Local Area Network Overview
Chapter 11 in Stallings 10th Edition
CS420/520 Axel Krings Page 1 Sequence 11
LAN Applications (1) • Personal computer LANs —Low cost —Limited data rate • Back end networks —Interconnecting large systems (mainframes and large storage devices) • High data rate • High speed interface • Distributed access • Limited distance • Limited number of devices
CS420/520 Axel Krings Page 2 Sequence 11
1 LAN Applications (2) • Storage Area Networks — Separate network handling storage needs — Detaches storage tasks from specific servers — Shared storage facility across high-speed network — Hard disks, tape libraries, CD arrays — Improved client-server storage access — Direct storage to storage communication for backup • High speed office networks — Desktop image processing — High capacity local storage • Backbone LANs — Interconnect low speed local LANs — Reliability — Capacity — Cost
CS420/520 Axel Krings Page 3 Sequence 11
Storage Area Networks
CS420/520 Axel Krings Page 4 Sequence 11
2 LAN Architecture • Topologies • Transmission medium • Layout • Medium access control
CS420/520 Axel Krings Page 5 Sequence 11
Bus • Multipoint medium • Transmission propagates throughout medium • Heard by all stations — Need to identify target station • Each station has unique address • Full duplex connection between station and tap — Allows for transmission and reception • Need to regulate transmission — To avoid collisions — To avoid hogging • Data in small blocks - frames • Terminator absorbs frames at end of medium
CS420/520 Axel Krings Page 6 Sequence 11
3 A
A B C Frame C transmits frame addressed to A Transmission
on Bus LAN A
Figure 11.1 A B C
Frame is not addressed to B; B ignores it
A
A B C
A copies frame as it goes by
CS420/520 Axel Krings Page 7 Sequence 11
Figure 11.1 Frame Transmission on a Bus LAN
Star Topology • Each station connected directly to common central node —Usually via two point to point links • Central node can broadcast —Physical star, logical bus —Only one station can transmit at a time (hub) • Central node can act as frame switch
CS420/520 Axel Krings Page 8 Sequence 11
4 Central Hub, switch, Star or repeater Topology
Figure 11.2
CS420/520 Axel Krings Page 9 Sequence 11
Figure 11.2 Star Topology
Protocol Architecture • Lower layers of OSI model • IEEE 802 reference model • Physical • Logical link control (LLC) • Media access control (MAC)
CS420/520 Axel Krings Page 10 Sequence 11
5 IEEE 802 vs OSI
Figure 11.3 OSI Reference Model
Application
Presentation IEEE 802 Reference Session Model
Transport Upper Layer LLC Service Protocols Access Point Network (LSAP)
( ) ( ) ( ) Logical Link Control Data Link Medium Access Control Scope of Physical Physical IEEE 802 Standards Medium Medium CS420/520 Axel Krings Page 11 Sequence 11
Figure 11.3 IEEE 802 Protocol Layers Compared to OSI Model
802 Layers - Physical • Encoding/decoding • Preamble generation/removal • Bit transmission/reception • Transmission medium and topology
CS420/520 Axel Krings Page 12 Sequence 11
6 IEEE 802 Layers • Logical Link • Media Access Control Layer (LLC) Control (MAC) —Provide interface to —On transmit assemble higher levels data into frame —Perform flow and error —On reception control disassemble frame, perform address recognition and error detection —Govern access to LAN transmission medium
CS420/520 Axel Krings Page 13 Sequence 11
LAN Protocols in Context
Figure 11.4
CS420/520 Axel Krings Page 14 Sequence 11
7 Logical Link Control
Ø Transmission of link level PDUs between stations Ø Must support multi-access, shared medium Ø Relieved of some details of link access by the MAC layer Ø Addressing involves specifying source and destination LLC users l Referred to as service access points (SAPs)
CS420/520 Axel Krings Page 15 Sequence 11
LLC Services Unacknowledged connectionless service • Data-gram style service • Delivery of data is not guaranteed Connection-mode service • Logical connection is set up between two users • Flow and error control are provided Acknowledged connectionless service • Datagrams are to be acknowledged, but no logical connection is set up
CS420/520 Axel Krings Page 16 Sequence 11
8 LLC Service Alternatives Unacknowledged connectionless service • Requires minimum logic • Avoids duplication of mechanisms • Preferred option in most cases Connection-mode service • Used in simple devices • Provides flow control and reliability mechanisms Acknowledged connectionless service • Large communication channel needed • Time critical or emergency control signals
CS420/520 Axel Krings Page 17 Sequence 11
MAC MAC Destination Source LLC PDU CRC Frame Control MAC Address MAC Address
1 octet 1 1 or 2 variable LLC DSAP PDU SSAP LLC Control Information
LLC I/G DSAP value C/R SSAP value Address Fields
I/G = Individual/Group C/R = Command/Response
