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Question 1

Serial Line Internet Protocol (SLIP)

Answer 1

Serial Line Internet Protocol (SLIP) is an early protocol used on modems to encapsulate IP traffic over serial (telephone) lines

Question 2

size of SLIP frame

Answer 2

296b good compromise

Question 3

SLIP problems

Answer 3

Only IP in the next layer is supported (no type field in frame)
The ends must have pre-agreed IP addresses: no mechanism for agreeing addresses
No checksum: telephone lines were noisy and created data corruption
No authentication: no way to check who is connecting

Question 4

describe PPP’s data frame

Answer 4

set first 3 headers

• Cyclic redundancy check to spot corruption
• But no address fields

Question 5

PPPoA

Answer 5

It offers standard PPP features such as authentication, encryption, and compression. It also supports (as does PPPoE) the encapsulation types: VC-MUX and LLC

Question 6

ATM

Answer 6

Asynchronous transfer mode (ATM) is a switching technique used by telecommunication networks that uses asynchronous time-division multiplexing to encode data into small, fixed-sized cells. This is different from Ethernet or internet, which use variable packet sizes for data or frames.

The fixed cell structure of ATM means it can be easily switched by hardware without the delays introduced by routed frames and software switching. This is why some people believe that ATM is the key to the internet bandwidth problem. ATM creates fixed routes between two points before data transfer begins, which differs from TCP/IP, where data is divided into packets, each of which takes a different route to get to its destination.

Question 7

MPLS and how BT uses it in its 21C Network

Answer 7

The 21st Century Network (21CN) programme is the network transformation project of the UK telecommunications company BT Group plc for data and voice. It was intended to move BT’s telephone network from the AXE/System X Public Switched Telephone Network (PSTN) to an Internet Protocol (IP) system. As well as switching over the PSTN, BT planned to deliver many additional services over their new data network, such as on-demand interactive TV services.

The most significant differences between the legacy 20th century network (20CN) and 21CN are:

The use of VoIP landline telephone services served digitally, rather than the previous analogue PSTN system. This does not mean telephone services are provided over the Internet with 21CN. In reality this will make no difference to the end user.
The availability of ADSL2+ (including ADSL2+ Annex M) broadband services. The 20CN allows services up to ADSL Max with a maximum download throughput of 8Mbit/s whereas ADSL2+ allows a maximum theoretical download throughput of 24Mbit/s.
The retirement of legacy IPStream data services previously available with 20CN.

Question 8

Multiprotocol Label Switching (MPLS)

Answer 8

replaces ATM (alien time machine)

Question 9

what do hardware independent addresses solve (IP)

Answer 9

We can’t simply treat the world as a shared medium and broadcast the packet to everybody

thats why

Question 10

where does IP sits within the talk of standards

Answer 10

it’s just part of the most famous implementation of the 4 layer internet model, the Internet Protocol (ironically called exactly the same as the IP in the question which is the implementation of the implementation of the network layer)

Question 11

what does ip do

Answer 11

It has the major function of dealing with routing, determining where a packet should go

Question 12

contents of IP header:

Answer 12

4 bit version
4 bit header length
8 bit type of service
16 bit total length
16 bit identification
13 bit fragment offset
8 bit time to live
8 bit protocol
16 bit checksum
32 bit source address
32 bit destination address
Header options
Data

Question 13

describe the structure of the IP header address

Answer 13

In this example, 138.38.32.14. the first half 138.38 is a 16-bit network address, which identifies the University of Bath
And 32.14 is a 16-bit host address, which identifies a single machine on the University’s network

Question 14

ARP

Answer 14

This is done by the Address Resolution Protocol (ARP)
ARP is a very simple link-layer protocol that essentially broadcasts a special frame on the local medium to the effect of “who has IP address 138.38.32.14?”
All hosts on the local network hear this broadcast and the host with that address replies “Me: and I have Ethernet address 08:00:20:9a:34:dd”

We don’t want to use ARP for every packet we send, so there is an ARP cache kept by the OS kernel that records the relation 138.38.32.14 ↔ 08:00:20:9a:34:dd