2 Internetworking With Ethernet Issue Sep 2

11TOPC-E4011E-02-ＸＸＸＸＸＸ

Basics of InternetworkingBasics of Internetworking

Ethernet Based LAN


General InternetworkingGeneral Internetworking

LAN

Ethernet

LANLAN

LAN

LANEthernet

LAN

WAN

File SVR

File SVR

Switch

Router

HUB

LANNE

NE

NE

i.e. Ethernet over RadioEthernet

Ethernet

Ethernet

The LAN EnvironmentThe WAN Environment


ContentsContents

1. General Description of an Ethernet Based LAN

2. Layer 2 Switching

3. Layer 3 and Routing

4. VLAN Technology

5. Principles of Flow Control and Auto Negotiation


Chapter 1Chapter 1

General Description of an Ethernet Based LAN


LAN based on HubLAN based on Hub

Fm A

to C MSG

Fm A

to C MSG

Fm A

to C MSG

Ethernet HUB

A

B

C

•Hosts connect to a LAN through their Ethernet Board. Each board has a unique address called MAC Address

•Ethernet Hub acts as repeater

•Any message input through any hub port is replicated on all remaining hub ports

•All Hosts share the bandwidth of the Ethernet Network (for example, 10 Mbps)

•Stations cannot send and receive at the same time (Half Duplex mode of operation is mandatory)

: Ethernet Board


Shared Media: Collisions!Shared Media: Collisions!

Fm A

to C MSG Fm

Cto B MSG

Ethernet HUB

A

B

C

•Collision: Simultaneous transmission by multiple Hosts

•Information is lost, and must be re-transmitted

•Possibility of collision increases with the number of hosts connected to the same shared Ethernet segment (known as the size of the Collision Domain)


UnicastUnicast TransmissionTransmission

A B C

OS OS OS

Ethernet BoardMAC A

Ethernet BoardMAC B

Ethernet BoardMAC C

Fm A

to C Message

1

2

3 4

Message Message

When a user needs to deliver a message to another specific host

(Unicast):

•User A requests its Ethernet Board to send a message to User C

•Ethernet Board A places frame containing message on shared medium (addressed only to User C)

•Ethernet Board B receives frame, but discards it without interrupting OS of User B

•Ethernet Board C receives frame, and hands over message to User C

1

2

3

4

: Operative System, running on PC’s CPUOS


Broadcast TransmissionBroadcast Transmission

A B C

OS OS OS

Ethernet BoardMAC A

Ethernet BoardMAC B

Ethernet BoardMAC C

Fm A

to All Message

1

2

3 3

Message MessageMessage

At times, a user needs to deliver a message to all other hosts in the LAN

(Broadcast):

•User A requests Ethernet Board to send a message to All Hosts.

•Ethernet Board A places frame containing message on shared medium (addressed to All Hosts)

•Ethernet Boards B and C receive the frame, and forward the message to their respective users.•Interrupts processing of all computers on Local

Network (Broadcast Domain)

•Excessive broadcasting can affect performance of all machines on local network

1

2

3


MAC AddressMAC Address

48 Bits (6 Octets)1 2 3 4 5 6

FF – FF – FF – FF – FF – FF Broadcast Address All ‘1’s

00 – 01 – 24 – 80 – 4a – 7F Example of Unicast Address

Each Ethernet Board has one unique Unicast Address

01 – 80 – c2 – 00 – 00 – 00 Example of Multicast Address

Set to 1(LSB of first Octet set to 1)

• Allows delivery of information to selected groups of hosts on the LAN

• Ethernet Board can belong to several Multicast Groups at a time


MAC Frame FormatMAC Frame Format

Preamble SFD Frame IFG7 1 12 (Oct) •Preamble: Alternating 1s and 0s for sync

of receiver

•SDF = Start Frame Delimiter = ‘10101011’

•IFG = Inter Frame Gap

DEST MAC FCS (Extension)

6 6

Source MAC

Length Type

Client Data (+PAD)

2 46 ~ 1500 4 (Oct) •Client Data:Must be padded if shorter than 46 Oct

Must not exceed 1500 Octets

•FCS: Frame Check SequenceFor corrupted frame detection

•Extension: (when required)