Figure 11.5 LLC PDU in a Generic MAC Frame Format
CS420/520 Axel Krings Page 18 Sequence 11
9 LLC Protocol
Ø Modeled after HDLC Ø Asynchronous balanced mode l Connection mode (type 2) LLC service Ø Unacknowledged connectionless service l Using unnumbered information PDUs (type 1) Ø Acknowledged connectionless service l Using 2 new unnumbered PDUs (type 3) Ø Permits multiplexing using LSAPs
CS420/520 Axel Krings Page 19 Sequence 11
Media Access Control • Where —Central • Greater control • Simple access logic at station • Avoids problems of co-ordination • Single point of failure • Potential bottleneck —Distributed • How —Synchronous • Specific capacity dedicated to connection, not optimal —Asynchronous • In response to demand, round robin, reservation, contention CS420/520 Axel Krings Page 20 Sequence 11
10 Asynchronous Systems • Round robin — Good if many stations have data to transmit over extended period • Reservation — Divide medium into slots. — Good for stream traffic • Contention — Good for bursty traffic — All stations contend for time — Distributed — Simple to implement — Efficient under moderate load — Tend to collapse under heavy load
CS420/520 Axel Krings Page 21 Sequence 11
MAC Frame Handling
Ø MAC layer receives data from LLC layer Ø PDU is referred to as a MAC frame Ø MAC layer detects errors and discards frames Ø LLC optionally retransmits unsuccessful frames
CS420/520 Axel Krings Page 22 Sequence 11
11 Bridges Ø Connects similar LANs with identical physical and link layer protocols Ø Minimal processing Ø Can map between MAC formats Ø Reasons for use: l Reliability l Performance l Security l Geography
CS420/520 Axel Krings Page 23 Sequence 11
LAN A
Frames with addresses 11 through 20 are accepted and repeated on LAN B
Bridge
Station 1 Station 2 Station 10
Frames with addresses 1 through 10 are accepted and repeated on LAN A LAN B
Station 11 Station 12 Station 20
Figure 11.6 Bridge Operation
CS420/520 Axel Krings Page 24 Sequence 11
12 Bridge Design Aspects Ø Makes no modification to the content or format of the frames it receives Ø Should contain enough buffer space to meet peak demands Ø Must contain routing and addressing intelligence Ø May connect more than two LANs Ø Bridging is transparent to stations
CS420/520 Axel Krings Page 25 Sequence 11
User User t1 t8 LLC LLC t2 t7 MAC MAC MAC t3 t4 t5 t6 Physical LAN Physical Physical LAN Physical
(a) Architecture
t1, t8 User Data
t2, t7 LLC-H User Data
t3, t4, t5, t6 MAC-H LLC-H User Data MAC-T
(b) Operation
Figure 11.7 Connection of Two LANs by a Bridge
CS420/520 Axel Krings Page 26 Sequence 11
13 Station 1 Station 2 Station 3
LAN A
Bridge Bridge 101 Bridge 102 107
LAN B LAN C
Bridge Bridge Bridge Bridge 105 106 103 104
LAN D LAN E LAN F LAN G
Station 4 Station 5 Station 6 Station 7
Figure 11.8 Configuration of Bridges and LANs, with Alternate Routes CS420/520 Axel Krings Page 27 Sequence 11
Fixed Routing • Simplest and most common strategy • Suitable for small internets and internets that are relatively stable • A fixed route is selected for each pair of LANs • Usually least hop route Ø Only change when topoloy changes Ø Widely used but limited flexibility
CS420/520 Axel Krings Page 28 Sequence 11
14 Spanning Tree • Bridge automatically develops routing table • Automatically update in response to changes • Algorithm consists of 3 mechanisms: —Frame forwarding —Address learning —Loop resolution
CS420/520 Axel Krings Page 29 Sequence 11
Frame forwarding • Maintain forwarding database for each port —List station addresses reached through each port • For a frame arriving on port X: —Search forwarding database to see if MAC address is listed for any port (except port X) —If address not found, forward to all ports (except X) —If address listed for some port Y, check port Y for blocking or forwarding state • Blocking prevents port from receiving or transmitting —If Y is not blocked, transmit frame through port Y
CS420/520 Axel Krings Page 30 Sequence 11
15 Address Learning
• Can preload forwarding database • When frame arrives at port X, it has come from the LAN attached to port X • Use source address to update forwarding database for port X to include that address • Have a timer on each entry in database —If timer expires, entry is removed • Each time frame arrives, source address checked against forwarding database —If present timer is reset and direction recorded —If not present entry is created and timer set
CS420/520 Axel Krings Page 31 Sequence 11
Spanning Tree Algorithm • Address learning works for tree layout if there are no alternate routes in the network —Alternate route means there is a closed loop • For any connected graph there is a spanning tree maintaining connectivity with no closed loops • Algorithm must be dynamic
IEEE 802.1 Spanning Tree Algorithm:
