Download 14 - UTRGV Faculty Web

14-MAC Sub-layer
Dr. John P. Abraham
Professor
UTPA
A taxonomy of mechanisms for
multi-access
• For this you must refer to figure 14.1 on
p.242
The data link layer
• LLC
– Logical Link Control
– Refers upward to higher layers
• MAC
– Media Access Control
– refers downward to lower layers
Media Access
• Determine how to get access when there is
competition for the media.
• MAC (Medium Access Control) sub-layer takes
care of this problem
• MAC is important in LANs where broadcast
channels are used
• MAC is the lower part of the data link layer (next
to physical layer)
• Mac sub-layer does not guarantee delivery
Static and Dynamic Channel Allocation
Static is discussed below:
• Channelization to refer to a mapping (between
communication and a channel in the underlying
transmission system).
• Traditional way to allow more than one person to
use the medium is to use FDM
• In Frequency division multiplexing, the total
bandwidth is divided among the total number of
users, each pair is assigned to a unique
frequency. This is known as 1-to-1 static.
• FDM works well when there is a small number of
users
• When users grow a fast busy signal is issued
Dynamic Channel Allocation
• Need dynamic if the set of entities using the channels
change frequently. Think of cellular phones. In dynamic
a mapping can be established when a new station
appears, or removed when it disappears.
• underlying assumptions of dynamic channel allocation
– 1. Station Model
– 2. Single Channel Assumption
– 3. Collision Assumption
– 4. Continuous time
– 5. Slotted time
Station Model
• Consists of N independent STATIONS
• Each has programs that produce frames
for transmission
• Frames are generated at intervals
• Once a frame is generated the station is
locked until the frame is transmitted
Single Channel Assumption
• Only one channel is available for all
communication
• All stations transmit on it and all stations
receive on that channel
Collision Assumption
• If two frames are transmitted
simultaneously, they overlap in time and
resulting signal is garbled.
• All stations can detect collisions.
• A collided frame must be retransmitted.
Continuous time
• Frame transmission can start any time
• There is no master clock controlling
transmission (as opposed to slotted time
discussed next)
Slotted time
• Time is divided into discrete intervals
(slots)
• Frame transmission begins at the start of a
time slot
Channelization Protocols
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•
•
•
FDMA
TDMA
Code Division Multi-Access
Already covered these
Controlled Access Protocols –
Collision free
• Polling: A centralized controller cycles
through all stations on the network and
gives each an opportunity to transmit a
packet, either uses round robin order or
priority order
• .
Reservation – Collision free
Often used with satellite transmission, employs a two-step process. Each
transmission is planned in advanced. In the first step, each potential sender
specifies whether they have a packet to send during the next round and the
controller transmits a list of stations that will be transmitting. In the second
step, stations transmit upon their turn.
• Bit-map protocol
– A bit map with enough slots for all stations is passed
around
– Each station wanting to send a frame and if the frame
is ready in the queue, inserts a 1 bit into its reserved
slot in the bit map.
– Once station numbers of all who want to send is
known they take turns in order.
Reservation – collision free. Binary
count down
• Each station is given a binary address
• If a station wants to transmit a frame it
broadcasts its address one bit at a time
starting with the high order bit.
• Bits from each station are Ored together
the station address starting with the
resulting 0 or 1 bit as agreed upon is
allowed to go on. If two or more has the
same bit then go to the next bit and so on.
Token Passing – collision free
• Token bus
– Each station knows the address of the station to its
left and right
– The highest numbered station may send the first
frame
– Then it passes permission to its immediate neighbor
by send a special frame called a token.
– The first station passes the token to the highest
numbered one.
•
Token Ring
– Physical Ring
– Token circulates
Random Access Protocols
• Many networks do not use collision free protocols, especially LANs
(Token passing is an exception).
• Instead, a set of computers attached to a shared medium attempt o
access the medium without coordination, like the old CB radio.
Random - Multiple Access
Protocols
• ALOHA
– PURE ALOHA
– SLOTTED ALOHA
• CARRIER SENSE MULTIPLE ACCESS
PROTOCOLS (CSMA)
– Persistent and Nonpersistent CSMA
– CSMA with collision detection
ALOHA
• 1970 - Norman Abramson – Alohnet.
