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.

Channel Allocation Problem Multiple Access Protocols Content Channel Allocation Problem Multiple Access Protocols

Two types of transmission technology Point-to-Point Link Broadcast Link

Channel Allocation Problem Who's first?

Channel Allocation Problem Static Method Dynamic Method

Static Channel Allocation in LANs and MANs KEY: Each user is assigned with a equal-portion of the bandwidth Minus: 1. Ineffective bandwidth utilization 2. Some users will be denied in high traffic Plus: 1. Simple 2. No interference

Dynamic Channel Allocation in LANs and MANs Key assumptions 1. Station Model 2. Single Channel Assumptions 3. Collision Assumption 4. Continuous Time vs Slotted Time 5. Carrier Sense vs No Carrier Sense

Multiple Access Protocols ALOHA Carrier Sense Multiple Access Protocols Collision-Free Protocols

ALOHA Idea: Users can transmit whenever they have data to send Aloha!! Two types of ALOHA: Pure ALOHA Slotted ALOHA The main difference between them is time and time and time

Pure ALOHA continuous time no global time synchronization frame Send & then wait for collision collision? end no wait a random time yes continuous time no global time synchronization

Pure ALOHA – vulnerable period t – frame time, to – starting time

Slotted ALOHA 1. Time is divided to equal intervals (slots) 2. Need time synchronization 3. Frames can only be transmitted at starts of time slots

Slotted ALOHA – who's next?

Channel utilization

CSMA with Collision Detection (CSMA/CD) How? Idea: A station stop transmitting its frame immediately after a collision is detected to save time and bandwidth This is the basis of Ethernet LAN But: half-duplex

Propagation Delay & CD

Collision-free Protocols The basic bit-map control Idea: At the contention period, all station announce their needs to transmit. And then at the transmission period all registered stations take turn to send their frames. Drawback: suffer n bits delay in contention period

Collision-Free Protocols The binary countdown protocol Unique binary station addresses with the same length Rules: Compete by “OR” with other bits in the same order If there is a “0 to 1 transformation” in the result → give up

The Adaptive Tree Walk Protocol Distributed Depth-first tree walk

The Adaptive Tree Walk Protocol - improvement At heavy traffic, skip nodes father nodes 1,2,3,... Number of nodes at level i: 2^i Assume that, p is number of ready stations. Then k = p/2^i is number of ready stations per node at level i Optimal: k = 1 or p/2^i= 1 from this i = log2(p)

Wavelength Division MA Protocols (WDMA) To allow multiple transmissions the spectrum is divided into channels (wavelength bands) Total internal reflection: n1>n2 i1>ilimit

WDMA Each station is assigned two channels: control & data and two transmitters and two receivers 1. A fixed-wavelength receiver for listening to its own control channel. 2. A fixed-wavelength transmitter for outputting data frames. 3. A tunable transmitter for sending on other stations' control channels. 4. A tunable receiver for selecting a data transmitter to listen to.

WDMA Require global time synchronization for both channels

Wireless LAN Protocols CSMA is not appropriate Interference at the receiver, not at the sender Hidden station problem Exposed station problem

Wireless LAN Protocols - MACA A sends RTS to B and begins transmission only after receiving CTS reply from B

Wireless LAN Protocols - MACA 1. C doesn't see CTS and thus is free to send its RTS 2. C and B may want to send RTS at the same time so cause collision.

Wireless LAN Protocols - MACAW Improvements: 1. ACK frame after each successful data frame 2. Use CSMA for RTS frame

Ethernet Ethernet Cabling Manchester Encoding The Ethernet MAC Sublayer Protocol Switched Ethernet Fast Ethernet Gigabit Ethernet

The most common kinds of Ethernet cabling.

Ethernet Cabling (2) Three kinds of Ethernet cabling. (a) 10Base5, (b) 10Base2, (c) 10Base-T.

Cable topologies. (a) Linear, (b) Spine, (c) Tree, (d) Segmented. Ethernet Cabling (3) Cable topologies. (a) Linear, (b) Spine, (c) Tree, (d) Segmented.

Manchester Encoding Using voltage to encode binary code: 0 volts ~ 0 bit and 5 volts ~ 1 bit 0001000 = 10000000 or 01000000, because the difference between an idle sender (0 volts) and a 0 bit (0 volts) is ambiguous -1 and +1? out of synchronization, long sequence of 0 or 1

Manchester Encoding Using two voltages to encode one bit: 1 ~ high – low; 0 ~ low - high 010 = (-1,1)(1,-1)(-1,1) Plus: every bit has a transition in the middle → determine bit boundary & sync Minus: double bandwidth, 10Mbs ~ 20 mil changes per second

Differential Manchester encoding 1 bit ~ absence;0 bit ~ presence of a transition at the start of the interval. In both cases, there is a transition in the middle. Plus: complex equipment Minus: better noise immunity. (the 802.5 token ring) All Ethernet systems use Manchester encoding due to simplicity.(+ 0.85 volts; - 0.85 volts)

Manchester Encoding (a) Binary encoding, (b) Manchester encoding, (c) Differential Manchester encoding.

Ethernet MAC Sublayer Protocol http://en.wikipedia.org/wiki/MAC_address

Ethernet MAC Sublayer Protocol Unicast: 1 receiver Multicast: group of receivers Broadcast: all 281,474,976,710,656 possible MAC addresses

802.3 Ethernet frame structure

802.3 Ethernet frame structure Header: source and destination MAC address; Ethertype protocol identifier field; optional IEEE 802.1Q VLAN tag Frame check sequence: 32-bit CRC (cyclic redundancy check)

In addition to there being a maximum frame length, there is also a minimum frame length. While a data field of 0 bytes is sometimes useful, it causes a problem. When a transceiver detects a collision, it truncates the current frame, which means that stray bits and pieces of frames appear on the cable all the time. To make it easier to distinguish valid frames from garbage, Ethernet requires that valid frames must be at least 64 bytes long, from destination address to checksum, including both. If the data portion of a frame is less than 46 bytes, the Pad field is used to fill out the frame to the minimum size.

A simple example of switched Ethernet.

Fast Ethernet 802.3u vs 802.12 802.3u: Ideas: backward compatible unforeseen problems technology change period Ideas: keep all the old stuffs reduce the bit time from 100 nsec to 10 nsec

The original fast Ethernet cabling.

Gigabit Ethernet (a) A two-station Ethernet. (b) A multistation Ethernet.

Gigabit Ethernet cabling.

Wireless LAN Protocols Hidden station problem Exposed station problem A wireless LAN. (a) A transmitting B. (b) B transmitting A.

The 802.11 MAC Sublayer Protocol (1) The use of virtual channel sensing using CSMA/CA

The 802.11 MAC Sublayer Protocol (2) A fragment burst.

The 802.11 Frame Structure The 802.11 data frame.

802.11 Services Distribution Services Association Disassociation Reassociation Distribution Integration

802.11 Services Intracell Services Authentication Deauthentication Privacy Data Delivery