• Each bridge assigned unique identifier • Cost assigned to each bridge port • Exchange information between bridges to find spanning tree • Automatically updated whenever topology changes
CS420/520 Axel Krings Page 32 Sequence 11
16 Station B
LAN Y
t1 t2
Bridge Bridge
t LAN X t0 0
Station A
Figure 11.9 Loop of Bridges
CS420/520 Axel Krings Page 33 Sequence 11
Hubs
Ø Active central element of star layout Ø Each station connected to hub by two lines Ø Hub acts as a repeater Ø Length of a line is limited to about 100m Ø Optical fiber may be used to about 500m Ø Physically a star, logically a bus Ø Transmission from any one station is received by all other stations Ø If two stations transmit at the same time there will be a collision
CS420/520 Axel Krings Page 34 Sequence 11
17 HHUB Two cables (twisted pair or optical fiber)
IHUB IHUB Station Transmit
Receive
Station Station Station Station
Figure 11.10 Two-Level Star Topology
CS420/520 Axel Krings Page 35 Sequence 11
Shared Bus - 10 Mbps 10 Mbps 10 Mbps 10 Mbps 10 Mbps
A B C D
(a) Shared medium bus
Total capacity up to 10 Mbps
10 Mbps 10 Mbps
10 Mbps 10 Mbps
A B C D
(b) Shared medium hub
Total capacity N 10 Mbps
10 Mbps 10 Mbps
10 Mbps 10 Mbps
A B C D
(c) Layer 2 switch
Figure 15.11 LAN Hubs and Switches CS420/520 Axel Krings Page 36 Sequence 11
18 Layer 2 Switch Benefits Ø No change is required to the software or hardware of the attached devices to convert a bus LAN or a hub LAN to a switched LAN Ø Have dedicated capacity equal to original LAN l Assuming switch has sufficient capacity to keep up with all devices Ø Scales easily l Additional devices attached to switch by increasing capacity of layer 2
CS420/520 Axel Krings Page 37 Sequence 11
Types of Layer 2 Switch • Store-and-forward switch — Accepts frame on input line — Buffers it briefly, — Then routes it to appropriate output line — Delay between sender and receiver — Boosts integrity of network • Cut-through switch — Takes advantage of destination address appearing at beginning of frame — Switch begins repeating frame onto output line as soon as it recognizes destination address — Highest possible throughput — Risk of propagating bad frames • Switch unable to check CRC prior to retransmission
CS420/520 Axel Krings Page 38 Sequence 11
19 Layer 2 Switch vs. Bridge • Differences between switches and bridges: • Layer 2 switch can be viewed as full- duplex hub Bridge Switch • Incorporates logic to Performs frame function as multiport Frame handling forwarding in bridge done in software hardware • New installations Analyzes and Can handle typically include forwards one multiple frames frame at a time at a time layer 2 switches with bridge functionality Uses store-and- Can have cut- rather than bridges forward operation through operation
CS420/520 Axel Krings Page 39 Sequence 11
Inaho for takeout. Love to All Tricia
Internet
Z Server
Router Ethernet switch Workstation
One big LAN Printer
No separation for broadcasts W
X Y
Figure 11.12 A LAN Configuration CS420/520 Axel Krings Page 40 Sequence 11
20 Internet
Z Server
Router Ethernet switch Workstation
Router V: Printer
Now 4 separate LANS V
W
X Y
Figure 11.13 A Partitioned LAN CS420/520 Axel Krings Page 41 Sequence 11
Problems with Routers • Routers do all IP-level processing in software —High-speed LANs and high-performance layer 2 switches pump huge # of packets per second —Software-based router are slower • Solution: layer 3 switches —Implement packet-forwarding logic of router in hardware • Two categories —Packet by packet —Flow based
CS420/520 Axel Krings Page 42 Sequence 11
21 VLAN A VLAN Internet B Server
Z
VLAN Workstation A VLAN: VLAN (Virtual LAN) D Printer
Logical subgroup W Ethernet VLAN switch with VLAN VLAN and within LAN A IP routing E capability
VLAN A
VLAN X B Y VLAN C
Figure 11.14 A VLAN Configuration
CS420/520 Axel Krings Page 43 Sequence 11
Defining VLANs Ø Broadcast domain consisting of a group of end stations not limited by physical location and communicate as if they were on a common LAN Ø Membership by: l Port group l MAC address l Protocol information
CS420/520 Axel Krings Page 44 Sequence 11
22 Communicating VLAN Membership
Switches need to know VLAN membership
Ø Configure information manually Ø Network management signaling protocol Ø Frame tagging (IEEE802.1Q)
CS420/520 Axel Krings Page 45 Sequence 11
Summary
• Bus and tree topologies and transmission media • Bridges —Topologies —Functions of a bridge —Choice of topology —Bridge protocol —Choice of transmission architecture medium —Fixed routing • LAN protocol —The spanning tree architecture approach —IEEE 802 reference model • Virtual LANs —Logical link control —The use of virtual LANs —Medium access control —Defining VLANs • Hubs and switches —Communicating VLAN —Hubs membership —Layer 2 switches
CS420/520 Axel Krings Page 46 Sequence 11
23