Example DLL Protocols
1. High-Level Data Link Control
(HDLC).
2. Point-to-Point Protocol (PPP).
HDLC
Flag
Identifies
ACK,
host.
seq. #
CRC
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 (dialup, dedicated point-to-point connections).
Why MAC?
• Point-to-point versus shared-medium networks.
• Shared-medium networks use “broadcast”
channels.
– multi-access or random access channels.
MAC layer protocols regulate access to medium
in shared-medium networks.
MAC and LANs
• LANs typically use shared-medium.
• Examples?
• MAC layer critical!
– BTW, in wireless networks also!
• WANs typically use point-to-point
connections
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!
MAC Protocols
• Contention-based
– ALOHA and Slotted ALOHA.
– CSMA.
– CSMA/CD.
• Round-robin : token-based protocols.
– Token bus.
– Token ring.
Pure ALOHA
• In pure ALOHA, frames are transmitted at
completely arbitrary times.
Carrier Sense Multiple Access
• The capacity of ALOHA or slotted ALOHA
is limited by the large vulnerability period
of a packet.
• By listening before transmitting, stations try
to reduce the vulnerability period to one
propagation delay.
• This is the basis of CSMA.
CSMA
• Station that wants to transmit first listens to
check if another transmission is in progress
(carrier sense).
• If medium is in use, station waits; else, it
transmits.
• Collisions can still occur.
• Transmitter waits for ACK; if no ACKs,
retransmits.
CSMA Protocol
• CSMA
Packet
ready
Channel
Busy?
no
yes
transmit
delay packet
transmission
k times
wait for a
round-trip time
yes
positive
ack?
no
compute random
backoff integer k
CSMA (cont’d)
• Collisions can occur only when 2 or more
stations begin transmitting within short
time.
• If station transmits and no collisions during
the time leading edge of frame propagates
to farthest station, then NO collisions.
CSMA Flavors
• After detecting carrier, a station can persist
trying to transmit after the channel is idle again.
• 1-persistent CSMA (IEEE 802.3)
– If medium idle, transmit; if medium busy, wait until
idle; then transmit.
– If collision, waits random period and starts again.
• Non-persistent CSMA: if medium idle, transmit;
otherwise wait a random time before re-trying.
– Thus, station does not continuously sense channel when
it is in use.
• P-persistent: when channel idle detected, transmits
packet in the first slot.
CSMA versus Aloha
Comparison of the channel utilization versus
load for various random access protocols.
CSMA/CD
• CSMA with collision detection.
• Problem: when frames collide, medium is
unusable for duration of both (damaged) frames.
• For long frames (when compared to propagation
time), considerable waste.
• What if station listens while transmitting?
CSMA/CD Protocol
1. If medium idle, transmit; otherwise 2.
2. If medium busy, wait until idle, then
transmit with p=1.
3. If collision detected, transmit brief jamming
signal and abort transmission.
4. After aborting, wait random time, try again.
CSMA/CD Performance
• Wasted capacity restricted to time to
detect collision.
• Time to detect collision < 2*maximum
propagation delay.
• Rule in CSMA/CD protocols: frames
long enough to allow collision detection
prior to end of transmission.
CSMA with Collision Detection
• CSMA/CD can be in one of three states:
contention, transmission, or idle.
Ethernet
• IEEE 802. family.
• Standards for LANs and MANs.
• Ethernet defined in the IEEE 802.3 standard.
Ethernet MAC
• CSMA/CD.
• Binary exponential back-off.
• CRC
Ethernet Frame
• Frame formats. (a) DIX Ethernet, (b) IEEE
802.3.
Ethernet MAC (Cont’d)
Collision detection can take as long as 2 .
Ethernet Frame Length
• At 10Mbps with 2,500 m maximum distance:
–
–
–
–
RTT ~ 50 microsec.
Thus, at least 500-bit frames.
It is actually 512 bits.
If fewer bits than that, add “padding”.
Ethernet Cabling
• The most common kinds of Ethernet cabling.
Switched Ethernet
• A simple example of switched Ethernet.
Fast Ethernet
The original fast Ethernet cabling.
Gigabit Ethernet
• Gigabit Ethernet cabling.
Intro to 802.5 and ATM
• 802.5 Token ring networks
• ATM – Asynchronous Transfer Mode