信道分配策略.ppt

WELCOME TO
COMPUTER NETWORKS
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Review
● Specific Functions
● Services
● Framing
● Error Control
● Flow Control
● CRC
● Elementary Data Link Protocols
● Sliding Window Protocols
● HDLC
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● So far: We have discussed the Data Link Layer’s functionality and some protocols related to point-to-point communication.
● A large class of networks is built on top of broadcast channels: a number of stations share the same “wire.” If one station sends, all the others get to hear it.
● What’s the problem: if you’re sharing a channel, then two stations may decide to start frame transmission at the same time => frame collision
Chapter 4. The Medium Access Control Sublayer
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● Solution: Allocate the channel to one of the competing stations.
● Problem: You’ll have to use that same channel to figure out the competition and the allocation.
● The protocols used to determine who goes next on a multiaccess channel belong to a sublayer of the data link layer called the MAC (Medium Access Control) sublayer.
Chapter 4. The Medium Access Control Sublayer
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Main topics:
● MULTIPLE ACCESS PROTOCOLS
ALOHA
CSMA/CD
● ETHERNET
● WIRELESS LANS
● DATA LINK LAYER SWITCHING
Chapter 4. The Medium Access Control Sublayer
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信道分配策略
随机访问 (争用)---负载较轻时
控制访问
(负载较重时)
静态分配
(将频带或时间片等
固定分给各站点)
站点少且固定，数据量大时
频分多路复用
同步时分多路复用
波分多路复用
码分多路复用
动态分配
(异步时分多路复用)
仅当有数据发时,才占信道发数据 。
轮转
(每个站轮流获得发送机会)
预约
(各站先声明有数据要发送, 然后按预约顺序发送)
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Three strategies for channel allocation:
1. Exercise no control at all: simply let a station try to use the channel, and do something when a collision happens. Applied in contention（竞争） systems.
2. Employ a round-robin technique: each station in turn is allowed to use the channel. Applied in token-based systems – the station that has the token may use the channel.
3. Let a station place a reservation（预约） for the channel. Used in slotted systems. The problem is how to make a reservation.
The Channel Allocation Problem
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ALOHA
Carr