Lecture 3 #1

LAN Technologies

Completing Lecture 2

Lecture 3 #2

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!

Lecture 3 #3

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

Lecture 3 #4

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

Lecture 3 #5

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

Lecture 3 #6

Hubs, Bridges and Switches

Lecture 3

Lecture 3 #7

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 (Ethernet) specifies

maximum cable length large “collision domain” (can collide with many

stations) limited number of stations: 802.5 (token ring)

have token passing delays at each station

Lecture 3 #8

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

Lecture 3 #9

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)

Lecture 3 #10

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) Why?

Lecture 3 #11

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

Lecture 3 #12

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

Lecture 3 #13

Backbone Bridge

Lecture 3 #14

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

Lecture 3 #15

Bridges: frame filtering, forwarding

bridges filter packets same-LAN -segment frames not forwarded

onto other LAN segments forwarding:

how to know on which LAN segment to forward frame?

Lecture 3 #16

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)

Lecture 3 #17

Bridge Operation

bridge procedure(in_MAC, in_port,out_MAC)lookup in filtering table (out_MAC) receive out_portif (out_port not valid) /* no entry found for destination */

then flood; /* forward on all but the interface on which the frame arrived*/

if (in_port = out_port) /*destination is on LAN on which frame was received */then drop the frame

Otherwise (out_port is valid) /*entry found for destination */then forward the frame on interface indicated;

Lecture 3 #18

Bridge Learning: example

Suppose C sends frame to D and D replies back with frame to C

C sends frame, bridge has no info about D, so floods to both LANs bridge notes that C is on port 1 frame ignored on upper LAN frame received by D

Lecture 3 #19

Bridge Learning: example

D generates reply to C, sends bridge sees frame from D bridge notes that D is on interface 2 bridge knows C on interface 1, so selectively

forwards frame out via interface 1

C 1

Lecture 3 #20

What will happen with loops?Incorrect learning

A

B

1 1

22

A , 1 A , 122

Lecture 3 #21

What will happen with loops?Frame looping

A

C

1 1

22

C,?? C,??

Lecture 3 #22

What will happen with loops?Frame looping

A

B

1 1

22

B,2 B,1

Lecture 3 #23

Introducing Spanning Tree

Allow a path between every LAN without causing loops (loop-free environment)

Bridges communicate with special configuration messages (BPDUs)

Standardized by IEEE 802.1D

Note: redundant paths are good, active redundant paths are bad (they cause loops)

Lecture 3 #24

Spanning Tree Requirements Each bridge is assigned a unique

identifier A broadcast address for bridges on a

LAN A unique port identifier for all ports on

all bridges MAC address Bridge id + port number

Lecture 3 #25

Spanning Tree Concepts:Root Bridge The bridge with the lowest bridge ID

value is elected the root bridge One root bridge chosen among all

bridges Every other bridge calculates a path to

the root bridge

Lecture 3 #26

Spanning Tree Concepts:Path Cost A cost associated with each port on

each bridge default is 1

The cost associated with transmission onto the LAN connected to the port

Can be manually or automatically assigned

Can be used to alter the path to the root bridge

Lecture 3 #27

Spanning Tree Concepts:Root Port The port on each bridge that is on the

path towards the root bridge The root port is part of the lowest cost

path towards the root bridge If port costs are equal on a bridge, the

port with the lowest ID becomes root port

Lecture 3 #28

Spanning Tree Concepts:Root Path Cost The minimum cost path to the root

bridge The cost starts at the root bridge Each bridge computes root path cost

independently based on their view of the network

Lecture 3 #29

Spanning Tree Concepts: Designated Bridge Only one bridge on a LAN at one time is

chosen the designated bridge This bridge provides the minimum cost

path to the root bridge for the LAN Only the designated bridge passes

frames towards the root bridge

Lecture 3 #30

Example Spanning Tree

B3

B5

B7B2

B1

B6 B4

Protocol operation:1. Picks a root2. For each LAN,

picks a designated bridgethat is closest to the root.

3. All bridges on a LANsend packets towards the root via the designated bridge.

B8

Lecture 3 #31

Example Spanning Tree

B3

B5

B7B2

B1

B6 B4

Root

B8

B2 B4 B5 B7

B8

B1

Spanning Tree:

Designated Bridge

root port

Lecture 3 #32

Spanning Tree Algorithm:An Overview 1. Determine the root bridge among all bridges 2. Each bridge determines its root port

The port in the direction of the root bridge 3. Determine the designated bridge on each

LAN The bridge which accepts frames to forward towards

the root bridge The frames are sent on the root port of the

designated bridge

Lecture 3 #33

Spanning Tree Algorithm:Selecting Root Bridge Initially, each bridge considers itself to

be the root bridge Bridges send BDPU frames to its

attached LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge

considers root The root path cost for the sending bridge

Best one wins (lowest root ID/cost/priority)

Lecture 3 #34

Spanning Tree Algorithm:Selecting Root Ports Each bridge selects one of its ports

which has the minimal cost to the root bridge

In case of a tie, the lowest uplink (transmitter) bridge ID is used

In case of another tie, the lowest port ID is used

Lecture 3 #35

Spanning Tree Algorithm:Select Designated Bridges

Initially, each bridge considers itself to be the designated bridge

Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge

considers root The root path cost for the sending bridge

3. Best one wins (lowest ID/cost/priority)

Lecture 3 #36

Forwarding/Blocking State Root and designated bridges will

forward frames to and from their attached LANs

All other ports are in the blocking state

Lecture 3 #37

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

Lecture 3 #38

Routers vs. Bridges

Bridges + and - + Bridge operation is simpler requiring less

processing- 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)

Lecture 3 #39

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

bridges do well in small (few hundred hosts) while routers used in large networks (thousands of hosts)

Lecture 3 #40

Ethernet Switches

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!

