18: Ethernet, Hubs, Bridges, Switches

5: DataLink Layer 5a-1

18: Ethernet, Hubs, Bridges, Switches

Last Modified: 04/19/23 04:47 PM


Ethernet“dominant” LAN technology: First widely used LAN technology Kept up with speed race: 10, 100, 1000 Mbps

Metcalfe’s Ethernetsketch


Ethernet Frame Structure

Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

Preamble: 7 bytes with pattern 10101010 followed by one

byte with pattern 10101011 used to synchronize receiver, sender clock

rates


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

Data Link Layer 5-5

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


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}

Data Link Layer 5-8

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!


Ethernet Technologies: 10Base2 10: 10Mbps; 2: under 200 meters max cable length thin coaxial cable in a bus topology

repeaters used to connect up to multiple segments repeater repeats bits it hears on one interface to its other interfaces: physical layer device only!


10BaseT and 100BaseT

10/100 Mbps rate; latter called “fast ethernet” T stands for Twisted Pair Hub to which nodes are connected by twisted

pair, thus “star topology” CSMA/CD implemented at hub


10BaseT and 100BaseT (more) Max distance from node to Hub is 100 meters Hub can disconnect “jabbering adapter” Hub can gather monitoring information,

statistics for display to LAN administrators


Gbit Ethernet

use standard Ethernet frame format allows for point-to-point links and shared

broadcast channels in shared mode, CSMA/CD is used; short

distances between nodes to be efficient uses hubs, called here “Buffered Distributors” Full-Duplex at 1 Gbps for point-to-point links


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

802.3 specifications for distance limitation of 328 feet or 100 meters

Examples – outdoor installations, mine shafts, remote locations, etc.


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)

Data Link Layer 5-15

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


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)


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


Switch: allows multiple simultaneous transmissions

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)

A

A’

B

B’

C

C’

1 23

45

6

Collision 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


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)



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)


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?


Switch: self-learning

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

A

A’

B

B’

C

C’

1 23

45

6

A A’

Source: ADest: A’

MAC addr interface TTL

Switch table (initially empty)

A 1 60


Switch: frame filtering/forwardingWhen frame received:

1. record link associated with sending host2. index switch table using MAC dest address3. 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


Self-learning, forwarding: example

A

A’

B

B’

C

C’

1 23

45

6

A A’

Source: ADest: A’

MAC addr interface TTL

Switch table (initially empty)

A 1 60

A A’A A’A A’A A’A A’

frame destination unknown:flood

A’ A

destination A location known:

A’ 4 60

selective send

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)



Interconnecting switches

switches can be connected together

A

B

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!)

S1

C D

E

FS2

S4

S3

H

I

G


Switches vs. Routers

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

application

transportnetwork

linkphysical

networklink

physical

linkphysical

switch

datagram

application

transportnetwork

linkphysical

frame

frame

frame

datagram


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)


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


Network Diagrams

Shared

Sample Icons

Icons for in network diagrams



Summary

Layer 3 Devices (Network Layer) Router

Layer 2 Devices (Link Layer) Bridge Switch

Layer 1 Devices (Physical Layer) Repeaters Hubs


Outtakes


Institutional network

to externalnetwork

router

IP subnet

mail server

web server


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


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


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


Spanning Tree Algorithm

VLAN tagging



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


Backbone Switch


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


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)


Ethernet Limitations

Q: Why not just one big Ethernet? 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)

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18: Ethernet, Hubs, Bridges, Switches