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Wireless
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Wireless
• Wireless hosts: end system devices; may or may not be mobile
• Wireless links: A host connects to a base station or host through a communication link
• Base station: Responsible for sending and receiving data (access point)
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Standards
Letter Speed Range Frequency
No letter 2 Mbps 150 ft 2.4 GHz
A 54 Mbps 150 ft 5 GHz
B 11 Mbps 300 ft 2.4 GHz
G 54 Mbps 300 ft 2.4 GHz
N 108 Mbps 300 ft 2.4 GHz
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Range Limiters
• Cordless telephones
• Large electrical appliances such as refrigerators
• Fuse boxes, metal plumbing, metal studing and air conditioning units
• Sun spots
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Not wired is…
• Decreasing signal strength
• Interference from other sources
• Multipath propagation (bounce)
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BasicServiceSet
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MAC Protocol
• CSMA with collision avoidance: sense first, then send
• Collision avoidance (can’t send/receive at same time)
• Link-layer ACKs and retransmissions due to high bit-error rates
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802.11 frames
• 3 address fields– Address 2 is MAC address of sending station
(host or AP)– Address 1 is MAC address of destination– Address 3 is MAC address of router interface
• CRC value
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We are still working on this questionAnswer is easy if hub is involvedSwitch has “learned” that H1 is in BSS1 and has to be “taught” to use BSS2
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Wireless Security
• Configure a unique SSID then block transmission of it– Unique name is cute, but so what?– If you block transmission, network does not show up
and no way to specify name
• Use MAC filtering. This one makes total sense• Change administrator account name and
password• Why go the extra distance to use WPA2 if you
have done the above?
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Secure Communication
• Confidentiality: only the sender and intended receiver should be able to understand the contents of transmitted message
• Authentication: Both the sender and receiver shoul be able to confirm the identity of the other party
• Message integrity and nonrepudiation: Make sure message is not altered in transit
• Availability and access control: communication can occur in the first place – only lock out the “bad guys”
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Cryptography
• Dates back to Julius Caesar• Allow sender to disguise data so that an intruder
can gain no information from the data intercepted
• Send ciphertext (not cleartext or plaintext)• Symmetric key systems both keys are identical
and are secret • Public key systems use two keys. One is known
(public); the other is known only by Alice or Bob
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Symmetric Key
• Caesar cipher: substitute letter that is k letters removed (alphabet wraps)
• Monoalphabetic cipher: substitute random letter for letter (fixed chart)
• Polyalphabetic encryption: two or more mono’s with a random C1,C2,C2,C1 pick pattern
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DES
• Data Encryption Standard• Encodes plaintext in 64-bit chunks using a
64-bit key (8 bits are odd parity bits; 56 bits long)
• Two (first and last steps) permutation steps; 16 identical steps in the middle
• How well does it work? No one knows for sure. First crack was 4-months, then 22 hours
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3DES
• Encrypt with one key; decrypt with second key; encrypts with third key
• Advanced Encryption Standard (AES): processes data in 128-bit blocks using keys that are 128, 192 and 256 bits long
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A core problem
• Both sides have to know secret key
• How is this key communicated? Verified?
• Alternative is the idea of a public key
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Public Key Cryptography
• Bob has two keys: one public and one private to him
• Alice gets Bob’s public key; encrypts message
• Bob then decrypts message using private key
• Does this make sense?