Chapter 4
The Medium Access Control
Sublayer
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The Channel Allocation Problem
Static Channel Allocation in LANs and MANs
Dynamic Channel Allocation in LANs and
MANs
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Dynamic Channel Allocation in LANs and MANs
Station Model.
Single Channel Assumption.
Collision Assumption.
(a) Continuous Time.
(b) Slotted Time.
(a) Carrier Sense.
(b) No Carrier Sense.
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Multiple Access Protocols
ALOHA
Carrier Sense Multiple Access Protocols
Collision-Free Protocols
Limited-Contention Protocols
Wavelength Division Multiple Access Protocols
Wireless LAN Protocols
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Pure ALOHA
In pure ALOHA, frames are transmitted at completely
arbitrary times.
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Pure ALOHA (2)
Vulnerable period for the shaded frame.
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Pure ALOHA (3)
Throughput versus offered traffic for ALOHA systems.
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Persistent and Nonpersistent CSMA
Comparison of the channel utilization versus load for
various random access protocols.
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CSMA with Collision Detection
CSMA/CD can be in one of three states:
contention, transmission, or idle.
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Collision-Free Protocols
The basic bit-map protocol.
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Wireless LAN Protocols
A wireless LAN. (a) A transmitting. (b) B transmitting.
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Ethernet
Invented at XEROX by Dr. R. Metcalfe in 1975
Formal specifications published in 1980 by DEC-IntelXerox
Became IEEE 802.3 standard in 1985 - adopted by ISO
10 Mbps
Coaxial cable
Updated to include new technologies
Twisted pair
100 Mbps Fast Ethernet - IEEE 802.3U
Gigabit Ethernet - IEEE 802.3Z
Wireless versions
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Elements of the Ethernet System



Physical Medium
Medium Access Control (MAC)
Ethernet Frame
Operation of Ethernet
 No central control
 Stations are connected to a shared medium
 Ethernet signals are transmitted serially, one bit at a time,
to every connected station
 To send data a station listens to the channel and if it is idle
it transmits its data in the form of an Ethernet frame.
 Access to the medium is determined by the MAC
•
CSMA/CD protocol
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CSMA/CD Protocol
The Ethernet MAC is based on the Carrier Sense Multiple
Access / Collision Detection protocol
A station that wants to transmit
•
Listens to the channel
 If the channel is idle, it transmits its data
 If the channel is busy it waits until it is free and then
transmits its data.
•
If two stations transmit at the same time we have a
collision ---- Jam signal
 Truncated binary exponential backoff algorithm
• nth re-transmission will be attempted with a delay
between 0 and 2n-1 time units (ex. 53s)
• maximum of 16 attempts to re-transmit
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Collision detection
Collision detections can take as long as 2t
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Ethernet MAC Sublayer Protocol
Frame formats. (a) DIX Ethernet, (b) IEEE 802.3.
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Ethernet Addresses
Ethernet addresses (48 bits) are unique and controlled by
IEEE

24 bit Organizationally Unique Identifier (OUI) by the IEEE

24 bit by the organization
Multicast and Broadcast addresses

High order address bit is 1 for multicast and broadcast

A destination address of only 1s is accepted by all stations
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Higher level addresses
Ethernet is a trucking system and can operate with different higher
level protocols, like TCP/IP, AppleTalk, Novel etc.
Higher level protocols have their own addressing schemes.
They must find the right Ethernet address in order to communicate
with each other.
Example in TCP/IP : Address Resolution Protocol (ARP)
•
Station A sends a broadcast requesting the Ethernet address the
station B that has the specific IP address
•
All stations receive the message
•
Only the station with the requested IP address reply
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Ethernet Topology
The total size of an Ethernet LAN is defined by the round trip propagation
delay.
•
All stations should by able to respond to signals within a specified
amount of time.
•
Ethernet segments can be connected at any physical topology (star,
tree, bus ...) as long as the timing restrictions are fulfilled.
Repeaters
•
A repeater is simply re-transmits the signal extending the overall size
of the LAN
Switching hubs
•
Divides a set of Ethernet segments to multiple LANs

Packet switching
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Ethernet Cabling
The most common kinds of Ethernet cabling.
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Ethernet Cabling (2)
Three kinds of Ethernet cabling.
(a) 10Base5, (b) 10Base2, (c) 10Base-T.
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Ethernet Cabling (3)
Cable topologies. (a) Linear, (b) Spine, (c) Tree, (d)
Segmented.
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Ethernet Cabling (4)
(a) Binary encoding, (b) Manchester encoding,
(c) Differential Manchester encoding.
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Ethernet Performance
Efficiency of Ethernet at 10 Mbps with 512-bit slot times.
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Switched Ethernet
A simple example of switched Ethernet.
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Fast Ethernet
The original fast Ethernet cabling.
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Gigabit Ethernet
(a) A two-station Ethernet. (b) A multistation
Ethernet.
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Gigabit Ethernet (2)
Gigabit Ethernet cabling.
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Ethernet problems
Capture effect
A station with high traffic load can capture the network
for a considerable amount of time
Security
Packets are received by all stations
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IEEE 802.2: Logical Link Control
(a) Position of LLC. (b) Protocol formats.
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The 802.11 (WLAN) Protocol Stack
Part of the 802.11 protocol stack.
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The 802.11 MAC Sublayer Protocol
(a) The hidden station problem.
(b) The exposed station problem.
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Bluetooth Architecture
Two piconets can be connected to form a scatternet.
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Bluetooth Applications
The Bluetooth profiles.
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Data Link Layer Switching
Bridges from 802.x to 802.y
Local Internetworking
Spanning Tree Bridges
Remote Bridges
Repeaters, Hubs, Bridges, Switches, Routers,
Gateways
Virtual LANs
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Data Link Layer Switching
Multiple LANs connected by a backbone to handle a total
load higher than the capacity of a single LAN.
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Bridges from 802.x to 802.y
Operation of a LAN bridge from 802.11 to 802.3.
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Bridges from 802.x to 802.y (2)
The IEEE 802 frame formats. The drawing is not to scale.
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Repeaters, Hubs, Bridges, Switches,
Routers and Gateways
(a) Which device is in which layer.
(b) Frames, packets, and headers.
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Repeaters, Hubs, Bridges, Switches,
Routers and Gateways (2)
(a) A hub. (b) A bridge. (c) a switch.
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Virtual LANs
A building with centralized wiring using hubs and a switch.
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Virtual LANs (2)
(a) Four physical LANs organized into two VLANs, gray
and white, by two bridges. (b) The same 15 machines
organized into two VLANs by switches.
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The IEEE 802.1Q Standard
Transition from legacy Ethernet to VLAN-aware Ethernet. The
shaded symbols are VLAN aware. The empty ones are
not.
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The IEEE 802.1Q Standard (2)
The 802.3 (legacy) and 802.1Q Ethernet frame formats.
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