1. Dr. Mozafar Bag-Mohammadi University of Ilam
Link Layer 2
Dr. Mozafar Bag-Mohammadi
University of Ilam

2. Contents Multiple Access Protocols Local Area Network (LAN) Ethernet
Hubs, Bridges, and Switches

3. 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.)
switched (e.g., switched Ethernet, ATM etc)

4. 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
communication about channel sharing must use channel itself!
type of protocols:
synchronous or asynchronous
information needed about other stations
robustness (e.g., to channel errors)
performance

5. 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

6. Random Access protocols
When node has packet 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

7. CSMA: Carrier Sense Multiple Access
CSMA: listen before transmit:
If channel sensed idle: transmit entire pkt
If channel sensed busy, defer transmission
human analogy: don’t interrupt others!

8. CSMA/CD (Collision Detection)
CSMA/CD: carrier sensing, deferral as in CSMA
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
human analogy: the polite conversationalist

9. Collisions A B A B A B A B

10. CSMA/CD collision detection

11. 2. Local Area Network (LAN)
addressing
Ethernet
hubs, bridges, switches
802.11
PPP
ATM

12. LAN 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

13. IEEE 802 Standards
IEEE 802 is a family of standards for LANs, which defines an LLC and several MAC sublayers

14. 3. Ethernet “dominant” LAN technology:
first widely used LAN technology
Simpler, cheaper than token LANs and ATM
Kept up with speed race: 10Mbps, 100Mbps, 1Gbps, 10Gbps
Metcalfe’s Ethernet
sketch

15. Ethernet Speed: 10Mbps -10 Gbps Standard: 802.3, Ethernet II (DIX)
Most popular physical layers for Ethernet:
10Base5 Thick Ethernet: 10 Mbps coax cable
10Base2 Thin Ethernet: 10 Mbps coax cable
10Base-T 10 Mbps Twisted Pair
100Base-TX 100 Mbps over Category 5 twisted pair
100Base-FX 100 Mbps over Fiber Optics
1000Base-FX 1Gbps over Fiber Optics
10000Base-FX 10Gbps over Fiber Optics (for wide area links)

16. Bus Topology
10Base5 and 10Base2 Ethernets has a bus topology

17. Star Topology
Starting with 10Base-T, stations are connected to a hub in a star configuration

18. Frame format
Dest
addr 64 48 32
CRC
Preamble
Src
Type
Body 16

19. Ethernet uses CSMA/CD A: sense channel, if idle then {
transmit and monitor the channel;
If detect another transmission
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}

20. 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}

21. 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: specifies maximum cable length
large “collision domain” (can collide with many stations)

22. 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

23. 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)

24. 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)

25. 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

26. 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

27. 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

28. Bridges: frame filtering, forwarding
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!)

29. Interconnection with Backbone Bridge

30. 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

31. 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)

32. 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

33. 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

34. 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)

35. 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)

36. 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, star-connected into switch
Ethernet, but no collisions!

37. Ethernet Switches (more)
Shared