Saint's Log

Introduction to Networks – Part III

The focus of part 3 of this series is on the different types of wired networks. A key aspet of many networks is that QoS is just one concern, another key issue being how to meet guarantees for delivery of voice services. How is voice delivered? Integrated Services Digital Network (ISDN) is an international standard for voice, video, and data transmission over digital telephone or copper lines. It has two service levels. The first is Basic Rate Interface (BRI), which supports 2 bearer channels at 64kbps each and 1 D channel at 16 kbps. The second is the Primary Rate Interface (PRI), which supports 23 bearer channels (in the US) at 64 kbps each and 1 D channel at 16 kbps. The signaling/data (D) channel runs the ISDN signaling protocol based on
Q.931. This video is a good high level introduction of ISDN. T1 and ISDN are used in access networks, together with technologies like IP and MPLS.

Optical Networks

In newer generations of networks, the core is fiber (instead of copper) because it can deliver terabits per second. Installation and management of fiber networks is also much easier than copper networks. Fiber optic signals are analog – (in the infrared range).

Light sources used for fiber optic communication include light-emitting diodes (LEDs), laser diodes, vertical cavity surface emitting lasers (VCSELs), Fabry-Perot lasers, and distributed feedback lasers.

The packet transport network is another key piece to understand. Customers send traffic to metro access, aggregation, and core portions of the network where voice and data are converged. In the packet core, wavelengths are being added and dropped by add-drop multiplexers. There are several types of ADMs with links to explanations about them from various vendors:

1. Fixed OADM (FOADM)
2. Reconfigurable OADM (ROADM)
3. Flexible ROADM
4. Open ROADM, which works to address the fact that optical systems have been proprietary (e.g. because SD FEC algorithms) on transponders are not interoperable and there are proprietary control loops between transponders and other optical components).

The next video gave me a better understanding of customer concerns with ROADMs and FOADMs.

Other major types of network components include amplifiers, regenerators, and equalization nodes. Transponders map client side signals to wavelengths for high speed transport. They can be contained in a field-replaceable unit (FRU). Common types of pluggable optics include SFP+ (Small Form-factor Pluggable), CFP4, and QSFP28. Amplification is an analog process of boosting signal strength and is done in the optical domain (no conversion to electrical). Any impairments in the signal are boosted as well. A single pump laser is used for this. Regeneration can reshape and retime the optical signal but requires conversion to the electrical domain then back to the optical domain, making it more expensive to implement.

Major types of amplifiers in optical networks include EDFA (Erbium Doped Fiber Amplifer), Raman amplifier, and Hybrid Raman-EDFA amplifier. These are great explanations of these amplifiers:

Wavelength Selective Switch (WSS) was first implemented using MEMS but did’t work well because the hinges would fail. Liquid Crystal on Silicon (LCoS) is now commonly used to implement WSS since it has no moving parts. It can also support Flexgrid.

Optical patch panels are another component in fiber networks. They are used to join optical fibers where a connect/disconnect capability is required.

Handling Failure

There are 2 types of protection in networks:

1. Network protection: ensures that customer SLAs are met by preventing failures. Optical protection examples include mesh restoration (GMPLS, SDN), SNCP (OTN), UPSR & BLSR for SONET, and 1+1 or 1:1 circuits (active vs inactive backup circuit). Packet protection examples include MPLS fast reroute, LAG, G.8031, G.8032.
2. Equipment protection: focuses on protecting individual nodes.

I couldn’t emphasize this enough: this is such a broad field with so many technologies! What an introduction to networking!

Introduction to Networks – Part II

The previous post introduced different types of networks and some of their architectural details. In this post, we look at the biggest problem network engineers work on: congestion. How are networks designed to address it? The professor starts tackling this area with a discussion of Quality of Service (QoS). Quality is defined in terms of the underlying requirements e.g. throughput, delay, jitter, packet loss, service availability, and per-flow sequence preservation. Services can be best effort, or other classes like gold service. Cisco’s Quality of Service (QoS) document discusses four levels of metal policy (platinum, gold, silver, and bronze), for example.

Class of Service (CoS) is a traffic classification that enables different actions to be taken on individual classes of traffic. Contrast this to type of service (ToS), which is a specific field in the IPv4 header (used to implement CoS). Juniper Networks post on Understanding Class of Service (CoS) Profiles equates QoS and CoS, but the professor explains that QoS is a more abstract concept than CoS.

QoS is a set of actions that the network takes to deliver the right delay, throughput, etc. QoS timeframes affect the way congestion is handled. For example, scheduling and dropping techniques and per-hop queuing are useful for the low millisecond time regime common in web traffic. Congestion over hundreds of milliseconds typically affects TCP (e.g. round trip times, closed-loop feedback) and this is addressed via methods like active queue management (AQM) and congestion control techniques like random early detection (RED). Congestion that occurs in the tens of seconds to minutes range is addressed by capacity planning.

How is QoS achieved in the data and control planes? By queuing, scheduling, policing, and dropping. The roles of the data and control planes are quite extensive as per the router diagram used to describe them. This is without getting into the details of the management plane e.g. the element management systems (per node) and the network management systems they communicate with. Control plane QoS mechanisms handle admission control and resource reservation and are typically implemented in software. Resource Reservation Protocol (RSVP) is the protocol mostly used in practice for control plane QoS. There are many explanations on RSVP, e.g. this Introduction to RSVP and this RSVP Overview. The primary QoS architectures are integrated services (Intserv) and differentiated services (Diffserv). Intserv uses RSVP and although it doesn’t scale, it is useful when guaranteed service is required.

