的bs连接到同一个msc上。假设由 ms检测到切换的需求；然后产生以下动作：1.ms立即挂起通话，并通过给新的bs中一个空闲信道（目前空闲的）发送信号启动切换过程。然后重新恢复原来 bs上进行的通话（图（a）。2.一旦收到信号，msc就将密码信息发送到新bs所选择的空闲信道上，并由该信道建立新的通话路径。交换器将新路径桥接到原路径上，并通知ms从原信道转移到新的信道上（见图（b）。13wireless network layer3.当ms转接到新的 bs之后，即给

8、网络发信号（见图（c），并用新的信道重新开始通话。4.一旦收到切换完成信号，网络从路径上去除所搭接的桥，并释放与原信道相关的资源（见图（d）。此切换过程采用了移动台控制的切换策略。对于网络控制的切换策略，所有的切换信令消息均通过正在失效的链路在ms和原 bs之间交换。整个过程必须尽快完成，以保证在原链路失效之前建立新的链路。14wireless network layer原 bs新bs原bs新bs原 bs新 bsmscmscmscmscmscmscmsc原bs新 bs（ a）第 1步（ d）第 4步（ c）第 3步（ b）第 2步基站间链路转换15wireless network layer切

9、换 基站 (bs) 间切换 (续)如果新的 bs没有空闲信道，切换呼叫被中断（或强制中断）。强制中断率是通信网络性能评价的一个重要指标。与阻塞一个新的呼叫申请相比，我们更不希望一个正在进行中的呼叫被强制中断。大多数通信网络处理切换与新呼叫请求一视同仁。也就是说，如果没有信道，切换就要被阻塞，而且呼叫保留在原信道上，直到呼叫完成或链路失效。这就称为非优先级方案。为了减少强制中断和提高呼叫完成率，目前有三种改进的信道分配方案：16wireless network layer1.预留信道方案 ：除了在每个 bs中一些信道预留作切换呼叫之用外，其它与非优先级方案类似。2.排队优先方案 ：此方案是基于相

10、邻bs覆盖区域重叠这一事实。因此，呼叫在相当的一部分区域内可以由任何一个bs操纵。这个区域就叫作“ 切换区域” 。如果在切换期间新bs没有空闲信道，那么新bs就在一个 “ 等待队列 ” 中缓存该切换请求。ms继续延用原 bs信道，直到新 bs中有一个信道可用（切换呼叫被连接上）为止，或ms从切换区域离开（和呼叫被强制中断）为止。3.半速率方案 （不适用于 cdma系统） ：如果新 bs无可用信道，则通过与现有的呼叫共享资源来创建一个新的信道。半速率的意思是一个被占用的全速率信道以原来一半的速率暂时分裂成两个信道：一个服务于现有的呼叫，而另一个服务于切换呼叫请求。当有一个占用的信道释放时，被半速

11、率化的信道立即恢复成全速率信道。问题 :cdma系统如何计算系统的剩余容量 ?有没有半速率对应的方案 ?17wireless network layer切换 系统间切换在系统间切换中，新bs和原来的 bs分别连接到不同的 msc上。在下面的描述中，我们跟踪 is-41 的系统间切换过程，其中假设采用网络控制的切换。下图表示在切换前和切换后的中继连接。在图中，一个通信中的移动用户移出msc a服务的 bs,并进入到 msc b覆盖的区域。系统间切换须遵循以下步骤：18wireless network layer1.msc a请求 msc b在呼叫进行中执行切换测量。然后， msc b选择一个候选

12、 bs bs2，并询问有关正在进行的呼叫的信号质量参数。msc b向msc a返回信号质量参数值以及其它相关信息。2.msc a检查 ms是否最近进行过太多的切换（避免ping-pang效应，即在 bs1和bs2之间进行过多切换，其中 ms在重叠区域移动）或没有系统间可用中继线。如果如此，msc a退出这一过程；否则， msc a请求 msc b建立一个话音信道。假定在bs2中能得到一个话音信道，msc b指示msc a开始无线链路的转移。3.msc a向ms发出切换命令。ms与bs2进行同步操作。在 ms连接到 bs2之后， msc b通知 msc a切换成功了。然后，msc a与msc b

13、建立呼叫路径（中继线）的连接，并完成切换过程。19wireless network layervlrpsdn中继线bs1(a) 切换前vlr中继线(b) 切换后psdn基站bs2bs1bs2基站mscamscbmscbmsca公共数据交换网移动交换中心系统间切换电话机电话机20wireless network layer切换转送、切换回送和切换到第三方mscamscb(a) 切换转送mscamscbmscbmscbmscamscamsccmscc(c) 切换到第三方(d) 路径最小化(b) 切换回送蜂窝电话蜂窝电话蜂窝电话蜂窝电话电话机电话机电话机电话机21wireless network l

