Chapter 5 Hardware Layers Backbone Networks Networking in

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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 3

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 4

Introduction 5

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. 6

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. 7

Figure 5 -1 Backbone network design layers 8

Backbone Architectures 9

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

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. 11

Figure 5 -2 Hierarchical backbone architecture 12

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-1980 s 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. 13

Figure 5 -3 Flat backbone architecture 14

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 200 -600% 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. 15

Figure 5 -4 Collapsed backbone architecture 16

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. 17

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

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. 19

Figure 5 -7 Central Parking’s collapsed backbone 20

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 IEEE 802. 1 q and IEEE 802. 1 p. • The two basic designs are: – Single-switch VLANs – Multiswitch VLANs 21

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. 22

Figure 5 -8 Single-switch VLAN architecture 23

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 IEEE 802. 1 p standard in the hardware layers and the RSVP standard in the internetwork layers. • IEEE 802. 1 p works with the IEEE 802. 11 ac frame definition which includes a special priority field. 24

Figure 5 -9 Multiswitch VLAN architecture 25

Figure 5 -10 IONA VLAN 26

Fiber Distributed Data Interface 27

Fiber Distributed Data Interface (FDDI) • FDDI (standardized as ANSI X 3 T 9. 5) backbone protocol was developed in the 1980 s and popular during the 80 s and 90 s. • 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. 28

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). 29

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

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. 31

Figure 5 -12 Managing a broken circuit 32

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. 33

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 0 -4500 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. 34

Figure 5 -13 FDDI frame layout 35

Asynchronous Transfer Mode 36

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. 37

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-5 e twisted pair cables. 38

Figure 5 -14 ATM mesh architecture 39

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 (Qo. S). 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. • E-mail gets a lower priority, since small delays don’t matter very much. 40

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). 41

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

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. 43

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). 44

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 45 but only on the header portion of the field.

Figure 5 -16 ATM cell layout 46

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. 47

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. 48

Figure 5 -17 ATM in the backbone 49

The Best Practice Backbone Design 50

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. 51

Technology Effective Data Rate Full Duplex 1 Gb. E 1. 8 Gbps Full Duplex 10 Gb. E 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 52

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 5 e or cat 6 twisted pair cables (cat 6 is needed for 1000 Base. T). – 5. The distribution layer uses layer-3 Ethernet switches that use 1000 Base. T or fiber, Cat 6 or Cat 7 TP. – 6. The core layer uses layer-3 Ethernet switches running 10 Gb. E or 40 Gb. E over fiber. – 7. Network reliability is increased using redundant switches and cabling. 53

Figure 5 -20 The best practice network design 54

End of Chapter 5 55