Sometime back, the company I worked for asked me to help and prepare a technical assessment to be used by HR in headhunting for an entry level IP/Packet Core Network Engineer. In most telecom companies, the IP and Packet Core roles are combined especially if the company is small or mid-sized. Having so far spent about 10 years working in IP/PS operations, I thought these questions were fair and relevant, please let me know what you think by commenting on this article.
[Note: Answers are in italic format]
Section A: IP Questions & Answers
What is the difference between a private IP address and a public IP address? Give examples of where these two categories are applicable.
- Private IP address – These are addresses which are NOT routable on the internet.
- Public IP address – These are addresses which are routable on the internet.
With reference to the OSI model, at what layer would you place the following devices?
What do you understand by the term VLSM? And why is it so important in IP address planning and management?
VLSM – Variable Length Subnet Masking
VLSM is a technique used by network planners to effectively manage and save IP space. Using VLSM technique you can be able to use a subnet mask of varying length to break down a network address into different subnets of different sizes. This allows a planner to assign just enough addresses as required by different network sizes.
What is OSPF? And briefly explain why it is important do divide your OSPF network into multiple areas?
OSPF – Open Shortest Path First
Areas in OSPF help in reducing the routing table size and the LSDB (Link State DataBase) size hence reducing the time required for network convergence and also saving on system resources like the router CPU which is used to run the SPF algorithm.
Define MPLS and tell us the main difference between MPLS and the usual interior routing protocols like OSPF?
MPLS – Multi Protocol Label Switching
OSPF – Open Shortest Path First
MPLS uses labels to switch/route packets as opposed to OSPF which uses the IP address in the packet header to route packets to the destination. When a packet enters an MPLS domain, a label is added, this label is what is used to guide the packet through the MPLS domain. When a packet enters an IP routing domain say (OSPF), the routers use the IP address information contained in the packet header to guide the packet to the next hop/destination.
What is a VLAN? And what protocol standard is used for VLAN encapsulation?
VLAN – Virtual Local Area Network
VLAN uses protocol standard 802.1Q for encapsulation
What do you understand by the technique NAT? and why is NAT so important in IP networks?
NAT – Network Address Translation
NAT is a technique that allows multiple private addresses to use one or a few public addresses to access the internet.
This technique was developed to save on the scarce public IP space.
What are some of the parameters that you can use to effectively measure performance of an IP transmission PATH?
- Latency – Also known as packet delay is measured in milliseconds and it’s the time it takes for a packet to move from a source to a destination and back to the source (round trip)
- Jitter – this is the delta in packet delay
- Packet Loss – This a percentage of the number of packets received on an IP Path (round-trip) to the total number of packets transmitted on that IP Path.
What is the difference between HTTP and HTTPS and what layer of the OSI would you place these two protocols?
HTTP is Hyper Text Transfer Protocol whereas HTTPS is secure-HTTP
HTTP is transferred as plain text and is susceptible to man-in-the-middle attack where as HTTPS is transferred over a secure layer called SSL (Secure Socket Layer).
HTTP – Layer 7 (Application Layer)
HTTPS – layer 7 (Application Layer) and Layer 6 (Presentation Layer)
This is a real-life scenario; A business client who has a dedicated internet link with our company (ISP Company) has called and escalated to the NOC that he/she is unable to browse the internet! The NOC has in turn escalated to you. Briefly explain the troubleshooting steps you would quickly go through to help the customer reconnect. Please be brief and precise.
- Gather some basic information from customer for example; Client IP address, Gateway IP address, DNS IPs. This information can be gathered by asking the client to run ipconfig/all on his/her computer command terminal.
- Ask the client to try and ping the default gateway, this is to eliminate connectivity issues towards the last mile.
- Ask the client to try and ping a domain name for example www.google.com this is to help rule out DNS issues.
Section B: Packet Core Questions & Answers
With reference to GSM, define the term GPRS?
General Radio Packet Service – is a technology used to deliver packet oriented mobile data services over 2G/3G in GSM.
Mention at least two core network elements in the GPRS (2G/3G) domain and briefly state their respective roles?
SGSN – Serving GPRS Support Node is used for mobility management, session management, and authenticating mobile users to the packet network.
GGSN – Gateway GPRS Support Node is used to interface the GPRS domain with the external IP networks. It handles the conversion from IP to GTP and GTP to IP.
Define the term APN?
APN – Access Point Name is used as a gateway between a GPRS mobile network and another external network. It’s configured on the GGSN.
With reference to 3GPP packet core domain (IuPS), what protocol is used between the SGSN and RNC to carry:
- Signalling traffic? – RANAP (Radio Access Network Application Part)
- Payload traffic? – GTP (GPRS Tunnelling Protocol)
Define the term DNS and briefly explain the difference between the Gn DNS and Gi DNS with reference to the 3GPP Packet core architecture.
DNS means Domain Name Service, it’s a service that translates domain names strings like www.facebook.com into its equivalent IP address.
Gn DNS sits inside the packet core domain and is used to translate access point names into the GGSN address.
Gi DNS sits at the edge of the packet core domain and is used to translate public domain names into IP addresses.
With reference to 3GPP, what is a AAA server and what protocols can be used by a client to communicate with a AAA server?
AAA – Authentication Authorization and Accounting server is used to handle user requests to access protected resources.
Protocols to communicate with a AAA server include; Diameter and Radius
Mention at least two key performance indicators used to measure the performance of a 3G network, go ahead to briefly describe what each counter means?
GPRS attach success rate – percentage of the successful GPRS attach requests over the total number of attach requests reaching the SGSN.
GPRS PDP success rate – percentage of the successful GPRS PDP activation requests over the total number of GPRS PDP activation requests.
Mention at least three core network elements in a 4G network architecture?
MME – Mobility Management Entity
SGW – Serving GateWay
PGW – Packet GateWay
What do you understand by the term VoIP and mention at least one protocol used in VoIP?
VoIP – Voice over IP
SIP – Session Initiation Protocol is used in VoIP networks
With reference to 3GPP, explain the technique “Direct Tunnel” and what would be the advantages of implementing DT?
Direct Tunnel is a technique whereby after a successful PDP activation, the GGSN initiates a direct connection to RNC do deliver GTP payload.
DT is used to improve network delay and improve user experience.