For collision detection of GbE Half Duplex

Appended when required, to guarantee minimum frame length of 512 Octets

Minimum Frame size: 6+6+2+46+4 = 64 Octets

Maximum Frame size: 6+6+2+1500+4 = 1518 Octets

MTU of Ethernet: 1518 Octets (Maximum Transfer Unit)

•Length/Type:If ≤ 1500: Length of Client Data

If ≥ 1536: Type of Client Protocol ex: Case IP, Type=2048 (0800Hex)


Minimum Frame Lengths for Collision DetectionMinimum Frame Lengths for Collision Detection

By appending Extension at end of frame, whenever required

4095 bits (512 Oct)1000 Mbps

By PAD of data field, added whenever necessary

512 bits (64 Oct)10/100 Mbps

MethodMin. LengthRate

Runt Frame: Frame shorter than 64 bytes. When received, it is considered a collision fragment, and is automatically discarded


Chapter Chapter 22

Layer 2 Switching


LAN Based on Switch (Layer 2 SW)LAN Based on Switch (Layer 2 SW)

Fm A

to D MSG

Fm A

to D MSG

1

2

3

4

A

B

C

D

4D

3C

2B

1A

PortHost MAC

Forwarding Database

Switch Vs Hub:

•Switch learns about hosts connected to its physical ports

•Learned information is stored in FDB (Forwarding Database)

•Learning process usually achieved dynamically (by inspection of source addresses of incoming frames)

•Switch can establish multiple simultaneous point to point connections, improving the throughput of the local network

•Ethernet Switch functionality (also called MAC Bridge) is defined by IEEE Standard 802.1D


Environment Free of CollisionsEnvironment Free of Collisions

Fm A

to C MSG1

1

2

3

4

A

B

C

D

Fm A

to C MSG1

Fm D

to A MSG2

Fm D

to A MSG2

•Switch provides dedicated bandwidth to the users connected to each port

•When a port connects to only one user, the user can transmit and receive frames simultaneously

•Ethernet Port may operate in Full Duplex Mode

•No risk of frame collision: No Collision Domain


Switch ports shared through HubSwitch ports shared through Hub

1

2

3

4

A

B

C

D

E

Full Duplex

Full Duplex

Full Duplex

Half Duplex

Half Duplex

Half Duplex

•Only directly connected hosts can benefit from Full Duplex mode of operation

•Users sharing a switch port through a hub must make use of Half Duplex mode of operation

•Half/Full duplex settings must be identical on both ends of one link.

•The operation mode is typically decided automatically during port start up, through a feature of Ethernet called Autonegotiation


Learning process of SwitchLearning process of Switch

1

2

3

4

A

B

C

DFm D

to A

Fm A

to D

Fm D

to A

PortHost MAC

Forwarding Database

A 1

D 4

1

2

3

4

5

6

7

8 4

ENTRY

ENTRY

1

2

3

4

5

6

7

8

•A sends Unicast frame to D

•Switch examines Source MAC Address of frame, and creates entry in FDB: ‘User A is in Port 1’

•Switch examines Destination MAC Address of frame, and attempts to find location of User D in FDB. When location is unknown, the switch forwards the frame to all remaining ports (Unicast Flooding)

•Users B and C discard the unnecessarily received frame

•User D receives his frame. Let’s assume now that he sends a reply to User A.

•Switch learns location of User D, as in Step 2: ‘User D is in Port 4)

•Switch finds location of Destination User A in FDB, and forwards the frame to Port 1 only (Unicast Forwarding)

•User A receives his reply

Assume, to begin, that the FDB is empty:

Fm A

to D

Fm A

to D

Fm A

to D


Aging of FDB EntriesAging of FDB Entries

4

3

2

1

Entry No.

300

27

Age

4D

1A

PortHost MAC

•Entries of FDB are typically not permanent

•If a host becomes ‘silent’, his corresponding entry will be deleted after the expiration of an Aging Timer

•Typical Aging time is 300 seconds

•Every second, the age of each FDB entry is increased by ‘1’.

•Every time a frame is received from a known user, the age of his entry is reset back to ‘0’.

For Example:

•If user D stops sending frames, after 300 seconds of silence his FDB entry will be deleted

•After this moment, if User D sends a frame again, the switch learning process will be repeated.

Forwarding Database

Aged Entry!!


