Download Lecture 16

ICOM 6115 – Computer Networks
and the WWW
Manuel Rodriguez-Martinez, Ph.D.
Lecture 16
ICOM 6115
©Manuel Rodriguez-Martinez
Lecture Objectives
• Look at Stop-and-Wait pseudo code
– Sliding window comes on Tuesday
• Introduction to the Medium Access Control
(MAC) Sublayer
– Addressing
– Channel allocation
– IEEE 802 family of protocols
• Ethernet
• WiFi
• Broadband
ICOM 6115
©Manuel Rodriguez-Martinez
Multi-access Networks
• Issue
– Same channel is shared by many host
– Only one can be using it at any given time
• Problem
– How to control access to the channel and be fair?
ICOM 6115
©Manuel Rodriguez-Martinez
Medium Access Control (MAC) Sublayer
• Lower part of the Data Link Layer for multiaccess networks
– e.g. Bus networks like Ethernet
• MAC has the protocols to control which
host get access to the network
• Logical Link Control (LLC)
– Actual Data Link Services
• Connectionless No ACKs
• Connectionless with ACKs
• Connection-oriented with ACKs
ICOM 6115
©Manuel Rodriguez-Martinez
A View of the Data Link Layer
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Why have a LLC?
• Split Data Link Layer complexity
– MAC is specific to the physical network
• Various Flavors of Ethernet
• Various Flavors of Wireless (802.11a or 802.11b)
– LLC is common to all
• Build just one version of this component
– MAC is specific to interface with physical layer
• Permits backward compatibility or interoperability
– 802.11a y 802.11b share LLC
ICOM 6115
©Manuel Rodriguez-Martinez
Example LLC
• IEEE 802.2
• LLC receive packets from network layer
• Adds header with
– Frame sequence number
– Frame ACK number (if any)
• Call upon MAC sublayer to actually put data on
wire
• At Receiver side, MAC delivers frames to LLC
– LLC decides to ACK or not
– Send data up to the network layer
ICOM 6115
©Manuel Rodriguez-Martinez
Example MAC sublayers
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IEEE 802.3 – Ethernet
IEEE 802.5 – IBM Token Ring
IEEE 802.11 – Wireless LAN
IEEE 802.15 – Personal LAN (Bluetooth)
IEEE 802.16 – Broadband Wireless
– Wireless Local Loop
• All these share a common LLC
– IEEE 802.2
ICOM 6115
©Manuel Rodriguez-Martinez
Services provided by MAC sublayer
• Addressing
– MAC Address
• Given to the network interface card
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Frame generation
Checksum computation
Error detection
Arbitration for channel access
– Conflict detection and resolution
ICOM 6115
©Manuel Rodriguez-Martinez
Static Channel Allocation
• Allocate each host a chance to send
• FDM or TDM can be used here
– Reserve each host a part of the bandwidth
• But these do not work well in LANs
– Bursty traffic means channel is poorly used
• Idle sender eats bandwidth that can be given to
busy sender
ICOM 6115
©Manuel Rodriguez-Martinez
Dynamic Channel Allocation
• Distributed Algorithm is run to determine
who gets to send
– Get permission to send
• Token-based approach
– IBM Token Ring or FDDI
– Send first and then fix any problem
• Typical problem: Collisions
– Two frames are send at the same time, thus distorting
the signals
ICOM 6115
©Manuel Rodriguez-Martinez
Ethernet MAC Sublayer
• Invented at Xerox
• Standardized as IEEE 802.3
• Properties
– Best effort delivery
• LLC must do re-transmission in case of timeout or
CRC error
– Carrier Sense with Collision Detection
– Unique address for network Card
– Support for multicasting and broadcasting
ICOM 6115
©Manuel Rodriguez-Martinez
Ethernet Types
• Cable type provides the type of Ethernet
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Connectivity of Ethernet styles
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Ethernet Frame Format
• Preamble – 64 bits long, each byte has pattern
10101010
– Synchronize start of frame
• Dest addr – 48-bit destination address
• Src addr – 48-bit source address
• Type – 16-bit flag used to indicate which upper layer
protocol gets the frame
• Body – variable length area to put the data from LLC
• CRC – 32-bit error detection code
ICOM 6115
©Manuel Rodriguez-Martinez
Ethernet Manchester Data Encoding
• Problem: How to discern an idle link from the signal for bit 0?
• Each bit period is divided into two sub-periods
– 1 is encoded as transition from high to low voltage
– 0 is endoced transition from low to high voltage
ICOM 6115
©Manuel Rodriguez-Martinez
Ethernet MAC Address
• Every Ethernet card has a unique MAC address
– Stored (burned) in ROM
– 6 bytes long
– Usually printed on card in hexadecimal digits, one
digit per 4-bit group
• Example:
– 8:0:2B:E4:B1:2 represents MAC address
00001000 00000000 00101011
11100100 10110001 00000010
– Can also be written as: 08:00:2B:E4:B1:02
ICOM 6115
©Manuel Rodriguez-Martinez
Ethernet Addressing
• Unicast
– Frame is addressed to one network card
• Broadcast
– Frame is addressed to all network cards
– Special MAC Address for this
• 11111111 11111111 11111111 11111111
• Multicast
– Frame is addressed to a group of cards
– Each address has the first bit set to 1
• 10101010 01001010 11110111 01011011
ICOM 6115
©Manuel Rodriguez-Martinez
Transmitter Algorithm
• Ethernet is Carrier Sense system
– Adaptor can “listen” to signal on wire
• If card has a frame to send it listens to
wire
– If signal is detected, then it waits and tries
again later when channel is idle
– Otherwise, frame is sent and card starts
listening as it sends
• Send and listen simultaneously to detects collision
ICOM 6115
©Manuel Rodriguez-Martinez
Collision Detection
• If the card listens a different signal pattern
than the one its sending, it assumes a
collision
• Card waits a random amount of time t
• Card attempts to send the frame again
• This is called collision detection
– Often the system is term
• CSMA/CD – Carrier Sense Multiple-Access with
Collision Detection
ICOM 6115
©Manuel Rodriguez-Martinez
Collisions
• How much can it take to detect a collision?
– 1 RTT
ICOM 6115
©Manuel Rodriguez-Martinez