1. Chapter 5 Hardware Layers: Backbone Networks Networking in the
Internet Age
by Alan Dennis
Copyright © 2002 John Wiley & Sons, Inc.

2. Copyright ã 2002 John Wiley & Sons, Inc. All rights reserved
Copyright ã 2002 John Wiley & Sons, Inc. All rights reserved. Reproduction or translation of this work beyond that named in Section 117 of the United States Copyright Act without the express written consent of the copyright owner is unlawful. Requests for further information should be addressed to the Permissions Department, John Wiley & Sons, Inc. Adopters of the textbook are granted permission to make back-up copies for their own use only, to make copies for distribution to students of the course the textbook is used in, and to modify this material to best suit their instructional needs. Under no circumstances can copies be made for resale. The Publisher assumes no responsibility for errors, omissions, or damages, caused by the use of these programs or from the use of the information contained herein.

3. Chapter 5. Learning Objectives
Understand hierarchical backbones and the devices they use
Understand flat backbones and the devices they use
Understand collapsed backbones and the devices they use
Understand VLANs and the devices they use
Be familiar with FDDI
Be familiar with ATM
Understand the best practice recommendations for backbone design

4. Chapter 5. Outline Introduction Backbone Architectures
Hierarchical backbones, Flat backbones, Collapsed backbones, Virtual LANs
Fiber Distributed Data Interface
Topology, Medium Access Control, Error Control, Message Delineation, Data Transmission in the Physical Layer
Asynchronous Transfer Mode
Topology, Medium Access Control, Error Control, Message Delineation, Data Transmission in the Physical Layer, ATMs and LANs
The Best Practice Backbone Design
Architectures, Effective Data Rates, Conversion Between Protocols, Recommendations

5. Introduction

6. Backbone Networks
Backbone networks are high speed networks that link an organization’s LANs and also provide connections to other backbones, MANs, WANs and the Internet.
A backbone that connects backbones in several buildings is also often called a campus network.
A backbone is also sometimes called an enterprise network if it connects all the networks within a company, especially if this includes large WAN segments.

7. Backbone Architecture Layers (Figure 5-1)
Network designers view networks as made of three technology layers:
The access layer which is the technology used in LANs.
The distribution layer which is the part of the backbone that connects the LANs together.
The core layer connects different backbone networks together, often between buildings.
Some organizations are not large enough to have a core layer. In such cases their backbone only spans the distribution layer.

8. Figure 5-1 Backbone network design layers

9. Backbone Architectures

10. Backbone Network Types
There are four basic types of backbone networks:
Hierarchical Backbones
Flat Backbones
Collapsed Backbones
Virtual LANs

11. Hierarchical Backbones
Figure 5-2 shows an example of a distribution layer hierarchical backbone.
Each LAN is a separate and isolated network, connected by a TCP/IP gateway (usually a router) to a shared media backbone network.
Within the LANs messages are sent based on the data link layer addresses.
To move between LANs, message traffic needs to be sent specifically to the router, which forwards the message based on its network layer address.

12. Figure 5-2 Hierarchical backbone architecture

13. Flat Backbones
Figure 5-3 gives an example of a distribution layer flat backbone with a bus topology.
With a flat backbone, LANs are connected using bridges or layer-2 switches.
Packets are forwarded based on their data link layer addresses, making the entire flat backbone a single subnet.
Flat backbones using bridges were developed in the mid-1980s to reduce costs, because at the time routers were very expensive.
Bridges have now become obsolete and are typically replaced by layer-2 switches, which have continued to fall in price.

14. Figure 5-3 Flat backbone architecture

15. Collapsed Backbones (Figure 5-4)
Collapsed backbones use a star topology, usually with a high speed switch at the center.
Collapsed backbones can use either layer-2 switches or layer-3 routing switches.
The two main advantages are:
1) each connection to the switch becomes a separate point-to-point circuit also giving much higher performance (from % higher)
2) the network has far fewer devices and so is much simpler to manage.
Two minor disadvantages are: 1) use more cable and the cable runs for longer distances, 2) if the central switch fails, the network goes down.

16. Figure 5-4 Collapsed backbone architecture

17. Rack-Mounted Collapsed Backbones
Rack-mounted backbones collapse the backbone into a single room, called a main distribution facility (MDF) where networking equipment is connected and mounted on equipment racks (Figure 5-5).
Devices are connected using short patch cables.
Moving computers between LANs is relatively simple since equipment is all in the same location.

18. Fig. 5-5 Rack-mounted collapsed backbone architecture

19. Chassis-based Collapsed Backbones
Chassis switch designs include a number of open slots and have an internal capacity capable of supporting all active modules.
A variety of modules (i.e., card-mounted networking devices) can be inserted into these slots providing a high level flexibility in network configuration.
By turning the backbone into an internal bus, chassis switches also can provide very high performance speeds capable of aggregate data rates in the Gbps range.