Redundancy in the Switched NetworkRedundancy in the Switched Network

1

2

3

4

A

B

D

E

4F

4E

4D

3C

2B

1A

PortHost MAC

FDB SW1

4C

4B

4A

3F

2E

1D

PortHost MAC

FDB SW2

C F

1

2

3

4

SW1 SW2

•A LAN may be composed of several Switches

•A fully learned configuration would generate FDBs as shown in the figure

•If the Inter-Switch link fails, both sides of the network would be isolated

•Is it possible to add redundant Inter-Switch links??


Switching Loops 1Switching Loops 1

1

2

3

4

A

B

D

E

C F

1

2

3

4

SW1 SW2

5 5

Fm A

to All

Fm A

to All

Fm A

to All

Observe the delivery of a Broadcast frame from User A:

•SW1 forwards the frame to all other ports. The diagram shows the particular case of the frame forwarded on port 4.

•SW2 receives this frame on port 4 and again forwards to all other ports, including its port 5.

•The frame that returns to SW1 is once again forwarded to all ports, including port 4, creating thus an endless loop, and a Broadcast Storm of frames affecting all Hosts.

1

23

1

2

3


Switching Loops 2Switching Loops 2

A

Fm A

to All

Fm A

to All Fm

A

to AllFm A to All

•A similar situation occurs in a network of 3 or more fully interconnected switches.

•Clearly, not all links can be operational at one same time.


Loop Prevention: The Spanning Tree ProtocolLoop Prevention: The Spanning Tree Protocol

Blocked by STP

Blocked by STP

•STP: Spanning Tree Protocolprovides an automatic method of detecting and blocking redundant links

•If a working link fails, STP can reactivate other blocked links in order to recover loop less connectivity

•The process of deciding which links become active and which ones become blocked takes some time, and is known as the process of Convergence of STP


Convergence of STPConvergence of STP

•STP is a language for switches to talk and discover other switches directly connected to their links

•Switches discover each other by sending multicast packets to the Well Known Multicast Address:

01-80-C2-00-00-00

•After STP convergence the resulting network becomes like a tree, with only one possible path between any two switches, and thus without switching loops.

•The detailed analysis procedure for STP convergence is beyond the scope of this manual.


Chapter Chapter 33

Layer 3 and Routing


Interconnecting LANs through IPInterconnecting LANs through IP

File SVR

Router

To Other Networks

MAC A MAC B

MAC C MAC D

MAC R1

MAC R2

IP AIP B

IP C IP DIP R1

IP R2

•LANs typically interconnect with each other through routers.

•Routers transfer packets between different LANs by examining the destination’s IP Address.

•Within a LAN, frame delivery is achieved by the use of MAC addresses (Layer 2) only.

•But communication between LANs requires a higher layer of network addressing and functionality (Layer 3), provided generally by the IP Protocol (Internet Protocol).


LAN

IP Addressing within a LANIP Addressing within a LAN

Router

To Other LANs

IP A IP B

IP R

Ethernet

Subnet Mask

•Each host of a LAN must be identified by a unique IP Address.

•All hosts of a same LAN must share a predetermined set of commonbits (the most significant bits, or MSB) in their IP Addresses.

•The number of MSBs in common is indicated by the so called Subnet Mask.


Subnet MaskSubnet Mask

i.e. MASK = 255 . 255 . 255 . 0 (Dotted Decimal Notation)

11111111 . 11111111 . 11111111 . 00000000 (Binary Representation)

Bits in ‘1’ indicate location of common bits of IP Address. All hosts of a LAN with this mask must have the same first 24 bits in

common (known as Subnetwork ID)

Bits in ‘0’ indicate location of host bits of IP Address. Each host of a LAN with this

mask must be identified by a unique value in the last 8 bits (known as Host ID)


LAN

IP Addressing ExampleIP Addressing Example

RouterTo OtherLANs

IP A = 10.10.24.1

MASK = 255.255.255.0

IP B = 10.10.24.4

MASK = 255.255.255.0

IP R = 10.10.24.100

MASK = 255.255.255.0

NW = 10.10.24.0MASK = 255.255.255.0

•All LAN members must share the same Subnetwork ID (in this case, with the above mask, the first three fields of all IP addresses within the LAN or Subnet).

•External LANs can be easily recognized because each one should have a unique Subnet ID.

