【正文】 l more expensive and higher data rate 22 Media Available (2) ?High performance UTP ?Cat 5 and above ?High data rate for small number of devices ?Switched star topology for large installations ?Optical fiber ?Electromagic isolation ?High capacity ?Small size ?High cost of ponents ?High skill needed to install and maintain ?Prices are ing down as demand and product range increases 23 Protocol Architecture ?Lower layers of OSI model ?IEEE 802 reference model ?Physical ?Logical link control (LLC) ?Media access control (MAC) 24 協(xié)議體系結(jié)構(gòu) ?LAN的標準化協(xié)議體系結(jié)構(gòu) ?物理層 ?邏輯鏈路控制 (LLC) 層 ?媒體接入控制 (MAC) 層 ?標準 ?IEEE 802 ?ISO 8802 ?OSI 參考模型的低層 ?IEEE 802比之 ISO/OSI ?高層協(xié)議與網(wǎng)絡體系結(jié)構(gòu)無關(guān) ?主要關(guān)心 OSI模型的低層 ?將數(shù)據(jù)鏈路層劃分為 MAC和 LLC 兩級子層 25 IEEE 802 vs. OSI 26 802 Layers – Physical 物理層的功能 ?Encoding/decoding信號的編碼 /解碼 ?Pre’amble generation/removal前導碼的產(chǎn)生 /去除 (用于同步 ) ?Bit transmission/reception比特傳輸 /接收 ?Transmission medium 傳輸媒體 27 802 Layers Logical Link Control LLC子層的功能 ?Interface to higher levels與高層進行接口 ?Flow and error control流量控制與差錯控制 為什么要將傳統(tǒng)的 DL層拆分為 LLC與 MAC兩個子層？ 28 Logical Link Control ?Transmission of link level PDUs between two stations ?Must support multiaccess, shared medium ?Relieved of some link access details by MAC layer ?Addressing involves specifying source and destination LLC users ?Referred to as service access points (SAP) ?Typically higher level protocol 29 邏輯鏈路控制 ?LLC的特性 ?兩站之間傳輸鏈路級不需中間交換節(jié)點 ?必須支持多點接入的共享媒體 ?由 MAC承擔了一些鏈路接入的細節(jié) ?尋址包括指定源 LLC用戶和目的 LLC用戶 ?稱為 LLC服務訪問點 (LSAP) ?典型的用戶是高層協(xié)議 30 LLC Services ?Based on HDLC ?Unacknowledged connectionless service ?Connection mode service ?Acknowledged connectionless service ?提供三種服務 ?不確認的無連接服務（ Unacknowledged connectionless service） ?連接方式服務（ Connectionmode service） ?帶確認的無連接服務（ Acknowledged connectionless service） 31 LLC Protocol ?Modeled after HDLC ?Asynchronous balanced mode to support connection mode LLC service (type 2 operation) ?Unnumbered information PDUs to support unAcknowledged connectionless service (type 1) ?Two new unnumbered information PDUs to support Acknowledged connectionless service (type 3) ?Multiplexing using LSAPs 32 LLC協(xié)議 ?以 HDLC為藍本 ?第 1類操作 ?無編號信息 PDU支持不確認的無連接服務 ?第 2類操作 ?異步平衡方式支持連接方式 LLC服務 ?第 3類操作 ?兩個新的無編號 PDU支持帶確認的無連接服務 ?利用 LSAP實現(xiàn)多路復用 33 802 Layers Media Access Control ?Assembly of data into frame with address and error detection fields ?Disassembly of frame ?Address recognition ?Error detection ?Govern access to transmission medium ?Not found in traditional layer 2 data link control ?For the same LLC, several MAC options may be available 34 MAC子層的功能 ?在發(fā)送方，將數(shù)據(jù)配以地址與差錯檢測字段組裝成幀 ?在接收方 , 拆解幀 ?地址辨識 ?差錯檢測 ?監(jiān)管傳輸媒體的接入 ?在傳統(tǒng)的第二層數(shù)據(jù)鏈路控制中無此功能 35 分裂數(shù)據(jù)鏈路層的原因 ?在傳統(tǒng)的數(shù)據(jù)鏈路 (DL)層中不具有管理共享訪問媒體的接入所需的邏輯 ?對于相同的 LLC, 可以提供幾種 MAC選項 36 LAN Protocols in Context 37 Media Access Control ?Where ?Central ?Greater control ?Simple access logic at station ?Avoids problems of coordination ?Single point of failure ?Potential bottleneck ?Distributed ?How ?Synchronous ?Specific capacity dedicated to connection ?Asynchronous ?In response to demand 38 媒體接入控制 ?重要參數(shù) ?Where ?集中式（ Central） – 更多的控制 – 簡單的站點接入邏輯 – 避免協(xié)商問題 – 單點故障 – 可能形成瓶頸 ?分布式（ Distributed） – 集體完成 MAC功能 – 動態(tài)確定哪個站發(fā)送 – 優(yōu)缺點是集中式方案的鏡像 ?How ?同步 – 為每個連接指定固有帶寬 ?異步 – 按需響應 39 Asynchronous Systems ?Round robin ?Good if many stations have data to transmit over extended period ?Reservation ?Good for stream traffic ?Contention ?Good for bursty traffic ?All stations contend for time ?Distributed ?Simple to implement ?Efficient under moderate load ?Tend to collapse under heavy load 40 異步系統(tǒng) ?循環(huán)（ Round robin） ?若許多站都需持續(xù)一段時間發(fā)送數(shù)據(jù)則很有效 ?輪詢：集中式技術(shù) ?令牌環(huán)：分布式控制 ?預約（ Reservation） ?適用于流式通信量 stream traffic ?爭用（ Contention） ?適用于 突發(fā)通信量 bursty traffic