14、ayer切换 系统间切换 (续)在此系统间切换过程中，msc a被称为 “ 归属msc” ，在切换前后它始终处在呼叫的路径上，图所示表示了切换的四种情况。这种“ 归属 ” 方案用在所有的现有移动电话系统中，因此在ms与新 msc之间一条新路径的重建需要 pstn额外释放或建立一条中继线，这是不可能或不划算的。如果 ms又回到 msc a，msc a 与msc b之间的连接被取消，如图（b）所示。如果ms移动到第三个mscmsc c ，那么 msc b 将处于呼叫路径之中，如图（c）所示。注意，当ms移动到第三个 msc时，第二个msc就可以从呼叫路径上退出，即msc a与msc b之间的连接被

15、中断，并且msc c 直接连接到 msc a，如图（d）所示。该过程被称为路径最小化。22wireless network layer漫游管理漫游管理的两个基本操作是注册（或位置更新）（ ms告诉系统其当前位置的过程）和位置跟踪（系统找到 ms位置的过程）。当网络要向一个移动用户发送一个呼叫时需要进行位置跟踪。is-41 和gsm map标准中提出的漫游管理策略是两级策略，其中采用归属和访问两种数据库两级系统。当一个用户在某一个pcs 网络中注册了服务时，在系统数据库中就建立了一个记录，称为归属位置寄存器（ hlr）。这也被称为移动用户的归属系统。hlr是一个网络数据库，它存储和管理由某个特别

16、运营商所有的移动注册信息。map: 移动应用部分(mobile application part) 23wireless network layer漫游管理(续)hlr是位置寄存器，为了保存记录，给每个ms分 配 了 一 个 识 别 符 ， 如 其 中 包 括 目 录 号 、profile 信息、当前位置和有效期等。当移动用户访问一个不是自己归属系统的pcs网络时，在被访问的系统vlr中就要为该移动用户建立一个临时记录。vlr暂时为来访的用户存储注册信息，以便对应的 msc为其提供服务。换句话说，vlr是“ 另外的 ” 位置寄存器，它用于检索处理来访移动用户呼叫的信息。注册过程如图所示。下面为

17、所描述的步骤：24wireless network layer1. 假定某个移动用户的归属系统是在a城。当移动用户从一个城市（如b城）移动到另一个城市（如 c城）时，他必须在这个新的来访系统中注册。2. 新vlr通知移动用户的 hlr此人当前的位置 新vlr的地址。 hlr向vlr发送一个确认新消息，其中包括ms的profile 。3. 新vlr通知 ms注册成功。4. 第2步之后， hlr还发送一个解除注册消息，删除原 vlr中过期的位置信息。原vlr对此作出确认。25wireless network layer原vlr1324c城a城b城新vlrhlr访问位置寄存器归属位置寄存器ms 注册

18、过程26wireless network layer漫游管理(续 )为了发起一个呼叫，ms首先联系所访问的pcs网络的 msc。呼叫请求发送到vlr，等待批准。如果呼叫被接受，msc按下列标准 pstn呼叫建立过程建立与被叫方的呼叫。ms的呼叫传递（或呼叫终接）过程如下图所示。27wireless network layer呼叫传递过程hlrmsc2323111pstnvlr电话机28wireless network layer1.如果一个有线电话用户试图呼叫一个移动用户，呼叫被转送到一个pstn的发起交换机，并请求hlr去 寻 找ms 当 前 的vlr。hlr询 问ms所 在的vlr，以获得

19、路由地址。2.如果发起交换机不能得到hlr（即它不支持移动性），则该呼叫通过pstn到用户的网关msc进行路由，并询问hlr以确定当前vlr是否正在为该ms服务。3.vlr通过 hlr向发起交换机返回路由地址。4.基于路由地址，通过被访问的msc，从发起交换机到 ms建立起一条中继线（话音电路）。29wireless network layer移动 ip（mobile ip ）30wireless network layerbackground of mobile iposi reference model and protocol layerdata link1networktranspor

20、tsessionpresentationapplicationphysical1applicationprogramcomputer aprotocollayer7654321physical1physical2data link1data link2networkrouterdata link2networktransportsessionpresentationapplicationphysical2applicationprogramcomputer bmedium1medium2informationtransferred at this layer:(files, video,e-m

21、ail, etc.)(segments)(packets)(frames)(bits)计算机计算机路由器31wireless network layerwhat does each layer do?physical layermoves “raw”bits across a communications facility, or medium. define the electrical and mechanical characteristics of the medium, bit rate, voltages, etc. data linkuses the bits to movefr

22、ames from one node to a neighboring node on the same link. define methods for ensuring the reliability of each frame .network layeruses the frames to move packetsfrom their original source to their ultimate destination, traversing one or more intermediate links if necessary. define how network devic

