Backbone Networks Copyright 2005 John Wiley Sons

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 Copyright 2005 John Wiley & Sons, Inc 2

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 Copyright 2005 John Wiley & Sons, Inc 3

Backbone Network Devices Device Bridge Router Gateway Operates at Packets Physical Layer Data Link Network Layer Data Link Layer Filtered using data link layer addresses Same or Different Same Network Layer Routed using network layer addresses Same or Different Same or Different Copyright 2005 John Wiley & Sons, Inc 4

Bridges • Data link layer devices • Connect LANs with the same Data Link and same Network layers Allows different types of cabling Operate in a similar way to layer 2 switches (learning bridges) Copyright 2005 John Wiley & Sons, Inc 5

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 Copyright 2005 John Wiley & Sons, Inc 6

Routers • Operate at the network layer • Connect LANS with different data link layer, but the same network layer protocol Allows different types of cabling Perform more processing than bridges or layer 2 switches Copyright 2005 John Wiley & Sons, Inc 7

Routers (Cont. ) • Operations – Strip off the header and trailer of the incoming L 2 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 L 3 messages (no changes made) • Form new L 2 messages for outgoing packets – Processes only messages specifically addressed to it Copyright 2005 John Wiley & Sons, Inc 8

Gateways Also 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 Copyright 2005 John Wiley & Sons, Inc 9

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 L 2 addresses of all messages • Can also process directly addressed (L 2) messages • Layer-3 switches – Similar to L 2 switches, but switch messages based on L 3 addresses – Can support many more simultaneous ports than routers Copyright 2005 John Wiley & Sons, Inc 10

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 Copyright 2005 John Wiley & Sons, Inc 11

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) Copyright 2005 John Wiley & Sons, Inc 12

Bridged Backbone bus topology Entire network is just one subnet Copyright 2005 John Wiley

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 Copyright 2005 John Wiley & Sons, Inc 14

Routed Backbone Example of a routed BB at the Distribution layer Usually a bus

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 Copyright 2005 John Wiley & Sons, Inc 16

Collapsed Backbone Most common type BB mainly used in distribution layer A connection to

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. Copyright 2005 John Wiley & Sons, Inc 18

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 Copyright 2005 John Wiley & Sons, Inc 19

Chassis-Based Collapsed Backbones • Use a “chassis” switch instead of a rack – A collection of modules • Number of hubs with different speeds • L 2 switches • Example of a chassis switch with 710 Mbps capacity – 5 10 Base-T hubs, 2 10 Base-T switches (8 ports each) – 1 100 Base-T switch (4 ports), 100 Base-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 Copyright 2005 John Wiley & Sons, Inc 20

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 Copyright 2005 John Wiley & Sons, Inc 21

Single Switch VLAN Collapsed Backbone acting as a large physical switch Switch Computers assigned

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 Copyright 2005 John Wiley & Sons, Inc 23

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, policybased 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 Copyright 2005 John Wiley & Sons, Inc 24

Multi-switch VLAN-Collapsed Backbone Switch Copyright 2005 John Wiley & Sons, Inc 25

Multi-switch VLAN Operations • Inter-switch protocols – Must be able to identify the VLAN to which the packet belongs • Use IEEE 802. 1 q (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. 1 q packet reaches its destination switch • Its header (VLAN tag) stripped off and Ethernet packet inside is sent to its destination computer Copyright 2005 John Wiley & Sons, Inc 26

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. 1 q VLAN tag) • Include in the tag: a priority code based on 802. 1 p • Can have Qo. S capability at MAC level – Similar to RSVP and Qo. S capabilities at network and transport layers • Drawbacks – Cost – Management complexity Copyright 2005 John Wiley & Sons, Inc 27

Backbone Technologies • Gigabit Ethernet • Fiber Distributed Data Interface (FDDI) • Asynchronous Transfer

FDDI • A set of standards designed in 80’s for MANs (ANSI X 3 T 9. 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 Copyright 2005 John Wiley & Sons, Inc 29

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 Copyright 2005 John Wiley & Sons, Inc 30

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) Copyright 2005 John Wiley & Sons, Inc 31

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 • Qo. S (prioritized transmissions) – Each VC assigned a specific class of service with a priority Copyright 2005 John Wiley & Sons, Inc 32

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 Copyright 2005 John Wiley & Sons, Inc 33

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 Copyright 2005 John Wiley & Sons, Inc 34

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 Copyright 2005 John Wiley & Sons, Inc 35

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 Copyright 2005 John Wiley & Sons, Inc 36

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 Copyright 2005 John Wiley & Sons, Inc 37

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) Copyright 2005 John Wiley & Sons, Inc 38

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 • 10/100 Base-T Later 2 switches with cat 5 e or cat 6 – Distribution Layer • 100 base-T or 1000 Base. T/F Layer 3 switches – Core Layer • Layer 3 switches running 10 Gb. E or 40 GBe Copyright 2005 John Wiley & Sons, Inc 39

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) Copyright 2005 John Wiley & Sons, Inc 40

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 Copyright 2005 John Wiley & Sons, Inc 41