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Link Layer
MAC
Dr. Mozafar Bag-Mohammadi
University of Ilam
Contents
1. Multiple Access Protocols
2. Local Area Network (LAN)
3. Ethernet
4. Hubs, Bridges, and Switches
1.Multiple Access Links and Protocols
Three types of “links”:
 point-to-point (single wire, e.g. PPP, SLIP)
 broadcast (shared wire or medium; e.g, Ethernet, Wavelan, etc.)
Multiple Access protocols
 single shared communication channel
 two or more simultaneous transmissions by nodes: interference


only one node can send successfully at a time
multiple access protocol:


distributed algorithm that determines how stations share channel,
i.e., determine when station can transmit
type of protocols:
• synchronous or asynchronous
• information needed about other stations
• robustness (e.g., to channel errors)
• performance
Multiple Access Control Protocols
Three broad classes:
 Channel Partitioning
 divide channel into smaller “pieces” (time slots, frequency, code)
 allocate piece to node for exclusive use
 TDMA, FDMA, CDMA
 Random Access
 allow collisions
 “recover” from collisions
 CSMA, ALOHA
 Taking turns
 tightly coordinate shared access to avoid collisions
 Token ring
Goal: efficient, fair, simple, decentralized
Random Access protocols
 When node has frame to send
transmit at full channel data rate R.
 no a priori coordination among nodes
 two or more transmitting nodes -> “collision”,
 random access MAC protocol specifies:
 how to detect collisions
 how to recover from collisions (e.g., via delayed retransmissions)
 Examples of random access MAC protocols:
 slotted ALOHA
 ALOHA
 CSMA and CSMA/CD

CSMA: Carrier Sense Multiple Access
CSMA: listen before transmit:
 If channel sensed idle: transmit entire packet
 If channel sensed busy, defer transmission
CSMA/CD (Collision Detection)
CSMA/CD: carrier sensing



collisions detected within short time
colliding transmissions aborted, reducing channel wastage
persistent or non-persistent retransmission
 collision detection:
 easy in wired LANs: measure signal strengths, compare
transmitted, received signals
 difficult in wireless LANs: receiver shut off while transmitting
CSMA/CD collision detection
MAC Address (more)
 MAC address allocation administered by IEEE
 manufacturer buys portion of MAC address space (to assure
uniqueness)
 Analogy:
(a) MAC address: like Social Security Number
(b) IP address: like postal address
 MAC flat address => portability

can move LAN card from one LAN to another
 IP hierarchical address NOT portable

depends on network to which one attaches
IEEE 802 Standards
 IEEE 802 is a family of standards for Local Area Network
(LAN), which defines an LLC and several MAC sublayers
IEEE 802 standard
IEEE
Reference
Model
802.1
Logical Link
Control
802.2
802.11
802.5
802.4
802.3
Medium
Access
Control
Physical
Layer
Higher
Layer
Data Link
Layer
Physical
Layer
Ethernet
 Speed:
 Standard:
10Mbps -10 Gbps
802.3, Ethernet II (DIX)
 Most popular physical layers for Ethernet:
•
•
•
•
•
•
•
10Base5
10Base2
10Base-T
100Base-TX
100Base-FX
1000Base-FX
10000Base-FX
links)
Thick Ethernet: 10 Mbps coax cable
Thin Ethernet: 10 Mbps coax cable
10 Mbps Twisted Pair
100 Mbps over Category 5 twisted pair
100 Mbps over Fiber Optics
1Gbps over Fiber Optics
10Gbps over Fiber Optics (for wide area
Bus Topology
 10Base5 and 10Base2 Ethernets has a bus
topology
Ethernet
Star Topology
 Starting with 10Base-T, stations are
connected to a hub in a star configuration
Hub
Frame format
64
48
48
16
Preamble
Dest
addr
Src
addr
Type
32
Body
CRC
Ethernet uses CSMA/CD
A: sense channel,
if idle
then {
}
transmit and monitor the channel;
If detect another transmission
then {
abort and send jam signal;
update # collisions;
delay as required by exponential backoff algorithm;
goto A
}
else {done with the frame; set collisions to zero}
else {wait until ongoing transmission is over and goto A}
Ethernet’s CSMA/CD (more)
Jam Signal: make sure all other transmitters are aware of collision;
48 bits;
Exponential Backoff:
 Goal: adapt retransmission attempts to estimated current load
 heavy load: random wait will be longer
 first collision: choose K from {0,1}; delay is K x 512 bit
transmission times
 after second collision: choose K from {0,1,2,3}…
 after ten or more collisions, choose K from {0,1,2,3,4,…,1023}
4. Hubs, Bridges and Switches
5. 1 Interconnecting LANs
Q: Why not just one big LAN?
 Limited amount of supportable traffic: on single LAN, all stations
must share bandwidth
 limited length: 802.3 specifies maximum cable length
 large “collision domain” (can collide with many stations)
Hubs
 Physical Layer devices: essentially repeaters operating at bit
levels: repeat received bits on one interface to all other
interfaces
 Hubs can be arranged in a hierarchy (or multi-tier design), with
backbone hub at its top
Hubs (more)
 Each connected LAN referred to as LAN segment
 Hubs do not isolate collision domains: node may collide with any
node residing at any segment in LAN
 Hub Advantages:
 simple, inexpensive device
 Multi-tier provides graceful degradation: portions of the LAN
continue to operate if one hub malfunctions
 extends maximum distance between node pairs (100m per Hub)
Hub limitations
 single collision domain results in no increase in max throughput
multi-tier throughput same as single segment throughput
 individual LAN restrictions pose limits on number of nodes in
same collision domain and on total allowed geographical
coverage
 cannot connect different Ethernet types (e.g., 10BaseT and
100baseT)

