10 IEEE 802 Architecture 11 4 1 3

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IEEE 802 Architecture 11

4. 1. 3局域网的拓扑结构 n n 星型（Star） 环型（Ring） 总线型（Bus） 树型（Tree） Central server Ring network Network backbone Concentrator (or hub) Bus network 13

树型 Network backbone Concentrator (or hub) 17

Star Layout - Bus Topology 19

Star layout - Ring Topology 20

Typical Frame Format LLC - Logical Link Control octet ：八位组 PDU - Packet Data Unit DSAP - Destination Service Access Point SSAP - Source Service Access Point SAP indicates the user 41

4. 4. 1 IEEE 802. 3 CSMA/CD n History n n n Developed by Bob Metcalfe and others at Xerox PARC in mid-1970 s Standardized by Xerox, DEC, and Intel in 1978 LAN standards define MAC and physical layer connectivity n n IEEE 802. 3 (CSMA/CD - Ethernet) standard – originally 2 Mbps IEEE 802. 3 u standard for 100 Mbps Ethernet IEEE 802. 3 z standard for 1, 000 Mbps Ethernet CSMA/CD: Ethernet’s Media Access Control (MAC) policy n CS = carrier sense n n n Send only if medium is idle MA = multiple access CD = collision detection n Stop sending immediately if collision is detected 45

是一种随机争用的 媒体访问控制方法 CSMA/CD技术 1 Two nodes transmit at the same time 2 3 Nodes transmit a jamming signal 4 Node detect there has been a collision Nodes wait a random period before retransmitting All computers have access to a common bus at the same time 46

CSMA/CD 流程图 47

Binary Backoff 二元指数后退 算法 50

802. 3帧格式 Preamble (7 B) Start Dest. Src. Len delimiter (1 B) address (6 B) Address (6 B) (2 B) FCS (4 B) Data field (Logical link control) Preamble (seven bytes) precedes the Ethernet 802. 3 frame. Each byte of the preamble has a fixed binary pattern of 1010 and each node on the network uses it to synchronise their clock and transmission timings. It also informs nodes that a frame is to be sent and for them to check the destination address in the frame. Start delimiter field (SDF) is a single byte (or octet) of 10101011. It follows the preamble and identifies that there is a valid frame being transmitted. 0 to 1500 bytes E. g. IP TCP HTTP Pad Others: 4 bytes for the CRC (32 bits) and 2 bytes for the LLC length (16 bits). The LLC part may be up to 1500 bytes long. 0 to 46 bytes Data Source/destination addresses (2 or 6 bytes, Most Ethernet systems use a 48 -bit MAC address for the sending and receiving node. Each Ethernet node has a unique MAC address, which is normally defined as hexadecimal digits, such as: 4 C-31 -22 -10 -F 1 -32 (4 C 31 : 2210: F 132) A 48 -bit address field allows 248 different addresses (or approximately 281474976710000 different addresses). 51

Ethernet MAC Address 52

Baseband IEEE 802. 3 53

(a) 10 Base 5, (b) 10 Base 2, (c) 10 Base-T 54

IEEE 802. 3与Ethernet帧格式的比较 55

Frame Reception 帧的接收 56

4. 4. 2 Fast Ethernet n n IEEE 802. 3 u 作组规范(1995, 6) 特点： n n 采用CSMA/CD媒体访问控制方式和802. 3帧 格式 100 BASE-TX采用两对 5类双绞线 100 BASE-T 4采用四对 3类双绞线 网络最大长度 250 m（10 M以太网是 2500 m， 51. 2 x 10 -6 x 106=512 bits） 60

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Gigabit Ethernet的物理规范 66

4. 5 无线LAN n n 802. 11，Adopted in 1997. Defines: n n n MAC sublayer MAC management protocols and services Physical (PHY) layers, 1 M, 2 Mbps n n n IR 红外: FHSS跳频扩频： 79信道，每个信道宽 1 MHz，从 2. 4 GHz开始往上 DSSS直接序列扩频： 67

Channel Overlapping 68

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IEEE 802. 11 Protocols HR-DSSS

IEEE 802. 11 Terminology Basic Service Set (BSS): 基本服务集 n n BSS A set of stations controlled by a single “Coordination Function （协调功能）” (=the logical function that determines when a station can transmit or receive) Similar to a “cell” in pre IEEE terminology A BSS can have an Access-Point（接入点）, or can run without Access-Point Diameter of the cell is twice the coverage-distance between two wireless stations

IEEE 802. 11 Terminology IBSS 自组织网络（Ad hoc） Independent Basic Service Set (IBSS): 自主的基本服务集 n n A Basic Service Set (BSS) which forms a self-contained network in which no access to a Distribution System is available A BSS without an Access-Point One of the stations in the IBSS can be configured to “initiate” the network and assume the Coordination Function Diameter of the cell determined by coverage distance between two wireless stations

IEEE 802. 11 Terminology Extended Service Set (ESS): n n A set of one or more Basic Service Sets interconnected by a Distribution System (DS) Traffic always flows via Access-Point Distribution System (DS): n A system to interconnect a set of Basic Service Sets n Integrated; A single Access-Point in a standalone network n Wired; Using cable to interconnect the Access-Points n Wireless; Using wireless to interconnect the Access-Points

BSS Extended Service Set (ESS) Di st Sy ribu st tio em n BSS’s and wireless Distribution System (DS) Extended Service Set (ESS) BSS Di st r Sy ibu st tio em n BSS’s with wired Distribution System (DS) BSS

Overview, 802. 11 Architecture ESS Existing Wired LAN AP STA BSS STA Infrastructure Network STA Ad Hoc Network STA BSS STA Ad Hoc Network STA 75

IEEE 802. 11 Terminology Basic Service Set Identifier (BSSID) n “cell identifier” n 6 octets long (MAC address format) n One BSS has one SSID n Value of BSSID is the same as the MAC address of the radio in the Access-Point

802. 11 MAC子层 n the problem“Hidden stations” A B C A sends to B C doesn’t detect that, so C might also start sending to B Collision of messages at B: both messages lost

“Hidden stations” the solution A B C RTS: I want to send to B 500 bytes CTS: OK A, go ahead, so everybody quiet Data: the 500 bytes of data from A to B ACK: B received the data OK, so an ACK n IEEE 802. 11 defines: n n MAC level RTS/CTS protocol (Request to Send / Clear to Send) Can be switched off to reduce overhead (when no hidden nodes exist) More robustness, and increased reliability No interruptions when large files are transmitted

802. 11支持的两种操作模式 n 点协调功能（PCF, Point Coordination Function) n n 分布式协调功能(DCF, Distributed Coordination Function) n n 由基站周期性地（每秒 10~100次）广播一个信标帧 （beacon frame)，邀请站点申请服务 CSMA/CA(CSMA with Collision Avoidance) DCF是必须的，PCF是可选的 79

网桥的类型 n n 透明网桥（transparent bridge） 源路由选择网桥（source routing bridge） 85

Transparent bridges with address tables A 1 A 2 A 3 A 4 C 1 C 2 C 3 Bridge Port 1 Bridge Port 2 Port 1 A 2 A 3 A 4 Port 2 B 1 B 2 B 3 B 4 Port 1 A 2 A 3 A 4 Port 2 C 1 C 2 C 3 Address tables which assign MAC addresses with ports B 1 B 2 B 3 B 4 87

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Network backbone connection Main server Desktop switch Workgroup nodes Local printer Local server 99