Lecture 3 #41

Ethernet Switches

cut-through switching: frame forwarded from input to output port without awaiting for assembly of entire frameslight reduction in latency

combinations of shared/dedicated, 10/100/1000 Mbps interfaces

Lecture 3 #42

Ethernet Switches (more)Dedicated

Shared

Lecture 3 #43

Optional: Wireless LAN and PPP

Lecture 3 #44

IEEE 802.11 Wireless LAN wireless LANs: untethered (often mobile)

networking IEEE 802.11 standard:

MAC protocol unlicensed frequency spectrum: 900Mhz,

2.4Ghz Basic Service Set (BSS)

(a.k.a. “cell”) contains: wireless hosts access point (AP):

base station BSS’s combined to

form distribution system (DS)

Lecture 3 #45

Ad Hoc Networks Ad hoc network: IEEE 802.11 stations can

dynamically form network without AP Applications:

“laptop” meeting in conference room, car

interconnection of “personal” devicesbattlefield

IETF MANET (Mobile Ad hoc Networks) working group

Lecture 3 #46

IEEE 802.11 MAC Protocol: CSMA/CA802.11 CSMA: sender- if sense channel idle for

DISF sec. then transmit entire frame

(no collision detection)-if sense channel busy

then binary backoff

802.11 CSMA receiver:if received OK return ACK after SIFS

Lecture 3 #47

IEEE 802.11 MAC Protocol

802.11 CSMA Protocol: others

NAV: Network Allocation Vector

802.11 frame has transmission time field

others (hearing data) defer access for NAV time units

Lecture 3 #48

Hidden Terminal effect

hidden terminals: A, C cannot hear each other obstacles, signal attenuation collisions at B

goal: avoid collisions at B CSMA/CA: CSMA with Collision Avoidance

Lecture 3 #49

Collision Avoidance: RTS-CTS exchange CSMA/CA: explicit

channel reservation sender: send short

RTS: request to send receiver: reply with

short CTS: clear to send

CTS reserves channel for sender, notifying (possibly hidden) stations

avoid hidden station collisions

Lecture 3 #50

Collision Avoidance: RTS-CTS exchange

RTS and CTS short: collisions less likely, of

shorter duration end result similar to

collision detection IEEE 802.11 allows:

CSMA CSMA/CA: reservations polling from AP

Lecture 3 #51

Point to Point Data Link Control one sender, one receiver, one link:

easier than broadcast link:no Media Access Controlno need for explicit MAC addressinge.g., dialup link, ISDN line

popular point-to-point DLC protocols:PPP (point-to-point protocol)HDLC: High level data link control

(Data link used to be considered “high layer” in protocol stack!)

Lecture 3 #52

PPP Design Requirements [RFC 1557] packet framing: encapsulation of network-layer

datagram in data link frame carry network layer data of any network layer

protocol (not just IP) at same time ability to demultiplex upwards

bit transparency: must carry any bit pattern in the data field

error detection (no correction) connection livenes: detect, signal link failure to

network layer network layer address negotiation: endpoint can

learn/configure each other’s network address

Lecture 3 #53

PPP non-requirements

no error correction/recovery no flow control out of order delivery OK no need to support multipoint links

(e.g., polling)

Error recovery, flow control, data re-ordering all relegated to higher layers!!!

Lecture 3 #54

PPP Data Frame

Flag: delimiter (framing) Address: does nothing (only one option) Control: does nothing; in the future possible

multiple control fields Protocol: upper layer protocol to which frame

delivered (eg, PPP-LCP, IP, IPCP, etc)

Lecture 3 #55

PPP Data Frame

info: upper layer data being carried check: cyclic redundancy check for

error detection

Lecture 3 #56

Byte Stuffing “data transparency” requirement: data field

must be allowed to include flag pattern <01111110> Q: is received <01111110> data or flag?

Sender: adds (“stuffs”) extra < 01111101> byte before each < 01111110> or <01111101> data byte

Receiver: Receive 01111101

• discard the byte, • Next byte is data

Receive 01111110: flag byte

Lecture 3 #57

Byte Stuffing

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

Lecture 3 #58

PPP Data Control ProtocolBefore exchanging network-

layer data, data link peers must

configure PPP link (max. frame length, authentication)

learn/configure network layer information

for IP: carry IP Control Protocol (IPCP) msgs (protocol field: 8021) to configure/learn IP address

Lecture 3 #59

Data Link: Summary

principles behind data link layer services: error detection, correction sharing a broadcast channel: multiple access link layer addressing, ARP

various link layer technologies Ethernet hubs, bridges, switches IEEE 802.11 LANs PPP

Chapter 5 Kurose and Ross

Lecture 3 #60

Configuration Messages: BPDU