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CSC465 ā Computer Networks
Spring 2004
Dr. J. Harrison
These slides were produced from material by Behrouz Forouzan for the text āTCP/IP Protocol Suite (2 nd^ Edition)ā and from Kurose & Ross, āComputer Networking; A Top-down Approach Featuring the Internetā
Chapter 3 (Part B)
Underlying Technologies
-Continued-
CONNECTING
DEVICES
Hubs, Bridges and Switches
- Used for extending LANs in terms of geographical coverage, number of nodes, administration capabilities, etc.
- Differ in regards to:
- collision domain isolation
- layer at which they operate
Internet
- Internet is neither single LAN or WAN
- Interconnected WANs and LANs
- Interconnection achieved via connecting devices
- Repeaters & Hubs operate in 1st^ TCP/IP Layer
- same as 1st^ ISO/OSI Layer
- Bridges operate in the first two layers
- Routers operate in the first three layers
- Switches
- Sophisticated bridge
- Sophisticated router
Connecting devices
Repeaters
- Signals attenuate over longer distances
- Before attenuation causes the signal to become too weak or become corrupted, repeater regenerates and transmits the original bit pattern
- Operates only at physical layer
- Simply retransmits bit with more energy
- no CSMS/CD performed
- No address filtering capability
- Partitions network into segments
- All segments in same collision domain.
Repeater
Hubs
- Overloaded term but means: multiport repeater
- Used to create connections in a physical star configuration
- All nodes must use same technology
- Example: 10BASE-T Ethernet LAN
- Hubs can be used to build hierarchies to overcome attenuation limitations
Hubs
Network still one LAN with logical bus topology Every packet sent is still received by all stations Hierarchy removes length limitations of 10BASE-T
LAN Segment
Backbone Hub with Point-to-Point Connection
LAN Segment
Multi-tier Hub Design
Hubs
- Physical Layer devices: essentially repeaters operating at bit levels: repeat received bits on one interface to all other interfaces
- Hubs can be arranged in a hierarchy (or multi-tier design ), with a backbone hub at its top
- Each connected LAN is referred to as a LAN segment
- Hubs do not isolate collision domains: a node may collide with any node residing at any segment in the LAN
Hubs (Cont.)
- Hub Advantages:
- Simple, inexpensive device
- Multi-tier provides graceful degradation: portions of the LAN continue to operate if one of the hubs malfunction +Extends maximum distance between node pairs (100m per Hub)
- Hub Limitations:
- Single collision domain results in no increase in max throughput; the multi-tier throughput same as the the single segment throughput
- Individual LAN restrictions pose limits on the number of nodes in the same collision domain (thus, per Hub); and on the total allowed geographical coverage
- Cannot connect different Ethernet types (e.g., 10BaseT and 100baseT)
Primary Functions of Bridge
CRC SA DA
IP Datagram DATA H
MAC frame
Bridge checks and recalculates CRC
BRIDGE examines these fields: learns SA for future decisions forwards orfilters based on DA
Primary Functions of a Bridge AddressA Port 1 BC (^11) DE (^22) MAC frame FG^22 A E
Bridge learn fwd/filter
Single Subnet Total Capacity = 20Mb/s
A (^) B C^
D (^) E F
G Segment 1 Segment 2
Hub
Hub (^) Hub
Further Segmenting: Multiport Bridges
Single Subnet Total Bandwidth = 30Mb/s
Bridge MAC frame A E
Segment 1 Segment 2 Segment 3
Hub Hub Hub
A (^) B C
D E F G
Typical Network topology with Bridges
B
B1 B
Router
Hub
B3 Hub
B
Self-learning Bridge
- Bridge table initially empty
- If a frame arrives and the destination is not in the table, the frame is copied to output buffers of all interfaces
- Each interfaces sends frame using CSMA/CD
- If every node eventually sends a frame, every node will eventually get recording in the table
- Bridge table removes entries after the aging time has elapsed (like e-mail)
- If PC gets new adapter, physical address of old adapter will eventually be purged from table
Bridges
- Unlike a hub, bridge runs the CSMA/CD algorithm - Refrains from sending if it hears traffic and employs exponential backoff
