1. 18: Ethernet, Hubs, Bridges, Switches
Last Modified:
4/6/2019 8:57:37 PM
5: DataLink Layer

2. Ethernet “dominant” LAN technology: First widely used LAN technology
Kept up with speed race: 10, 100, 1000 Mbps
Metcalfe’s Ethernet
sketch
5: DataLink Layer

3. Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble:
7 bytes with pattern followed by one byte with pattern
used to synchronize receiver, sender clock rates
5: DataLink Layer

4. Ethernet Frame Structure (more)
Addresses: 6 bytes, frame is received by all adapters on a LAN and dropped if address does not match
Type: indicates the higher layer protocol, mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC: checked at receiver, if error is detected, the frame is simply dropped
5: DataLink Layer

5. Ethernet: Unreliable, connectionless
connectionless: No handshaking between sending and receiving NICs
unreliable: receiving NIC doesn’t send acks or nacks to sending NIC
stream of datagrams passed to network layer can have gaps (missing datagrams)
gaps will be filled if app is using TCP
otherwise, app will see gaps
Ethernet’s MAC protocol: unslotted CSMA/CD
Data Link Layer

6. 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}
5: DataLink Layer

7. 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}
5: DataLink Layer

8. Manchester encoding used in 10BaseT each bit has a transition
allows clocks in sending and receiving nodes to synchronize to each other
no need for a centralized, global clock among nodes!
Hey, this is physical-layer stuff!
Data Link Layer

9. Repeaters
Physical Layer devices: operating at bit levels: repeat received bits on one interface to all other interfaces
Extend the range of a signal by amplifying
Useful when want to connect devices beyond the IEEE specifications for distance limitation of 328 feet or 100 meters
5: DataLink Layer

10. Hubs  Also physical layer device, but may have some management
Hubs can be arranged in a hierarchy (or multi-tier design), with backbone hub at its top
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)
5: DataLink Layer

11. Hubs … physical-layer (“dumb”) repeaters:
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another
no frame buffering
no CSMA/CD at hub: host NICs detect collisions
twisted pair
hub
Data Link Layer

12. Hub limitations
Single collision domain results in no increase in max throughput
multi-tier throughput same as single segment throughput
Also less secure – hear traffic from/to everyone on the hub
Individual LAN restrictions pose limits on number of nodes in same collision domain and on total allowed geographical coverage
Difficult to connect different Ethernet types, but can have dual speed hubs (e.g., 10BaseT and 100baseT)
5: DataLink Layer

13. Switch link-layer device: smarter than hubs, take active role
store, forward Ethernet frames
examine incoming frame’s MAC address, selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment, uses CSMA/CD to access segment
transparent
hosts are unaware of presence of switches
plug-and-play, self-learning
switches do not need to be configured
Data Link Layer

14. Switch: allows multiple simultaneous transmissionsB B’ C C’ 1 2 3 4 5 6
Switch isolates collision domains
Hosts have dedicated, direct connection to switch
A-to-A’ and B-to-B’ simultaneously, without collisions
not possible with dumb hub
Does not forward out all interfaces
Buffers frames
Ethernet protocol used on each incoming link, but no collisions; full duplex
each link is its own collision domain
switch with six interfaces
(1,2,3,4,5,6)
Data Link Layer

15. Collision domain Broadcast domain
When I speak, who else can I prevent from speaking at the same time
Hub = one collision domain; Switch = collision domain per port
Broadcast domain
When I deliberately send a broadcast address, who all hears it
VLANs separate broadcast domains
5: DataLink Layer

16. Managed vs Unmanaged
Switches more likely than hubs or repeaters to have sophisticated management features
Log in remotely and configure, get reports/statistics etc.
More control over what each port or group of ports can do (e.g. establish groups of ports into virtual LANs or VLANs that further divide the broadcast domain)
5: DataLink Layer

17. Switches (more) Switch advantages:
Isolates collision domains resulting in higher total max throughput and more security
Can connect different type Ethernet since it is a store and forward device ( dual speed hub is compromise between full switch and hub that does this)
5: DataLink Layer

18. Switch: frame filtering, forwarding
Switches filter packets
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?
5: DataLink Layer

