Understanding Network Devices: Hubs, Bridges, and Switches
HUBS, BRIDGES, SWITCHES
HUBS:
Active central element of star layout
- Physically star, logically bus
- If two stations transmit at the same time, collision occurs
- Transmission from any station received by all other stations
• Each station connected to hub by two lines
- Transmit and receive on two unshielded twisted pairs
• Hub acts as a repeater
- When a single station transmits, hub repeats the signal on the outgoing line to each station.
Backbone hub interconnects LAN segments
- Extends max distance between nodes
- But individual segment collision domains become one large collision domain
▫ if a node in CS and a node EE transmit at the same time: collision
• Can’t interconnect 10BaseT & 100BaseT
BRIDGES:
A bridge is a layer-2 device that connects LANs that may or may not be based on the same technology.
Bridges for Traffic Isolation
- Bridge installation breaks LAN into LAN segments
- Bridges filter packets:
- same-LAN-segment frames not usually forwarded onto other LAN segments
- segments become separate collision domains.
Overcomes the distance limitations of LANs
- Connects LANs that use different technologies/Organizations
SWITCHES:
Layer 2 Switches
- Central hub acts as a switch
- Incoming frame from a particular station switched to the appropriate output line
- Unused lines can switch other traffic
- More than one station transmitting at a time
- Multiplying capacity of LAN
- Can perform error detection by CRC
Advantages of layer 2 switches:
Layer 2 switch scales easily. For Ethernet LAN, each device uses Ethernet MAC protocol. Device has dedicated capacity equal to the original LAN
Types of LAYER 2 SWITCHES:
- STORE AND FORWARD SWITCH
- CUT-THROUGH SWITCH
Layer 2 Switch vs. Bridge
Layer 2 switch can be viewed as full-duplex hub + L2 functions
Can incorporate logic to function as a multiport bridge
- Bridge frame handling done in software
- Switch performs address recognition and frame forwarding in hardware
- Bridge only analyzes and forwards one frame at a time
- Switch has multiple parallel data paths
- Can handle multiple frames at a time
- Bridge uses store-and-forward operation
- Switch support both store-and-forward and cut-through operation
- Bridge suffered commercially▫ New installations typically include layer 2 switches with bridge functionality rather than bridges
Difference between Hub, Bridge and Switch:
A Hub is the simplest of these devices out of the five compared. Hubs cannot filter data so data packets are sent to all connected devices. Hubs do not have intelligence to find out the best path for data packets. This leads to inefficiencies and wastage. Pretty much repeat signal on one end to another. Hubs are used on small networks where data transmission is not very high.
A bridge is more complex than a hub. A bridge maintains a MAC address table for both LAN segments it is connected to. Bridge has a single incoming and outgoing port. Bridge filters traffic on the LAN by looking at the MAC address. used to separate parts of a network that do not need to communicate regularly, but need to be connected.
A switch when compared to a bridge has multiple ports. Switches can perform error checking before forwarding data. Switches are very efficient by not forwarding good packets selectively to correct devices only. Switches can support both layer 2 (based on MAC Address) and layer 3 (Based on IP address).Usually large networks use switches instead of hubs to connect computers within the same subnet.
Error Correction Codes
- Frame consists of m data bits and r check bits.
- The resulting n=m+r bit unit is called a code word
- The number of bits by which two code words differ is called the Hamming Distance.
- To detect d-bit errors, we need a distance of d+1 between any pair of code words.
- code words with a single parity bit have a minimum distance of 2
- To correct d-bit errors, we need a distance of 2d+1.
Collision Avoidance: RTS-CTS Exchange
- Sender transmits a short RTS (request to send) packet
- Indicating duration of transmission
- Receiver replies with a short CTS (clear to send) packet
- Notifying (possibly hidden) nodes
- Hidden nodes will not transmit for a specified duration: NAV
Collision Avoidance: RTS-CTS Exchange
- Sender transmits a short RTS (request to send) packet
- Indicating duration of transmission
- Receiver replies with a short CTS (clear to send) packet
- Notifying (possibly hidden) nodes
- Hidden nodes will not transmit for a specified duration: NAV
Frequency Hopping:
transmit over a random sequence of frequencies.
Direct Sequence:
for each bit, send XOR of that bit and n random bits.random sequence known to both sender and receiver, called n-bit chipping code.
Point Coordination Function (PCF):
Alternative access method implemented on top of DCF.Polling by centralized polling master (point coordinator). Can seize the medium and lock out all asynchronous traffic while it issues polls and receives responses.E.g. wireless network configured some number of stations with time-sensitive traffic controlled by a point coordinator. Point coordinator polls in round-robin to stations configured for polling.