- Like a NIC except there are no addresses
- Does not insert a physical address into the packet
- Can be used to combine Ethernet segments that use different technologies (10BASE-T,10BASE2)
Bridge (^) Backbone Bridge
- LAN segments should be connected with a backbone
- Backbone: network that has direct connection to LAN segments
- Each pair of LAN segments can communicate without passing through a third LAN segment
Interconnection Without Backbone Bridge
- Two disadvantages:
- Single point of failure at Computer Science hub
- All traffic between EE and SE must path over CS segment
Interconnection with Backbone Bridge
Bridges Spanning Tree
- For increased reliability, it is desirable to have redundant, alternate paths from a source to a destination
- With multiple simultaneous paths however, cycles result on which bridges may multiply and forward a frame forever
- Solution is organizing the set of bridges in a spanning tree by disabling a subset of the interfaces in the bridges
Bridges Spanning Tree
Disabled
Switched Ethernet Multiple transactions in a switch
Increased throughput needs increased forwarding capability
1 4
3
2
C
D
A
B
Switched Ethernet
Multiport BridgeBandwidth shared among all nodes on each segment
Address Port A 1 1 AA 2 1 B 3 1 B^1
B^2 3
. . . Else Broadcast
1 3
2 Hub
A 1 A 2 A (^3)
B 1 B 2 B 3
C 1 C 2 C 3
Hub
Hub
Ethernet Switches (Cont) Dedicated
Shared
Routers
- 3-layer device (physical, data link & network)
- Physical: regenerates signal it receives
- Data link: examines physical addresses in packet
- Network layer: checks the network layer addresses (IP)
- Router is an internetworking device:
- Can connect 2 LANs together
- Can connect 2 WANs together
- Can connect a LAN and WAN together
A repeater or a bridge connects segments A repeater or a bridge connects segments
of a LAN.of a LAN.
A router connects independent LANs orA router connects independent LANs or
WANs to create anWANs to create an internetworkinternetwork
(internet).(internet).
Routing example
A router changes the physical addresses inA router changes the physical addresses in
a packet.a packet.
Bridges Vs. Routers
- Both are store-and-forward devices, but Routers are Network Layer devices (examine network layer headers) and Bridges are Link Layer devices
- Routers maintain routing tables and implement routing algorithms, bridges maintain filtering tables and implement filtering, learning and spanning tree algorithms
Bridges Pros and Cons:
- Bridge operation is simpler requiring less processing bandwidth, i.e., faster
- Topologies are restricted with bridges: a spanning tree must be built to avoid cycles
- Bridges do not offer protection from ābroadcast stormsā (endless broadcasting by a host will be forwarded by a bridge)
Routers Pros and Cons:
- Arbitrary topologies can be supported, cycling is limited by TTL counters
- Can provide firewall protection against broadcast storms
- Often requires IP address configuration
- not plug and play; DHCP is exception
- Require higher processing bandwidth
- Bridges do well in small (few hundred hosts) while routers are required in large networks (thousands of hosts)
Routers vs. Repeaters/Bridges (conāt)
- A router has both a physical and logical (IP) address for each of its interfaces
- A router acts only on those packets in which the destination address matches the address of the interface at which the packet arrives - True for unicast, multicast and broadcast
- A router changes the physical address of the packet (both source and destination) when forwarding the packet
Routers vs. Repeaters/Bridges (conāt)
- A router separates collision domains
- Can improve performance
- But 3-layer processing more time consuming than 2- layer processing
- Transparent bridges are ā plug-and-playā
- Routers require configuration (more admin)
- Bridges implement spanning tree over nodes to eliminate cycles - Implemented by virtually disconnecting nodes - Results in self-imposed traffic increases - Frames concentrated on spanning tree links when it could be spread on all links in original topology