19. Switch: self-learning
Source: A
Dest: A’ A
A A’
switch learns which hosts can be reached through which interfaces
when frame received, switch “learns” location of sender: incoming LAN segment
records sender/location pair in switch table C’ B 1 2 6 3 4 5 C B’ A’
MAC addr interface TTL A 1 60
Switch table
(initially empty)
Data Link Layer

20. Switch: frame filtering/forwarding
When frame received:
1. record link associated with sending host
2. index switch table using MAC dest address
3. if entry found for destination then {
if dest on segment from which frame arrived then drop the frame
else forward the frame on interface indicated }
else flood
forward on all but the interface
on which the frame arrived
Data Link Layer

21. Self-learning, forwarding: example
Source: A
Dest: A’
Self-learning, forwarding: example A
A A’ C’ B
frame destination unknown: 1 2
flood 6 3
A A’
A A’
A A’
A A’
A A’ 4 5
destination A location known: C
A’ A
selective send B’ A’
MAC addr interface TTL A 1 60
Switch table
(initially empty) A’ 4 60
Data Link Layer

22. Generally on a switch only see traffic to/from your machine and broadcast traffic
Can attack switch by sending many MACs and overflowing its storage of which MACs on which port => will begin to act like hub ( flooding each packet out every port)
5: DataLink Layer

23. Interconnecting switches
switches can be connected together D E F S2 S4 S3 H I G S1 A B C
Q: sending from A to G - how does S1 know to forward frame destined to F via S4 and S3?
A: self learning! (works exactly the same as in single-switch case!)
Data Link Layer

24. Switches vs. Routers both store-and-forward devices
application
transport
network
link
physical
both store-and-forward devices
routers: Layer 3 or network-layer devices (examine network-layer headers)
switches are Layer 2 or link-layer devices (examine link-layer headers)
routers maintain routing tables, implement routing algorithms
switches maintain switch tables, implement filtering, learning algorithms
datagram
frame
link
physical
frame
switch
network
link
physical
datagram
frame
application
transport
network
link
physical
Data Link Layer

25. Switch Pros and Cons
+ Switch operation is simpler requiring less processing bandwidth
- Topologies are restricted with bridges: a spanning tree must be built to avoid cycles
- Switch do not offer protection from broadcast storms (endless broadcasting by a host will be forwarded by a bridge)
5: DataLink Layer

26. Routers Pros and Cons
+ 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
5: DataLink Layer

27. Summary Layer 3 Devices (Network Layer) Layer 2 Devices (Link Layer)
Router
Layer 2 Devices (Link Layer)
Bridge
Switch
Layer 1 Devices (Physical Layer)
Repeaters
Hubs
5: DataLink Layer

28. Outtakes
5: DataLink Layer

29. Institutional network
mail server
to external
network
web server
router
IP subnet
Data Link Layer

30. Switch Learning: example
Suppose C sends frame to D and D replies back with frame to C
C sends frame, switch has no info about D, so floods to both LANs
switch notes that C is on port 1
frame ignored on upper LAN
frame received by D
5: DataLink Layer

31. Switch Learning: example
D generates reply to C, sends
switch sees frame from D
switch notes that D is on interface 2
switch knows C on interface 1, so selectively forwards frame out via interface 1
5: DataLink Layer

32. 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
5: DataLink Layer

33. Spanning Tree Algorithm
5: DataLink Layer

34. VLAN tagging
5: DataLink Layer

35. 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
5: DataLink Layer

36. Backbone Switch
5: DataLink Layer

37. Ethernet Switches Sophisticated bridges
Switches usually switch in hardware, bridges in software
large number of interfaces
Like bridges, layer 2 (frame) forwarding, filtering using LAN addresses
Can support combinations of shared/dedicated, 10/100/1000 Mbps interfaces
5: DataLink Layer

38. Switching Switching: A-to-B and A’-to-B’ simultaneously, no collisions
cut-through switching: frame forwarded from input to output port without awaiting for assembly of entire frame
slight reduction in latency
Store and forward switching: entire frame received before transmission out an output port
Fragment-free switching: compromise, before send out the output port receive enough of the packet to do some error checking (ex. detect and drop partial frames)
5: DataLink Layer

39. Ethernet Limitations Q: Why not just one big Ethernet?
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)
5: DataLink Layer