Business Data Communications and Networking

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Business Data Communications and Networking

• Dr. Blaine Garfolo

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• Backbone Networks

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Outline

• Components of Backbone networks
• Bridges, Routers, Gateways
• Backbone network architectures
• Backbone technologies
• Best practice backbone design
• Improving backbone performance

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Backbone Networks

• High speed networks linking an organizations
  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

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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

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Backbone Network Devices
 
 
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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)
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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

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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
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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

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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
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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

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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

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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)

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Bridged Backbone
bus topology
Entire network is just one subnet
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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

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Routed Backbone
Example of a routed BB at the Distribution layer
Usually a bus topology
Each LAN is a separate subnet
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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

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Collapsed Backbone
Most common type BB mainly used in distribution
layer
Star topology
A connection to the switch is a separate
point-to-point circuit
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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.

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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

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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

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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

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Single Switch VLAN Collapsed Backbone
acting as a large physical switch
Computers assigned to different LANs by software
Switch
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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

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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

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Multi-switch VLAN-Collapsed Backbone
Switch
Switch
Switch
Switch
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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

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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

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Backbone Technologies

• Gigabit Ethernet
• Fiber Distributed Data Interface (FDDI)
• Asynchronous Transfer Mode (ATM)

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FDDI

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

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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

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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)

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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

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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

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Addressing and Forwarding in ATM
When a cell arrives, switch checks the cells VC
identifier at the table and determines where to
send it .
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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

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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

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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

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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

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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)

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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

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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)

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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