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Advanced Networking. Lecture for September 17 X.25, Frame and ATM. Many of these figures were adapted from Tanenbaum (Computer Networks) and from Forouzan (Data Communications and Networking) . B. A. G. C. F. D. E. Shared medium drawbacks. Shared-medium networks do not scale
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Advanced Networking Lecture for September 17 X.25, Frame and ATM Many of these figures were adapted from Tanenbaum (Computer Networks) and from Forouzan (Data Communications and Networking)
B A G C F D E Shared medium drawbacks • Shared-medium networks do not scale • Simple hub sends incoming frames to all output ports – (a layer 1 hub) • As more nodes are added, congestion becomes a problem >> it’s a shared medium 10BaseT 10Base2
Layer 2 Switching • Layer 2 switch is used to interconnect LAN segments • Usually Ethernet • Types of layer 2 switches • Simple Bridge • Multiport Bridge • Transparent Bridge • Remote Bridge • Possibly others??
Hub and Spoke (cont) • Filtering alleviates this problem • Why not just send the frames to the destination port and not the others? • This requires processing the destination fields in the frame • Drawbacks: • Requires the switching fabric to be much faster than before • 10 users all transmitting at 100Mbps >>> 1Gbps • Hub must be able to “Learn” the proper destination • i.e. How does the hub know to selectively forward frames?
Bridges • Bridges forward at Layer2 based on destination MAC address in Ethernet frame • When Ethernet frame comes in, sent out only on port corresponding to MAC address in table Problem: How is this table built???
B A G C F D E Learning Bridges • Bridges are Hubs that filter and forward selected layer 2 frames. Bridging Switch I H N J M K L
Learning Bridge (cont) • Old bridges required these tables to be built by hand. • The learning bridge builds and maintains a map of the physical port and the MAC address. • It does this by watching source MAC address of frames and from what physical port they come. Problem: What if we have multiple bridges connecting networks???
Spanning Tree Bridges Two parallel transparent bridges. These can create multiple copies of the frame. Can also cause forwarding loops.
Spanning tree (cont) • Often multiple bridges are used for redundancy. • Spanning tree algorithm is used to eliminate forwarding loops and multiple copies of frames. • Routes and bridges with lower numbers become primary elements of the spanning tree. • Other routes and bridges are used in the event of a failure • Exact algorithm is detailed but straightforward. • Algorithm also used at layer 3 for multicast IP and other applications.
Spanning Tree Bridges (2) (a) Interconnected LANs. (b) A spanning tree covering the LANs. The dotted lines are not part of the spanning tree.
Bridges from 802.x to 802.y Operation of a LAN bridge from 802.11 to 802.3. MAC layers are different for .11 and .3, LLC the same
Bridges from 802.x to 802.y (2) The IEEE 802 frame formats.
Remote Bridges • Remote bridges can be used to interconnect distant LANs.
Repeaters, Hubs, Bridges, Switches, Routers and Gateways (a) Which device is in which layer. (b) Frames, packets, and headers.
Figure 17-1 X.25
X.25 • Was the first popular packet switched network • Allowed for the setup of data connections at speeds between 300bps to about 56kbps. • Uses the “Virtual Circuit” concept. • Connection oriented packet switch. • Still used for low bandwidth transactions • credit cards / Point-of-Sale (POS) transactions. • Telemetry networks.
Figure 17-2 X.25 Layers in Relation to the OSI Layers
Figure 17-3 Format of a Frame
Figure 17-6 Frame Layer and Packet Layer Domains
Figure 17-7 Virtual Circuits in X.25 Virtual circuits allow “meshy” network with fewer physical links.
Figure 17-8 LCNs in X.25 LCN: Logical Channel Number, identifies the VC at different sections of the network.
Figure 17-10 PLP Packet Format Note the LCN is in the Layer 3 portion of the protocol. GFI: General Format Identifier-- defines which device should acknowledge the packet PTI: Packet Type Identifier – The type of packet
Figure 17-11 Categories of PLP Packets RR: Receive Ready RNR: Receive not ready REJ: Reject
Figure 17-12 Data Packets in the PLP Layer
Figure 17-13 RR, RNR, and REJ Packets
Figure 17-14 Other Control Packets
Figure 17-15 Control Packet Formats
Figure 17-17 Triple-X Protocols
Key points on X.25 • Was developed as a packet switching protocol. • Standard includes Layer 1,2,3 • Incorporates SVCs and PVCs • Limited in bandwidth • Not optimized for high quality links • Too much error checking for “good” networks • Not optimized for TCP / IP transport • Already has PLP defined at Layer 3
Chapter 18 Frame Relay
Figure 18-1 Frame Relay versus Pure Mesh T-Line Network
Figure 18-2 Fixed-Rate versus Bursty Data
Figure 18-3 X.25 Traffic
Figure 18-4 Frame Relay Traffic
Figure 18-5 Frame Relay Network
Figure 18-6 DLCIs: Data Link Connection Identifier
Figure 18-7 PVC DLCIs
Figure 18-8 SVC Setup and Release
Figure 18-9 SVC DLCIs
Figure 18-10 DLCIs Inside a Network Note that the overall connection is a mixture of different interfaces and DLCIs This allows DLCIs to be re-used on different interfaces
Figure 18-11 Frame Relay Switch
Figure 18-12 Frame Relay Layers
Figure 18-13 Comparing Layers in Frame Relay and X.25
Figure 18-14 Frame Relay Frame
Figure 18-15 BECN BECN assumes the source can reduce congestion by slowing down the transmission of data.
Figure 18-16 FECN FECN notifies the receiver that congestion is occurring. It can then be more patient and not request so much data.
Figure 18-17 Four Cases of Congestion
Frame Relay bandwidth management • Frame customers typically connect at T1 or T3 line rate. • They can “burst” up to this speed • They pay for something less • Normally pay based on CIR: Committed Information Rate • When Customers exceed CIR for an extended period, their traffic is subject to being discarded • Allows service providers to “oversubscribe” the network, but prevent individual users from “hogging” the resources.
Figure 18-18 Leaky Bucket
Figure 18-19 A Switch Controlling the Output Rate