20. Figure 5-7
Central
Parking’s
collapsed
backbone

21. Virtual LANs
VLANs are a new type of LAN/BN architecture using intelligent, high-speed switches.
Unlike other LAN types, which physically connect computers to LAN segments, VLANs assign computers to LAN segments by software.
VLANs have been standardized as IEEE802.1q and IEEE802.1p.
The two basic designs are:
Single-switch VLANs
Multiswitch VLANs

22. Single Switch VLANs (Figure 5-8)
With single switch VLANs, computers are assigned to VLANs using special software, but physically connected together using a large physical switch.
Computers can be assigned to VLANs in four ways:
Port-based VLANs assign computers according to the VLAN switch port to which they are attached
MAC-based VLANs assign computers according to each computer’s data link layer address
IP-based VLANs assign computers using their IP-address
Application-based VLANs assign computers depending on the application that the computer typically uses. This has the advantage of allowing precise allocation of network capacity.

23. Figure 5-8 Single-switch VLAN architecture

24. Multiswitch VLANs (Figure 5-9)
Multiswitch VLANs send packets between multiple switches, making VLANs with segments in separate locations possible.
When a frame is sent between switches it is modified and to include a tag field carrying VLAN information field. When the frame reaches the final switch, the tag field is removed prior to the frame being sent to its destination computer.
Multiswitch VLANs can also prioritize traffic using the IEEE802.1p standard in the hardware layers and the RSVP standard in the internetwork layers.
IEEE802.1p works with the IEEE802.11ac frame definition which includes a special priority field.

25. Figure 5-9 Multiswitch VLAN architecture

26. Figure 5-10 IONA VLAN

27. Fiber Distributed Data Interface

28. Fiber Distributed Data Interface (FDDI)
FDDI (standardized as ANSI X3T9.5) backbone protocol was developed in the 1980s and popular during the 80s and 90s.
FDDI operates at 100 Mbps over a fiber optic cable.
Copper Distributed Data Interface (CDDI) is a related protocol using cat 5 twisted wire pairs.
FDDI’s future looks limited, as it is now losing market share to Gigabit Ethernet and ATM.

29. FDDI Topology (Figure 5-11)
FDDI uses both a physical and logical ring topology capable of attaching a maximum of 1000 stations over a maximum path of 200 km. A repeater is need every 2 km.
FDDI uses dual counter-rotating rings (called the primary and secondary). Data normally travels on the primary ring.
Stations can be attached to the primary ring as single attachment stations (SAS) or both rings as dual attachment stations (DAS).

30. Figure 5-11 Optical cable topology for an FDDI
local area network.

31. FDDI’s Self Healing Rings
An important feature of FDDI is its ability to handle a breaks in the network by forming a single temporary ring out of the pieces of the primary and secondary rings.
Figure 5-12 shows an example of a cable break between two dual attachment stations.
Once the stations detect the break, traffic is rerouted through a new ring formed out of the parts of the primary and secondary rings not affected by the break.
The network then operates over this temporary ring until the break can be repaired.

32. Figure 5-12 Managing a broken circuit

33. FDDI Media Access Control
FDDI uses a token passing system. Computers wanting to send packets wait to receive a token before transmitting.
Multiple packets can be attached to the token as it moves around the network.
When a station receives the token, it looks for attached packets addressed to it and removes them from the incoming packet.
If the station wants to send a packet it attaches it to the token and sends the token with its attached packets to the next station.
This controlled access technique provides a higher performance level at high traffic levels compared to a contention-based technique like Ethernet.

34. FDDI Message Delineation (Fig. 5-13)
The FDDI frame can be broken into three parts:
Frame Start: like Ethernet, the frame begins with a preamble (8-bytes in this case) and a 1-byte start delimiter.
Frame Body: the main body of the frame includes the following fields:
1-byte frame control field (used for the token)
2 or 6 byte fields for the destination and source addresses (6 bytes is more common)
the data field contains bytes of data
the frame check sequence (FCS) used in error control.
Frame End: the frame ends with a 1-byte end delimiter and a 2-byte frame status field.

35. Figure 5-13 FDDI frame layout

36. Asynchronous Transfer Mode

37. Asynchronous Transfer Mode (ATM)
Asynchronous Transfer Mode (ATM) (also called cell relay) was originally designed to carry both voice and data traffic over WANs. It is also used in backbone networks.
In the WAN, ATM almost always uses SONET as its hardware layer. In backbones, ATM is often implemented as a standalone protocol.
On order to interconnect with the TCP/IP world, an ATM gateway is used that converts TCP/IP and Ethernet frames into ATM cells and then converts them back once they have reached their destination network.

38. ATM Topology ATM uses a mesh topology (see Figure 5-14)
This mesh is made up of point-to-point, full duplex circuits that interconnect ATM switches.
ATM circuits typically operate at 155 Mbps in each direction, although higher speeds, esp. 622 Mbps (1.24 Gbps total) is also possible.
Although originally designed to run on optical fiber, some versions of ATM can run on cat-5e twisted pair cables.