23、es discover each other and how packets are routed to their final destination.32wireless network layertransport layeris responsible for making reliable the end-to-end packet-transmission facility (streams). each individual transmission is called a segment.define the methods for detecting errors in tr

24、ansmission of segments and correcting when they occur.session layertakes the reliable streams and delivers application-oriented servicesto the higher layers. sometimes, resume the communication in the event of a catastrophic network failure.presentation layerdefines the syntax and semantics of the i

25、nformation being exchanged by an application (including encoding).application layerprovides the transfer of information that is specific to the computer program being run by a user. such as define how to exchange e-mail, files, and web pages.33wireless network layerwhich layers are relevant to mobil

26、e ip?network layer!the purpose of this layer is to hide the specifics of individual links and to provide the transport layer with the illusion of one large, logical network. it moves packets from a source all the way to the ultimate destination, traversing one or more links if necessary. thus, looki

27、ng down at the network layer, the transport layer sees a service which delivers segments from the source to the ultimate destination.34wireless network layerwhat is internet protocol (ip)?the internet model of communications, as embodies in the tcp/ip suite of protocols, resembles but varies slightl

28、y from the osi model. the internet model generally groups the highest three layersof the osi model together and considers them to be one layer, the application layer. the name given to the internets network-layer protocol is the internet protocol (ip). at the transport layer, there are two protocols

29、 in wide use on the internet: the transmissioncontrol protocol (tcp), which provides reliable, stream-oriented communications to the higher layer; and the user datagram protocol (udp), which provides connectionless communications to the higher layers without guaranteeing reliability. the combination

30、 of the internets reliable transport protocol (tcp) and its network protocol (ip) gives rise to the name of the internet suite of protocols.35wireless network layerwhat is icmp?the internet control message protocol (icmp)defines a set of error and controlmessages which provide indications that error

31、s have occurred in the transmission of a packet. other icmp messages provide diagnostic information to a requesting node. a node which implements ip must also implement icmp in order to a claim compliance with internet standards; thus when speaking of ip we are generally speaking about icmp as well.

32、see following figure for the tcp/ip reference model36wireless network layer802.3utpppprs-232fddifiberinternet protocol (ip)icmptcpudptelnetftphttpripnfsterminalemulatorwebbrowserremotefile accessuser interfaceapplication protocolstransport protocolsnetwork protocolslink protocolsapplicationsoperatin

33、g systemdevice driversand hardwarenetworkinternet (tcp/ip) reference model37wireless network layerwhat does ip do?ip, like all network-layer protocols, moves packets of information from the original source to the ultimate destination. this service is sometimes referred to as “end-to-end packet deliv

34、ery . ” the reliability of the service provided by ip is called “best-effort, ” which means that ip will try very hard to deliver a packet to the destination, but ip makes no guaranteethat the packet will arrive without error.ip addresses32-bit (4-byte) numbers assigned to each network interface of

35、a node. such as: 8network-prefixhost(p = 32-h)bitsh bitsipv6 是“ internet protocol version 6” 的缩写，也被称作下一代互联网协议，其最显著的特征是通过采用 128 位的地址空间替代ipv4 的32位地址空间来提高下一代互联网的地址容量。38wireless network layerwhat happens when a node changes link?ip nodes (hosts and routers) base on their packet-forwarding de

36、cisions on information contained within the ip packet header. routing decisions are made based on the network-prefix portion of the ip destination address. it implies that all nodes with interfaces on a given link must have identical network-prefix portions of their ip addresses on those interfaces.

37、see an example in following figure. host 4 s ip address has a network prefix of 2.0.0, but it connects to a link whose network prefix is 4.0.0. what will happen when host 1 tries to send a packet to host 4?39wireless network layeraccc“ direct ”“ direct ”5521.0.0. 0 / 243.0.0. 0 / 242.

38、0.0. 0 / 244.0.0. 0 / 24l/fnext hoptarget / pref-lenrouting tablerouter a s iprouter arouter crouter bhost 1host 2host 5host 3host 4host 555555aaacccbbbbbbethernet anetwork-prefix = 1.0.0high-speed fiber backb

39、onenetwork-prefix = 3.0.0ethernet bnetwork-prefix = 2.0.0ethernet cnetwork-prefix = 4.0.0host movement膝上型计算机膝上型计算机计算机计算机计算机计算机40wireless network layerwhat happens when a node changes link? (contd.)1.host 1 generates an ip packet in which the ip source address is and the ip destination addres

40、s is . the only match in the host 1s routing table for this destination is a default route, which specifies a next hop of router a (54 ) via interface “ a.” thus, host 1 forwards the packet to router a.2.router a finds an entry for targets with network-prefix 2.0.0 in the third row of