•Each LAN member must be assigned with a unique Host ID (in this case, with the above mask, the last field of any IP address).

•The router port connected to this LAN must also follow these same IP addressing rules.


The Layered NetworkThe Layered Network

MAC A

IP A

USER A

IP Layer

MAC Layer

Fm MAC A

To MAC Z

MAC Client Payload

Fm IP A

To IP Z

IP Client Payload

User Message

MAC Frame

IP Packet

Running by software, on

CPU of Host PC

Running by firmware, inside

controller of Ethernet Board

•The Upper Layers (represented in figure as User A, for simplicity) rely on the underlying IP Layer (Layer 3) for any information delivery (Upper Layers are clients of the IP Layer).

•IP Functionality (Layer 3) relies on underlying MAC Functionality (Layer 2) for any information delivery (IP Layer is a Client of the MAC Layer).

•But to request transmission services between IP and MAC Layers, there must be a method of converting IP Addresses into MAC Addresses.


Intra LAN Delivery of an IP PacketIntra LAN Delivery of an IP Packet

USER A

IP Layer

MAC Layer

Fm MAC A

To MAC B

MSG

MAC Frame

Please send this:

To IP DEST = 10.10.24.4 (IP B)•This Destination is on my same LAN!•I just need to find out the MAC address of IP B Fm

IP ATo

IP B MSGPlease send this IP Packet:

To MAC DEST = MAC B

•I will make UnicastDelivery to the host with MAC Address = MAC B

Fm IP A

To IP B MSG

IP Packet

1

2

3

45

User Request:

IP Request:

MAC Delivery:

MAC RIP R

MAC BIP B

MAC AIP A

MACIP

ARP Table (*)

(*) ARP Table: Address Resolution Protocol Table (See next slide)

Rtr

To Other Networks

IP R = 10.10.24.100MASK = 255.255.255.0

IP A = 10.10.24.1MASK = 255.255.255.0

DEF GW = 10.10.24.100

IP B = 10.10.24.4MASK = 255.255.255.0DEF GW = 10.10.24.100

A B

Direct Delivery (Case Intra LAN)

A B

MAC A MAC B

MAC R


Converting IP Address Converting IP Address -- MAC AddressMAC Address

Relies on a process called ARP (Address Resolution Protocol)

1. ARP dynamically builds a table that stores all known IP Addresses (of self LAN) and their corresponding MAC Addresses.

2. When IP receives a transmission request to an intra LAN IP destination, it consults first with ARP to find out the corresponding MAC Address of this destination.

3. IF ARP knows about the destination, it returns the MAC Address, and the IP Layer can proceed immediately with its packet delivery request to the MAC Layer.

4. If ARP does not know about the destination, it must make an ARP inquiry, as shown on the next slide.


Process of Address ResolutionProcess of Address Resolution

A B C

IP

ARP A ARP B ARP C

Fm A

to All

Where is IP C?

1 2 2

Address Resolution Request for IP C

IP IP

Fm C

to A

Here I am! My Add is MAC C

MAC CIP C

MACIP

4

5

Address Resolution Reply for IP C

3

1. ARP asks the MAC Layer to send a broadcast frame with an IP Address Inquiry message: “Where is the user with IP Address IP C?

2. All Ethernet boards on LAN receive the inquiry and transfer it to their corresponding ARP Process.

3. Non matching hosts discard the received package.

4. Only the matching host replies to the ARP Request: “Here I am, and my MAC Address is MAC C.

5. ARP process receives this reply, stores the information in its ARP Table for future use, and returns the requested MAC address to IP.


Default GatewayDefault Gateway

LAN

Router

To Other Networks

IP A = 10.10.24.1MASK = 255.255.255.0DEF GW = 10.10.24.100

IP B = 10.10.24.4MASK = 255.255.255.0DEF GW = 10.10.24.100

IP R = 10.10.24.100MASK = 255.255.255.0

NW = 10.10.24.0MASK = 255.255.255.0

Each host of a LAN must also have knowledge of the IP Address ofits corresponding router (known as the Default Gateway), in order

to be able to send packets to any other external subnetworks.


