8 - 1

Chapter 8

Backbone Networks

8 - 2

Outline

• Components of Backbone networks

– Bridges, Routers, Gateways

• Backbone network architectures

• Backbone technologies

• Best practice backbone design

• Improving backbone performance

8 - 3

Backbone Networks

• High speed networks linking an organization’s LANs

– Making information transfer possible between departments

– Use high speed circuits to connect LANs

– Provide connections to other backbones, MANs, and WANs

• Sometimes referred to as

– An enterprise network

– A campus-wide network

8 - 4

Backbone Network Components

• Network cable

– Functions in the same way as in LANs

– Optical fiber - more commonly chosen (provides higher data rates)

• Hardware devices

– Computers or special purpose devices used for interconnecting networks

• Bridges

• Routers

• Gateways

8 - 5

 

Device Operates at

Packets PhysicalLayer

Data Link Layer

NetworkLayer

 

BridgeData Link

LayerFiltered using data link layer

addresses

Same or

Different

 

Same 

Same

 

RouterNetwork

LayerRouted using network layer

addresses

Same or

Different

Same or

Different

 

Same

 Gateway

Network Layer

Routed using network layer

addresses

Same or

Different

Same or

Different

Same or

Different 

Backbone Network Devices

8 - 6

Bridges• Data link layer devices• Connect LANs with the same Data

Link and same Network layers

Operate in a similar way to layer 2 switches (learning bridges)

Allows different types of cabling

8 - 7

Learning Bridges

• Operate in a similar way to layer 2 switches: – Learn which computers are on each side of the bridge

• By reading the source addresses on incoming frames and recording this information in forwarding tables

• Data link layer devices – Connecting similar type of networks

• But they can connect different types of cable

• Not popular anymore – Losing market share to layer 2 switches as the latter

become cheaper and more powerful

8 - 8

Routers• Operate at the network layer • Connect LANS with different data

link layer, but the same network layer protocol

Perform more processing than bridges or layer 2 switches

Allows different types of cabling

8 - 9

Routers (Cont.)

• Operations – Strip off the header and trailer of the incoming L2 frame

– Examine the destination address of the network layer

– Build a new frame around the packet

– Choose the “best” route for a packet (via routing tables)

– Send it out onto another network segment

• Compared to Bridges– Perform more processing

• Process L3 messages (no changes made)

• Form new L2 messages for outgoing packets

– Processes only messages specifically addressed to it

8 - 10

GatewaysAlso operate at network layer (like routers)

Connect LANS with different data link layer and different network layer protocols

Some operate at the application layer as well

8 - 11

Other BB Network Devices

• Multiprotocol routers– Can handle several different protocols (no translation)

• In and out protocols must be the same

• Brouters– Combine bridge and router functions

• Examine L2 addresses of all messages

• Can also process directly addressed (L2) messages

• Layer-3 switches– Similar to L2 switches, but switch messages based on

L3 addresses

– Can support many more simultaneous ports than routers

8 - 12

Backbone Network Architectures

• Identifies the way backbone interconnects LANs

• Defines how it manages packets moving through BB

• Fundamental architectures– Bridged Backbones

– Routed Backbones

– Collapsed Backbones

• Rack-based

• Chassis-based

– Virtual LANs

• Single-switch VLAN

• Multiswitch VLAN

8 - 13

Backbone Architecture Layers• Access Layer (not part of BB)

– Closest to the users;

• Backbone Design Layers

– Distribution Layer

• Connects the LANs together (often in one building

– Core Layer (for large campus/enterprise networks)

• Connects different BNs together (building to building)

• <<<<<<<< Figure 8.5 goes here

8 - 14

Bridged Backbone

bus topologyEntire network is just one subnet

8 - 15

Bridged Backbones

• Move packets between networks based on their data link layer addresses

• Cheaper (since bridges are cheaper than routers) and easier to install (configure)– Just one subnet to worry

– Change in one part may effect the whole network

• Performs well for small networks – For large networks broadcast messages (e.g., address

request, printer shutting down) can lower performance

• Formerly common in the distribution layer– Declining due to performance problems

8 - 16

Routed Backbone

Usually a bus topology

Example of a routed BB at the Distribution layer

Each LAN is a separate subnet

8 - 17

Routed Backbones

• Move packets using network layer addresses

• Commonly used at the core layer– Connecting LANs in different buildings in the campus

– Can be used at the distribution layer as well

• LANs can use different data link layer protocols

• Main advantage: LAN segmentation– Each message stays in one LAN; unless addressed

outside the LAN

– Easier to manage

• Main disadvantages– Tend to impose time delays compared to bridging

– Require more management than bridges & switches

8 - 18

Star topology

Collapsed Backbone

A connection to the switch is a separate point-to-point circuit

Most common type BB mainly used in distribution layer

8 - 19

Collapsed Backbones

• Replaces the many routers or bridges of the previous designs– Backbone has more cables, but fewer devices

– No backbone cable used; switch is the backbone.

• Advantages:– Improved performance (200-600% higher)

• Simultaneous access; :switched” operations

– A simpler more easily managed network – less devices

• Two minor disadvantages– Use more and longer cables

– Reliability:

• If the central switch fails, the network goes down.

