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Bài giảng Computer Networks 1 (Mạng máy tính 1) - Lecture 3: Networking Technologies - Phạm Trần Vũ

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Computer Networks 1  
(Mạng Máy Tính 1)  
Lectured by: Dr. Phạm Trần Vũ  
Lecture 3: Networking Technologies  
Reference:  
Chapter 4 - “Computer Networks”,  
Andrew S. Tanenbaum, 4th Edition, Prentice Hall, 2003.  
Content  
ꢀ Channel allocation problem  
ꢀ Multiple access protocols  
ꢀ Ethernet LAN  
ꢀ Wireless LAN  
3
Channel Allocation Problem  
ꢀ How to allocate a single broadcast channel  
amongst competing users?  
ꢀ Static met
ꢀ Dynamic method  
4
Static Channel Allocation in LANs and  
MANs  
ꢀ Each user is assigned with a equal-portion  
of the bandwidth  
ꢀ No interfe
ꢀ Simple  
ꢀ In efficient if there are a lot of users in the  
network  
5
Dynamic Channel Allocation in  
LANs and MANs  
ꢀ Five key assumptions  
ꢁ
Station Model:  
The model consists of N stations  
ꢀ
ꢀ
Once a frame is generated, the station blocks until  
the frame has been successfully transmitted  
ꢁ
ꢁ
Single Channel Assumption  
Collision Assumption  
ꢀ
Collision happens when two frames are  
transmitted imultanousy and overlap in time  
ꢁ
ꢁ
(a) Continuous Time vs (b) Slotted Time  
(a) Carrier Sense vs (b) No Carrier Sense  
6
Multiple Access Protocols  
ꢀ ALOHA  
ꢀ Carrier Sense Multiple Access Protocols  
ꢀ Collision-Free Protocols  
ꢀ Limited-Contention Protocols  
ꢀ Wavelength Division Multiple Access  
Protocols  
ꢀ Wireless LAN Protocols  
7
ALOHA  
ꢀ Developed by Norman Abramson, in 1970s  
ꢀ Used ground-based radio broadcast  
ꢀ Pure ALOHA  
ꢁ
Use continuous time  
ꢁ
No need for global time  
ꢀ Slotted ALOHA  
ꢁ
Need global time synchronisation  
8
Pure ALOHA (1)  
ꢀ Users can transmit whenever they have data  
to send  
ꢀ If there is a collision, colliding frames will be  
damaged and will be destroyed  
ꢀ Senders need to wait for some time to know  
if there is a collision  
ꢀ Senders wait for a random time to transmit  
destroyed frames  
9
Pure ALOHA (2)  
In pure ALOHA, frames are transmitted at completely  
arbitrary times.  
10  
Pure ALOHA (3)  
Vulnerable period for the shaded frame.  
11  
Slotted ALOHA  
ꢀ Time is divided into fixed interval (slot)  
ꢀ Each slot is equal to a frame time  
ꢀ Need time synchronisation among stations  
ꢁ
E.g. use a special station for timing  
ꢀ Frames can only be transmitted at starts of  
time slots  
12  
ALOHA System Performances  
Throughput versus offered traffic for ALOHA  
systems.  
13  
Carrier Sense Multiple Access  
Protocols  
ꢀ
When there is data to send, a station senses  
carrier first  
ꢀ
ꢀ
ꢀ
If the carrier is free, it starts sending  
Else, it waits until the carrier becomes free  
Common carrier sense protocols  
ꢁ
ꢁ
ꢁ
1-Persistent  
Nonpersis
p-Persistent  
14  
Persistent and Nonpersistent  
CSMA  
Comparison of the channel utilization versus  
load for various random access protocols.  
15  
CSMA with Collision Detection  
ꢀ An improvement to CSMA  
ꢀ A station stops transmitting its frame  
immediate ected to  
save time and bandwidth  
ꢀ A basis for Ethernet LAN protocol  
16  
CSMA with Collision Detection  
CSMA/CD can be in one of three states:  
contention, transmission, or idle.  
17  
Collision-Free Protocols  
The basic bit-map protocol.  
18  
Collision-Free Protocols (2)  
The binary countdown protocol. A dash indicates silence.  
19  
Ethernet  
ꢀ Ethernet Cabling  
ꢀ Manchester Encoding  
ꢀ The Ethernet MAC Sublayer Protocol  
ꢀ Switched Ethernet  
ꢀ Fast Ethernet  
ꢀ Gigabit Ethernet  
20  
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