IP Packet Delivery to an External LANIP Packet Delivery to an External LAN

USER A

IP Layer

MAC Layer

Fm MAC A

To MAC R

MSG

MAC Frame

Please send this:

To IP DEST = 10.10.20.8 (IP Z)•This Destination is on an external LAN!•I must forward through Router•I just need to find out the MAC address of my Default Gateway Fm

IP ATo

IP Z MSGPlease send this IP Packet:

To MAC DEST = MAC R

•I will make UnicastDelivery to the host with MAC Address = MAC R

Fm IP A

To IP Z MSG

IP Packet

1

2

3

45

User Request:

IP Request:

MAC Delivery:

MAC RIP R

MAC BIP B

MACIP

ARP Table

Rtr

IP R = 10.10.24.100MASK = 255.255.255.0

IP A = 10.10.24.1MASK = 255.255.255.0

DEF GW = 10.10.24.100(IP R)

IP B = 10.10.24.4MASK = 255.255.255.0DEF GW = 10.10.24.100

A Z

Indirect Delivery (Case External LAN)

A B

MAC A MAC B

MAC RLAN

Z

IP Z = 10.10.20.8


LANLAN

IP Forwarding by RoutersIP Forwarding by Routers

Rtr

A B

Rtr

Z

Rtr

Rtr

LAN

LAN

LAN

NW = 10.10.24.0Mask = 255.255.255.0

NW = 10.10.20.0Mask = 255.255.255.0

NW = 10.10.28.0Mask = 255.255.255.0

NW = 10.10.21.0Mask = 255.255.255.0

NW = 10.10.25.0Mask = 255.255.255.0

Indirect DeliveryIndirect Delivery

Indirect Delivery

Direct Delivery

•Messages addressed to external LANs (i.e. from A to Z) are forwarded through the Default Gateway (Router)

•It is a function of a router to discover the location and reachability of all external subnetworks

•Routers discover and talk to each other periodically and frequently through the use of Routing Protocols

•Through Routing Protocols, routers inform each other the reachability to all the known subnetworks.

•With his network knowledge, R1 forwards the IP Packet efficiently through routers R3 and R4.

R1 R2

R3

R4


Chapter Chapter 44

VLAN Technology


Corporate network with multiple LANsCorporate network with multiple LANs

File SVR

File SVR

Router

•Different company divisions might want to have functionally separated LANs

•Communication between LANs achieved through Router

•But administration of physically independent cabling infrastructures is usually expensive and complicated!

Accounting Division LAN

Marketing Division LAN


LAN 1

VLAN SwitchVLAN Switch

File SVR

File SVR

Router

File SVR

File SVR

Router

LAN 2

VLAN1

VLAN2

•VLAN Switch can realize the virtual partitioning of a physical LAN into multiple independent Virtual LANs

•Relationship User VLAN is logical, not physical

•Communication between VLANs achieved through Router

SW1

SW2

VLAN SW


VLANsVLANs spanning multiple switchesspanning multiple switches

File SVR

File SVR

Router

VLAN1

VLAN1

VLAN2

VLAN2

•VLANs can also span multiple switches

•To optimize Switch to Switch and Switch to Router connections, actually only one physical link is required

•This link transports the traffic of two or more VLANs, and is known as a VLAN trunk

•But how to distinguish the frames that belong to different VLANS, when these go through the same VLAN trunk?

File SVR

File SVR

Router

VLAN1

VLAN2

VLAN2

VLAN1VLAN Trunk VLAN Trunk


Modification of the Ethernet FrameModification of the Ethernet Frame

VLAN1

VLAN2

VLAN2

VLAN1

VLAN Tag

Fm A

to C MSG1

A

B

C

D

Fm A

to C MSG1VLAN

1Fm B

to D MSG2VLAN

2

Fm A

to C MSG1

Fm B

to D MSG2

•Switch inserts VLAN tag on Ethernet Frame before forwarding a packet over a VLAN Trunk

•Receiving Switch examines the tag and keeps the frame within thecorresponding VLAN group

Fm B

to D MSG2


VLAN Tag (IEEE 802.1Q)VLAN Tag (IEEE 802.1Q)

DEST MAC FCS (Extension)

6 6Source MAC

Length Type

Client Data (+PAD)

2 46 ~ 1500 4 (Oct)VLANTag

4

Type Tag Control Info

PRI

2 2 (Oct)

CFI

VLAN ID

3 1 12 (Bits)