8 - 20

Rack-Based Collapsed Backbones

Figure 8-9 goes here

8 - 21

Rack-Based Collapsed Backbones

• Places all network equipment (hubs and switch) in one room (rack room)– Easy maintenance and upgrade

– Requires more cable (but cables are cheap)

• Main Distribution Facility (MDF) or Central Distribution Facility– Another name for the rack room

– Place where many cables come together

• Patch cables used to connect devices on the rack

• Easier to move computers among LANs– Useful when a busy hub requires offloading

8 - 22

Main Distribution Facility (MDF)

• >>>> Figure 8.10 goes here

8 - 23

Chassis-Based Collapsed Backbones

• Use a “chassis” switch instead of a rack– A collection of modules

• Number of hubs with different speeds

• L2 switches

• Example of a chassis switch with 710 Mbps capacity

– 5 10Base-T hubs, 2 10Base-T switches (8 ports each)

– 1 100Base-T switch (4 ports), 100Base-T router

( 5 x 10) + (2 x 10 x 8) + (4 x 100) + 100 = 710 Mbps

• Flexible– Enables users to plug modules directly into the switch

– Simple to add new modules

8 - 24

Virtual LANs (VLANs)

• A new type of LAN-BN architecture– Made possible by high-speed intelligent switches

– Computers assigned to LAN segments by software

• Often faster and provide more flexible network management– Much easier to assign computers to different segments

• More complex and so far usually used for larger networks

• Basic VLAN designs: – Single switch VLANs

– Multi-switch VLANs

8 - 25

Single Switch VLAN Collapsed Backbone

Switchacting as a large physical switch

Computers assigned to different LANs by software

8 - 26

Types of Single Switch VLANs

• Port-based VLANs (Layer 1 VLANs)– Use physical layer port numbers on the front of the

VLAN switch to assign computers to VLAN segments

– Use a special software to tell the switch about the computer - port number mapping

• MAC-based VLANs (Layer 2 VLANs)– Use MAC addresses to form VLANs

– Use a special software to tell the switch about the computer - MAC address mapping

• Simpler to manage

– Even if a computer is moved and connected to another port, its MAC address determines which LAN it is on

8 - 27

Types of Single Switch VLANs

• IP-based VLANs (Layer 3 VLANs, protocol based VLANs)– Use IP addresses of the computers to form VLANs

– Similar to MAC based approach (use of IP instead of MAC address)

• Application-based VLANs (Layer 4 VLANs, policy-based VLANs)– Use a combination of

• the type of application (Indicated by the port number in TCP packet) and

• The IP address to form VLANs

– Complex process to make assignments

– Allow precise allocation of network capacity

8 - 28

Multi-switch VLAN-Collapsed Backbone

Switch

Switch

Switch

Switch

8 - 29

Multi-switch VLAN Operations

• Inter-switch protocols– Must be able to identify the VLAN to which the packet

belongs

• Use IEEE 802.1q (an emerging standard)– When a packet needs to go from one switch to another

• 16-byte VLAN tag inserted into the 802.3 packet by the sending switch

– When the IEEE 802.1q packet reaches its destination switch

• Its header (VLAN tag) stripped off and Ethernet packet inside is sent to its destination computer

8 - 30

VLAN Operating Characteristics

• Advantages of VLANs– Faster performance

• Precise management of traffic flow

• Ability to allocate resources to different type of applications

– Traffic prioritization (via 802.1q VLAN tag)

• Include in the tag: a priority code based on 802.1p

• Can have QoS capability at MAC level– Similar to RSVP and QoS capabilities at network and

transport layers

• Drawbacks– Cost

– Management complexity

8 - 31

Backbone Technologies

• Gigabit Ethernet

• Fiber Distributed Data Interface (FDDI)

• Asynchronous Transfer Mode (ATM)

8 - 32

FDDI

• A set of standards designed in 80’s for MANs (ANSI X3T9.5)– Also used as BB and LAN technologies

• Limited future– Gigabit Ethernet’s strong presence

• A ring network operating at 100 Mbps over fiber cables– Assumes a mix of 1,000 stations and 200 Km path

• With repeaters at every 2 Km

– Uses 2 counter rotating rings: primary and secondary

• Data on the primary; secondary used as backup

8 - 33

FDDI Topology

• >>>> Figure 8.15

Two types of FDDI computers

secondary ring flows in opposite direction

8 - 34

Managing a Broken Ring in FDDI

• >>>> Figure 8.16

If a ring is broken, the ring can still operate in a limited fashion

8 - 35

FDDI Media Access Control• Uses a controlled access token passing scheme

– Sending computer

• Wait for the token, when receive it

• Attach the packet to the token and transmit them

– Receiving computer

• See if there is a packet attached to the token

• If there is process the packet

• If it needs to transmit a packet follow the steps above

• If no packet to send simply transmit the token to the next computer

• Very reliable and provide adequate response time until it almost reaches saturation at 100 Mbps

8 - 36

ATM

• Originally designed for use in WAN– Often used now in BNs

• Standardized; simple to connect BNs and WANs

• Also called cell relay

• Includes Layer 3, Layer 2 and Layer 1 technologies in the specifications– Compatible with TCP/IP and Ethernet as if ATM was