Ethernet Hubs vs. Ethernet Switches
 An Ethernet switch is a packet switch for Ethernet frames
• Buffering of frames prevents collisions.
• Each port is isolated and builds its own collision domain
 An Ethernet Hub does not perform buffering:
• Collisions occur if two frames arrive at the same time.
Hub
Switch
CSMA/CD
CSMA/CD
CSMA/CD
CSMA/CD
CSMA/CD
CSMA/CD
CSMA/CD
CSMA/CD
CSMA/CD
CSMA/CD
CSMA/CD
CSMA/CD
CSMA/CD
HighSpeed
Backplane
CSMA/CD
Input
Buffers
CSMA/CD
CSMA/CD
Output
Buffers
Bridges
 Link Layer devices: operate on Ethernet frames, examining
frame header and selectively forwarding frame based on its
destination
 Bridge isolates collision domains since it buffers frames
 When frame is to be forwarded on segment, bridge uses
CSMA/CD to access segment and transmit
Bridges (more)
 Bridge advantages:
 Isolates collision domains resulting in higher total max
throughput, and does not limit the number of nodes nor
geographical coverage
 Can connect different type Ethernet since it is a store and
forward device
 Transparent: no need for any change to hosts LAN adapters
Bridges: frame filtering, forwarding
 frame filtering
 same-LAN-segment frames not forwarded onto other LAN
segments
 forwarding:
 how to know which LAN segment on which to forward
frame?
 looks like a routing problem (more shortly!)
Interconnection with Backbone Bridge
Interconnection Without Backbone
 Not recommended for two reasons:
- single point of failure at Computer Science hub
- all traffic between EE and SE must path over CS segment
Bridge Filtering
 bridges
learn which hosts can be reached through which interfaces:
maintain filtering tables
 when frame received, bridge “learns” location of sender: incoming
LAN segment
 records sender location in filtering table
 filtering table entry:
 (Node LAN Address, Bridge Interface, Time Stamp)
 stale entries in Filtering Table dropped (TTL can be 60 minutes)
Bridges Spanning Tree
 for increased reliability, desirable to have redundant, alternate
paths from source to dest
 with multiple simultaneous paths, cycles result - bridges may
multiply and forward frame forever
 solution: organize bridges in a spanning tree by disabling subset
of interfaces
Disabled
Bridges vs. Routers
 both store-and-forward devices


routers: network layer devices (examine network layer headers)
bridges are Link Layer devices
 routers maintain routing tables, implement routing algorithms
 bridges maintain filtering tables, implement filtering, learning and
spanning tree algorithms
Routers vs. Bridges
Bridges + and + Bridge operation is simpler requiring less processing bandwidth
- Topologies are restricted with bridges: a spanning tree must be
built to avoid cycles
- Bridges do not offer protection from broadcast storms (endless
broadcasting by a host will be forwarded by a bridge)
Routers vs. Bridges
Routers + and + arbitrary topologies can be supported, cycling is limited by TTL counters
(and good routing protocols)
+ provide firewall protection against broadcast storms
- require IP address configuration (not plug and play)
- require higher processing bandwidth
 bridges do well in small (few hundred hosts) while routers used in large
networks (thousands of hosts)
Ethernet Switches
 Popular LAN device
 layer 2 (frame) forwarding, filtering
using LAN addresses
 Switching: A-to-B and A’-to-B’
simultaneously, no collisions
 large number of interfaces
 often: individual hosts, starconnected into switch
 Ethernet, but no collisions!
Ethernet Switches (more)
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