University of Hawaii
• One main transmitter with a large tower
• Smaller tower and transmitter, each can
reach the central transmitter, but not each
other.
• Used ground based radio broadcasting
• Two versions of Aloha
– Pure
– Slotted
Pure ALOHA
• Users may send whenever they have data to send
• If collisions occur, collided data will be destroyed
• Sender can determine if the data was destroyed by
listening to the channel (the sender can hear too).
• If data was destroyed, re-send after waiting random
amount of time
• Each station’s transmission is repeated by the central
station, which can be received by all.
Slotted ALOHA
• Divide time into discrete slots, each time
slot is enough for one frame
• Users agree on slot boundaries
• A special station emits a signal at the start
of each time slot to synchronize
Carrier Sense Multiple Access
Protocols (CSMA)
• Listen for a transmission
• If the line is clear then transmit
• Implementations:
– Persistent, Non Persistent and p-persistent
– CSMA with collision detection
Persistent
• Listen, if busy wait until line is free
• Transmit a frame
• If collision occurred, wait for a random
amount of time
• Transmission time delay between two
sending computers will cause the second
computer not to hear the transmission.
Non-Persistent
• Listen, if busy wait random amount of time
and listen again until the line is free
• This approach is less greedy than the
Persistent one
• This prevents two or more wanting to get
on the line from doing so at the same time
when the channel becomes free.
P-persistent CSMA
• Slotted channels.
• Listen, if free send at the beginning of the
next slot
CSMA with Collision Detection
(CSMA-CD)
• Abort transmission as soon as collision is
detected
• Collision is detected by comparing
received signal power to sent signal
• If collision is detected, stop transmission
and wait for random amount of time
• CSMA/CD is used widely in LAN IEEE
802.3 is an example.
Binary Exponential Backoff
• After a collision occurs, a computer must
wait, but how long? In Aloha
randomization was used.
• In exponential backoff, the computer must
wait twice the amount of time than the
previous time. This is repeated if collision
occur again.
CSMA-CA
• For wireless.
• May not be able to hear computers outside
the range, while the other party can hear.
This is known as the hidden station
problem.
• Ready to send and clear to send are
transmitted first before transmitting packet.
The clear to send or the ready to send will
be heard by all within range.
BRIDGES
• Connect multiple LANs
• Operate at the Data Link Layer
– do not examine the network layer header
– doe not care whether it is IP, IPX, or other
Purpose of a Bridge
– Connect dissimilar networks
– Have different networks (different floors) and
connect them all together with a bridge
– Isolate traffic
– Length of cable limitations
– Reliability - if one segment goes bad, others
work.
– Security (not all traffic go through all cables)
VLANs
• Virtual LAN. Operates under layer 2, 802.1Q. Physically they could
be anywhere, but virtually act like a LAN. Without a VLAN we have
to physically run wires as requirements change.
• a group of hosts with a common set of requirements that
communicate as if they were attached to the same LAN.
• 802.1Q header contains a 4-byte tag header containing a 2-byte tag
protocol identifier (TPID) and a 2-byte tag control information (TCI).
•
TCI contains Three-bit user priority, One-bit canonical format indicator
(CFI), and Twelve-bit VLAN identifier (VID)-Uniquely identifies the VLAN to
which the frame belongs
There are Static (port based), Dynamic (VLAN management software using
originating MAC address, etc.) and Protocol based VLANs (example ARP
traffic).
Link-layer Addressing and ARP
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•
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Link layer addressing is the MAC address
Sending device does not know MAC address of destination
ARP is used to resolve MAC address
Each host and router keeps an ARP table which includes TTL
More about ARP later
Point to point protocol (PPP)
• Traditionally used in dial up lines (derived
from High-Level Data Link Control –
HDLC)
• Now used in aDSL
• Within PPP there are two protocols: Link
control Protocol (LCP) and Network
Control Protocol (NCP)
• PPP over Ethernet (PPPoE) is used in
aDSL
Power-Line Networking
(HomePlug)
• Transmits data over power lines.
• Allows 14 Mbits/s in half-duplex.
• Another proprietary solution exists that
gives 85 Mbps to 189 Mbps.
Multiplexing protocols for fiber
• Sonet (synchronous optical network) – US
and Canada
• SDH (Synchronous digital Hierarchy) –
rest of the world.