41、its routing table, which specifics a next hop of router b (53 ) via interface “c.” thus, router a forwards the packet to router b.3.router b has a “direct ”route in its routing table for targets with network-prefix equal to2.0.0 , so router b transmits the packet via interface “b ” on etherne

42、t b.however, the packet is undeliverable, because host 4 is not connected to ethernet b. router b will then send an icmphost unreachableerror massage back to the source (host 1).41wireless network layerwhat happens when a node changes link? (contd.)conclusions :if a node is not located on the link w

43、here its network-prefix says its supposed to be located, then packets sent to that node will be undeliverable. this means that such a node is incapable of communicating with any other nodes. as a corollary ( 推论 ), a node may not move from one link to another if it wishes to communicate without minim

44、ally changing the network-prefix portion of its ip address to reflect its new point-of-attachment to the network.42wireless network layerwhy not just change the node s ip address?how about simply changing the nodes ip address as it moves from one link to another link?the widely used transport protoc

45、ols tcp and udp have the concept of ports .a port is a 16-bit (2-byte) integer which allows the receiving tcp and udp protocol entities to determine which of many possible higher-layer applications is supposed to receive the data portion of any segment that arrives over the network.this allows many

46、applications to be open simultaneously on a node and have the application traffic routed independently to the proper destination(s).a tcp connection within a node is uniquely identified by the 4 values(2 pairs of): ip source address, ip destination, tcp source port, and tcp destination port.ipv4 ass

47、umes these values will remain constant over the duration of a tcp connection.43wireless network layerwhy not just change the node s ip address? (contd.)conclusions:it would simply drop its connections to a destination node whose ip address was to change. all ongoing communications between a mobile n

48、ode and any of these existing nodes would have to be terminated, with new connections being initiated by the mobile node at its new address.thus, by the definition, changing a mobile nodes ip address as it moves does not solve the problem of node mobility!44wireless network layermobile ip overviewwh

49、at is mobile ip?mobile ip, a standard proposed by a working group within the internet engineering task force (ietf), was designed to solve this problem by allowing the mobile node to use two ip addresses : a fixed homeaddressand a care-ofaddressthat changes at each new point of attachment. 45wireles

50、s network layerwhat is mobile ip? (contd.)mobile ip can also be thought of as the cooperation of three major subsystems.1.there is a discovery mechanismdefined so that mobile nodes can determine their new attachment points (new ip addresses) as they move from place to place within the internet.2.onc

51、e the mobile node knows the ip address at its new attachment point, it registerswith an agent representing it at its home network.3.mobile ip defines simple mechanisms to deliver datagramsto the mobile node when it is away from its home network.46wireless network layerhow does mobile ip work?1.ip ro

52、utes packets from a source endpoint to a destination by allowing routers to forward packets from incoming network interfaces to outbound interfaces according to routing tables . 2.the routing tables typically maintain the next-hop (outbound interface) information for each destination ip address, acc

53、ording to the number of networks to which that ip address is connected. 3.the network number is derived from the ip address by masking off some of the low-order bits. thus, the ip address typically carries with it information that specifies the ip nodes point of attachment.47wireless network layerho

54、w does mobile ip work? (contd.)4.to maintain existing transport-layer connections as the mobile node moves from place to place, it must keep its ip address the same.5.in tcp, connections are indexed by 4 numbers that contain the ip addresses and port numbers of both connection endpoints. 6.changing

55、any of these 4 numbers will cause the connection to be disrupted and lost. 7.on the other hand, correct delivery of packets to the mobile nodes current point of attachment depends on the network number contained within the mobile nodes ip address, which changes at new points of attachment.8.to chang

56、e the routing requires a new ip address associated with the new point of attachment.48wireless network layerhow does mobile ip work? (contd.)mobile ip has been designed to solve this problem by allowing the mobile nodeto use two ip addresses.in mobile ip, the home addressis static and is used to ide

57、ntify tcp connections. the care-of addresschanges at each new point of attachment and can be thought of as the mobile nodes topologically significant address; it indicates the network number and thus identifies the mobile nodes point of attachment with respect to the network topology.the home addres

58、s makes it appear that the mobile node is continually able to receive data on its home network(hn) , where mobile ip requires the existence of a network node known as the home agent(ha) . whenever the mobile node is not attached to its home network (and is therefore attached to what is termed a fore

59、ign network(fa) ), the home agent gets all the packets destined for the mobile node and arranges to deliver them to the mobile nodes current point of attachment.49wireless network layerhow does mobile ip work? (contd.)whenever the mobile node moves, it registersits new care-of addresswith its ha. to

60、 get a packet to a mobile node from its ha, the ha delivers the packet from the home network to the care-of address. the further delivery requires that the packet be modified so that the care-of address appears as the destination ip address. this modification can be understood as a packet transforma