CMPE 150- Introduction to Computer Networks 1 CMPE 150 Fall 2005 Lecture 15 Introduction to Computer Networks

CMPE 150- Introduction to Computer Networks 2 Announcements Midterm moved to –In class, closed books/notes. Homework 3 is up. –Due on 11,07.05.

CMPE 150- Introduction to Computer Networks 3 Today MAC.

CMPE 150- Introduction to Computer Networks 4 Example DLL Protocols

CMPE 150- Introduction to Computer Networks 5 Example DLL Protocols High-Level Data Link Control (HDLC). Point-to-Point Protocol (PPP).

CMPE 150- Introduction to Computer Networks 6 HDLC Flag Identifies host. ACK, seq. # CRC ISO standard.

CMPE 150- Introduction to Computer Networks 7 PPP Internet’s DLL. Router-to-router. Home user-to-ISP. RFC 1661, etc. PPP is a multi-protocol framing mechanism that can be used over multiple PHYs (dial-up, dedicated point-to-point connections).

CMPE 150- Introduction to Computer Networks 8 The Data Link Layer in the Internet

CMPE 150- Introduction to Computer Networks 9 PPP Frame Default value; no need for addresses. Default value: unumbered frame; No reXmissions. Type of protocol in the payload

CMPE 150- Introduction to Computer Networks 10 Medium Access Control MAC. Tanenbaum, Chapter 4.

CMPE 150- Introduction to Computer Networks 11 Why MAC? Point-to-point versus shared-medium networks. Shared-medium networks use “broadcast” channels. –A.k.a. multi-access or random access channels. MAC layer protocols regulate access to medium in shared-medium networks.

CMPE 150- Introduction to Computer Networks 12 Where is the MAC Sub-Layer? MAC

CMPE 150- Introduction to Computer Networks 13 Where is the MAC Sub-Layer? MAC PHY DLL Link Control Network Transport Application

CMPE 150- Introduction to Computer Networks 14 MAC and LANs LANs typically use shared-medium. Examples? MAC layer critical! –BTW, in wireless networks also! WANs typically use point-to-point connections.

CMPE 150- Introduction to Computer Networks 15 Channel Allocation Problem How to allocate single shared, broadcast channel among several stations/users. If no arbitration, several stations/users may transmit at the same time: COLLISIONS! A.k.a., “floor control”.

CMPE 150- Introduction to Computer Networks 16 Multiplexing Sharing a link/channel among multiple source- destination pairs. Example: high-capacity long-distance trunks (fiber, microwave links) carry multiple connections at the same time. MUX... DEMUX...

CMPE 150- Introduction to Computer Networks 17 Multiplexing Techniques 3 basic types: –Frequency-Division Multiplexing (FDM). –Time-Division Multiplexing (TDM). –Statistical Time-Division Multiplexing (STDM). Static Dynamic

CMPE 150- Introduction to Computer Networks 18 FDM

CMPE 150- Introduction to Computer Networks 19 FDM Simple. But: –What if number of users is large? –What if number of users changes over time? –What if traffic is bursty?

CMPE 150- Introduction to Computer Networks 20 TDM

CMPE 150- Introduction to Computer Networks 21 TDM (Cont’d) Time divided into time slots. One or more slots assigned to a data source. But, also inefficient… 12N N frame Time U1U1 U2U2...UNUN

CMPE 150- Introduction to Computer Networks 22 Dynamic Multiplexing Dynamic allocation. In particular, statistical TDM. –Dynamically allocates time slots on demand. Increased channel utilization. But…

CMPE 150- Introduction to Computer Networks 23 Multiple Access Protocols Centralized approaches: –Controller grants access to medium. –Simple, greater control: priorities, QoS. –But, single point of failure and performance bottleneck. Decentralized schemes: –All stations collectively run MAC to decide when to transmit.

CMPE 150- Introduction to Computer Networks 24 Round-Robin MAC Each station is allowed to transmit; station may decline or transmit (bounded by some maximum transmit time). Centralized (e.g., polling) or distributed (e.g., token ring) control of who is next to transmit. When done, station relinquishes and right to transmit goes to next station. Efficient when many stations have data to transmit over extended period (stream).

CMPE 150- Introduction to Computer Networks 25 Scheduled-Access MAC Time divided into slots. Station reserves slots in the future. Multiple slots for extended transmissions. Suited to stream traffic.

CMPE 150- Introduction to Computer Networks 26 Contention-Based MAC No control. Stations try to acquire the medium. Distributed in nature. Perform well for bursty traffic. Can get very inefficient under heavy load.

CMPE 150- Introduction to Computer Networks 27 Contention-Based MACs ALOHA family. CSMA family.

CMPE 150- Introduction to Computer Networks 28 Pure ALOHA In pure ALOHA, frames are transmitted at completely arbitrary times.

CMPE 150- Introduction to Computer Networks 29 Pure ALOHA: Performance Vulnerable period for the shaded frame.

CMPE 150- Introduction to Computer Networks 30 Pure ALOHA Throughput versus offered traffic for ALOHA systems.