39. Figure 5-14 ATM mesh architecture

40. ATM Media Access Control
ATM uses full-duplex circuits, so media access control is less of an issue.
To handle circuit congestion, ATM prioritizes transmissions based on Quality of Service (QoS). Priorities are based on 5 ATM service class definitions.
Real time applications, such as voice, get a high priority, since it can not allow delays.
gets a lower priority, since small delays don’t matter very much.

41. ATM Addressing (Figure 5-15)
ATM addressing uses virtual channels (VCs).
Each cell’s VC has two parts: a virtual path identifier and a virtual channel identifier.
Virtual channels are also assigned a service class when they are created.
When a cell reaches an ATM switch, the switch looks up the VC number in its VC table to determine where to send it next (similar to how a routing table works).

42. Figure 5-15 Addressing and forwarding
with ATM virtual circuits

43. ATM virtual circuits
ATM is connection-oriented: all packets travel in order in the same virtual channel.
VCs can be set up in one of two ways:
Permanent Virtual Circuits (PVCs) – permanent virtual circuits set up for long periods.
Switched Virtual Circuits (SVCs) - temporary virtual circuits set up for one transmission and deleted when the transmission is completed.

44. ATM Error Control
ATM’s error control technique is called throw-it-on-the-floor.
Error checking is only done on the ATM header.
If an error is detected, the cell is discarded.
Full error control, including requests for retransmission are handled at the source and destination computers (on a LAN this is typically done using TCP).

45. ATM Message Delineation (Fig. 5-16)
ATM has a 53-byte frame called a cell.
The ATM header includes these fields:
Generic Flow Control: controls the flow of data across the circuit
Virtual Path Identifier: identifies the group of channels the data is moving with.
Virtual Circuit Identifier: identifies the specific channel.
Payload Type: indicates type of data in data field
Cell Loss Priority: whether or not the cell is discarded if the circuit gets busy.
Header Error Control: uses CRC-8 for error control but only on the header portion of the field.

46. Figure 5-16 ATM cell layout

47. ATM and LANs
Ethernet and TCP/IP use large variable length frames/packets with fixed addresses while ATM uses small fixed length cells addressed using virtual channels.
For that reason, Ethernet and TCP/IP must first be translated before being sent over ATM networks.
Two approaches for this are:
LAN Encapsulation (LANE), which splits frames into 48 byte pieces, reassembling them when they reach their destination LAN.
Multiprotocol Over ATM (MPOA) is an extension of LANE that uses both IP and Ethernet addresses.

48. LAN Encapsulation (LANE)
LANE works by breaking Ethernet frames into 48-byte chunks. This occurs in the LAN’s gateway ATM edge switch (see Figure 5-17).
The edge switch also creates a virtual channel identifier for the cells to use.
The cells are then sent over the ATM backbone using this virtual channel identifier.
When they reach the destination edge switch, the frame is reassembled.
LANE’s high overhead creates significant delays, lowering network performance as a consequence.

49. Figure 5-17 ATM in the backbone

50. The Best Practice Backbone Design

51. Current Backbone Technology Trends
The following trends in backbone technologies have been taking place in recent years:
Organizations are moving to collapsed backbones or VLANs.
Gigabit Ethernet use is growing.
FDDI seems to be on its way out.
ATM, while still popular in WANs, is losing ground to Gigabit Ethernet as a backbone technology.
Taken together, it appears that Ethernet use will dominate both the LAN and backbone environments.

52. Figure 5-19 Effective data rates for backbone technologies
Technology
Effective Data Rate
Full Duplex 1 GbE
1.8 Gbps
Full Duplex 10 GbE
18 Gbps
FDDI
7-70 Mbps depending on traffic
ATM (155 Mbps, Full Duplex)
160 Mbps
ATM (622 Mbps, Full Duplex)
760 Mbps
 Assumes: collapsed backbone connecting Ethernet LANs transmitting mostly large frames
Figure 5-19 Effective data rates
for backbone technologies

53. Backbone Recommendations (Fig. 5-20)
The best practices are recommended for backbones:
1. Architecture: collapsed backbone or VLAN.
2. Technology: gigabit Ethernet. ATM and FDDI use has started to fall off over the past year.
3. The ideal network design combines use of layer-2 and layer-3 Ethernet switches.
4. The access layer (LANs) uses 10/100 layer-2 switches using cat 5e or cat 6 twisted pair cables (cat 6 is needed for 1000BaseT).
5. The distribution layer uses layer-3 Ethernet switches that use 1000BaseT or fiber, Cat 6 or Cat 7 TP.
6. The core layer uses layer-3 Ethernet switches running 10GbE or 40GbE over fiber.
7. Network reliability is increased using redundant switches and cabling.

54. Figure 5-20 The best practice network design

55. End of Chapter 5