Always 8100 (Hex) for IEEE802.1Q Tag

•VLAN Identifier: 0 ~ 4095

•Canonical Form Identifier: 0=Ethernet / 1=Token Ring

•User Priority: 8 different Classes of Service (CoS) are supported, from PRI=0 (lowest) to PRI=7 (highest)


Native VLANNative VLAN

VLAN1

VLAN2

VLAN2

VLAN1

Tagged Frame

Fm A

to C MSG1

A

B

C

D

Fm A

to C MSG1

Fm A

to C MSG1

Untagged Frame

•For compatibility with Ethernet implementations not supporting the VLAN Tag, IEEE802.1Q allows delivery of untagged frames over VLAN Trunks.

•Untagged frames are assumed to belong by default to the Native VLAN.

•Assume that VLAN ID=1 has been assigned as Native VLAN on both switches. Untagged frames can be forwarded between both switches without any risk of inconsistency.

•Evidently, untagged frames cannot benefit from CoS distinction.

Native VLAN

Native VLAN

Fm B

to D MSG2 Fm

Bto D MSG2

Fm B

to D MSG2VLAN

2


New Field in Forwarding DatabaseNew Field in Forwarding Database

4Vlan 2Router

Vlan 1

Vlan 2

Vlan 1

Vlan 2

Vlan 1

Vlan 2

Vlan 2

Vlan 1

Vlan 1

VLAN Id

4Router

5SVR2

3SVR1

6F

2E

1D

1C

1B

1A

PortHost MAC

FDB SW2

File SVR

File SVR

Router

VLAN1

VLAN2

VLAN2

VLAN1VLAN Trunk VLAN Trunk

A

B

D

C

F

E

SVR1

SVR2

1

2

3

4

5

6SW2SW1

•The FDB of a VLAN Switch records not only the MAC Address but also the VLAN Id of each host connected to each Switch Port

•The fully learned configuration of SW2 would generate an FDB as shown in this figure

•The VLAN Id field in the FDB prevents traffic mixing between separate VLAN groups


One Spanning Tree per VLANOne Spanning Tree per VLAN

VLAN 1

VLAN 2

VLAN 1

VLAN 2

VLAN 1

VLAN 2

Blocked by STP

VLAN Trunk

VLAN Trunk

VLAN Trunk

•The most recent IEEE standards support multiple spanning tree instances on a same VLAN switch

•In these independent Spanning Trees, the operator can control the STP port blocking process

•Ports blocked for one VLAN can be set as working ports for other VLANs

•This mechanism allows load sharing over redundant links in the Layer 2 Network, despite the STP loop prevention process.


Chapter Chapter 55

Principles of Flow Control and Auto Negotiation


Flow Control through PauseFlow Control through Pause

SW

High Traffic

Congestion!!

PAUSE

•Full Duplex Ethernet supports the implementation of an optional Flow Control mechanism.

•During congestion on reception, an Ethernet port can request its connected partner to halt traffic for some time, through the delivery of a MAC Control Frame called PAUSE.

•If the connected partner honors the requested PAUSE, it will stop the data flow, thus relieving the congestion.

•Some Ethernet implementations have the capability of both sending and receiving pause frames (Symmetrical Flow Control)

•Other implementations can either only send, or will only honor a received PAUSE frame, but cannot support both directions (Asymmetrical Flow Control)

•Other implementations do not provide Flow Control functionality at all.

•The flow control capabilities of interconnected partners are typically agreed during link start up, during a process called Autonegotiation.


Pause Delivery by MulticastingPause Delivery by Multicasting

Globally assigned Multicast Address for PAUSE:

01-80-C2-00-00-01

•Pause frames may be sent to a specific Unicast Address

•Or they may be sent to a globally assigned Multicast Address that is not forwarded by bridges / switches

•Each individual PAUSE request specifies the duration of that particular pause event.


AutonegotiationAutonegotiation

Speed

1000 Mbps

100 Mbps

10 Mbps

Mode

Full Duplex

Half Duplex

Flow Control Method

Symmetric

Asymmetric

None

•Autonegotiation allows hosts to automatically negotiate link capabilities upon start-up, such as speed, mode of operation and Flow Control method.

•IEEE defines a priority scheme, that ensures selection of the highest common denominator abilities between negotiating partners.

2 Internetworking With Ethernet Issue Sep 2

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