Layer 2 technology

• A connection oriented technology

• ATM switches– Provide point-to-point full duplex circuits at 155 Mbps

(622 Mbps for switch-to-switch)

8 - 37

ATM vs. Ethernet

• Packet format:– Uses fixed-length packets (cells) of 53 bytes: 5-byte header,

48 byte data

– Designed to make switching faster (in hardware)

• Error Checking– Error checking done for header only (not on data)

• If error detected, cell is discarded

• Addressing– Uses a virtual channel(VC) between sender and receiver

• All cells use VC Identifier as addresses

• QoS (prioritized transmissions)– Each VC assigned a specific class of service with a priority

8 - 38

Virtual Channels in ATM

• Identified by a two-part number– Path number

– Circuit number within that path

• A physical port on a switch may have many paths– A path may have many circuits

• A switch may have thousands of VCs– A VC table is used to map the connections which can be

established either:

• Permanently: Permanent Virtual Circuit (PVC)

• Temporarily: Switched Virtual Circuit (SVC)

– Deleted when the connection is not needed

8 - 39

Addressing and Forwarding in ATM

• >>>Figure 8.17 goes here

When a cell arrives, switch checks the cell’s VC identifier at the table and determines where to send it .

8 - 40

Approaches of Using ATM in Backbone• LAN Emulation (LANE)

– Breaking LAN frame into 48-byte long blocks and transmit them in an ATM cell

– Called encapsulation and done by edge switches

– Reassembling done at the destination edge switch and LAN frame is sent to the LAN

– Requires translating of MAC addresses to VC Identifiers (assuming VCs are setup already)

– Performance suffers due to encapsulation and connection management

• Multiprotocol over ATM (MPOA)- LANE extension– Uses IP addresses in addition to MAC addresses

• If same subnet, use MAC address; otherwise use IP• ATM backbone operating like a network of brouters

8 - 41

Best Practice Backbone Design

• Architectures

– Performance and cost Collapsed backbone

• VLANs closer; but not mature enough

• Efficiency of data rates

– Data Link Protocol Efficiency

• FDDI with 99%: Overhead 29 bytes; up to 4500 byte data

• ATM with about 87%: Overhead: 5 bytes over 53 byte cell

– MAC Efficiency

8 - 42

FDDI MAC Efficiency

• Uses token passing controlled access– Imposes more fixed-cost delays initially in low traffic

– Increases response times only slowly up to 90-95% nominal capacity

– Total effective data rate = 89 Mbps

• 99% efficiency x 90% capacity x 100 Mbps

– >>>> Fig 8.19 goes here

8 - 43

ATM MAC Efficiency

• Uses full duplex transmission

– Efficiency ~ 100% of capacity

– Effective data rate = 135 Mbps each direction simultaneously

• 87% efficiency x 100% capacity x 155 Mbps

• Total for both directions: 270 Mbps

– An ATM network with 622 Mbps circuits

• Provides 540 Mbps capacity each direction

1080 Mbps total

8 - 44

Conversion between Protocols

• Both requires conversion from/to Ethernet frames

• FDDI uses translation– Remove Ethernet frame; replace it with FDDI frame

– Decreases efficiency 10-20%

– Actual total effective rate of FDDI 70 Mbps

• ATM uses encapsulation– Segment and surround Ethernet frames with ATM cell

headers Generally faster

– MAC Addresses must be translated to VC Identifiers and VC management 30-40% decreased efficiency

– Actual total effective rate of ATM 80 Mbps each direction (160 Mbps total)

8 - 45

Effective Data Rates of BB Technologies

• >>>Fig 8-20 goes here

8 - 46

Recommendations for BB Design

• Best architecture– Collapsed backbone or VLAN

• Best technology– Gigabit Ethernet

• Ideal design– A mixture of layer-2 and layer-3 Ethernet switches

– Access Layer

• 10/100Base-T Later 2 switches with cat5e or cat6

– Distribution Layer

• 100base-T or 1000BaseT/F Layer 3 switches

– Core Layer

• Layer 3 switches running 10GbE or 40GBe

8 - 47

Best Practice BB Design

• >>>>>Fig 8-21 goes here

8 - 48

Improving Backbone Performance

• Improve computer and device performance– Upgrade them to faster devices

– Use faster routing protocols

• Static routing is faster for small networks

– Use gigabit Ethernet as BB (eliminate translations)

– Increase memory in devices

• Improve circuit capacity– Upgrade to a faster circuit; Add additional circuits

– Replace shared circuit BB with a switched BB

• Reduce network demand– Restrict applications that use a lot of network capacity

– Reduce broadcast messages (placing filters at switches)

8 - 49

Implications for Management

• Increased traffic at backbone due to faster technologies– May requires that BN be replaced

Design BN to be easily upgradeable

• FDDI and ATM becoming as legacy technologies– Vendors stopping the production of these

Begin to invest more funds to replace these

• Ethernet moving into Backbone extensively– One standard technology used for both LANs and BN

Cheaper equipment; Easier management