We start a deep dive into the QoS techniques with queuing. There are different types of queues: first come first served (FCFS/FIFO), priority queues, and weighted queues. Packet schedulers can have a mix of these approches, e.g. 1 priority queue and N other weighted queues. Performance modeling can be done on queues. For voice traffic, the distribution of the arrival rate of traffic is a Poisson distribution. Therefore, the delay of packets and the length of the queue can be accurately modeled/predicted! See M/M/1 queues as a starting point (M/M/1 is Kendall notation and is more fully described in the next video).

Data Plane QoS Mechanisms

These data plane QoS mechanisms are applied at each network node: classification, marking, policing and shaping, prioritization, minimum rate assurance. Below are more details about each.

Classification

This is the process of identifying flows of packets and grouping individual traffic flows into aggregated streams such that actions can be applied to those flow streams. Up to this point, I have had a vague idea of what a flow is but not a proper definition. The instructor defines a flow as a 5-tuple of source & destination IP addresses and TCP/UDP ports and a transport protocol. What is a Network Traffic Flow? discusses various ways of defining a flow, and this is just one of many. Classification needs to avoid fragmentation because the 5-tuple information is only in the first packet. There are 4 ways of classifying traffic:

1. Simple classification – the use of fields designed for QoS classification in IP headers e.g. the type of service (TOS) byte in IPv4. There are complications with using the DTRM bits of the TOS (e.g. minimizing delay and maximizing throughput could conflict).
2. Implicit classification – done without inspecting packet header or content, e.g. by examining layer 1 or 2 identifiers.
3. Complex classification – using fields not designed for QoS classification or layer 2 criteria like MAC addresses.
4. Deep packet/stateful inspection – examination of actual payload and/or stateful inspection of a sequence of packets.

Marking/Coloring

This is simply setting the fields assigned for QoS classification in IP packet headers (DSCP field) or MPLS packet headers (EXP field).

• Source marking is applied at the source of the packets
• Ingress marking: used when source cannot mark correctly or cannot be trusted to do so.

Rate Enforcement

This is done to avoid congestion. Policing is a mechanism to ensure that a traffic stream does not exceed a defined maximum rate. It stands in constrast to shaping, which is typically accomplished by queuing (delays traffic, never drops it). One type of policer is the token bucket policer. It never delays traffic and cannot reorder or reprioritize traffic. See Cisco’s Policing and Shaping Overview and QoS Policing documents for details. This is one of the rate limiting algorithms discussed in the video below (I found this video’s explanation more intuitive).

The single rate three color marker and the two rate three color marker are two rate limiting approaches. Traffic metering can be implemented using such policers.

Prioritization

The next stage is prioritization of the traffic. 4 possible approaches: with prioritiy queues, e.g. where VoIP traffic always has highest priority, other queues can be starved by the scheduler. Weighted round robbin will take more packets from the high priority queues but still cycle through the other queues, taking fewer packets from them. Weighted bandwidth scheduling considers the packet sizes instead of just packet counts per queue (e.g. just taking 1 packet from a low priority queue can have negative impact if the packet is huge). Deficit round robbin is the one used in practice. It keeps track of the history of the number of packets services, and not just instantaneous values. I found the next video to expand on these brief explanations of scheduling algorithms.

One of the points that came up in discussion was that the schedulers use Run-to-completion scheduling, which means that a packet must be fully processed before starting on another packet. Routers have an interface FIFO (Tx buffer) on the physical link. When it fills up, this signals to the scheduler that there may be congestion downstream, thereby allowing for back pressure flow control. There is also multi-level strict policy queuing which allows for multiple priority queues instead of just 1 (e.g. voice & video) but not as common today.

Routers also drop packets to prevent unacceptable delays caused by buffering too many packets. There are different dropping strategies, e.g. tail dropping (dropping from the back of the queue), weighted tail dropping (>1 queue limit via heuristics), and head dropping (rare).

Active queue management (AQM) is a congestion avoidance technique. It works by detecting congestion before queues overflow. These are some techniques for AQM:

1. Random early detection (RED), which prevents TCP global synchronization
2. Weighted random early detection

These QoS mechanisms operate in the context of an overriding architecture, integrated services (Intserv) or differentiated services (Diffserv). IntServ can be used in the financial industry or medical health facilities, for example. These are delay sensitive applications where unbounded scaling is not a real requirement. IntServ explicitly manages bandwidth resources on a per flow basis. DiffServ was developed to support (as the name suggests) differentiated treatment of packets in large scale environments. It does this using a 6-bit differentiated services code point (DSCP) in the IPv4 ToS header or the IPv6 traffic class octet. Classification and conditioning happen at the edge of the DiffServ domain. Actions are performed on behavior aggregates (contrast this to the per flow actions of IntServ). The next technology we learn about is Multiprotocol Label Switching, defined as follows on Wikipedia:

Multiprotocol Label Switching is a routing technique in telecommunications networks that directs data from one node to the next based on labels rather than network addresses.

MPLS is similar to IntServ in that it lets you define an end-to-end path through the network for traffic but without reserving resources. It is a hop by hop forwarding mechanism, which stands in contrast to IP which works by making next hop routing decisions without regard to the end-to-end path taken by the packets. MPLS can be deployed on any layer 2 technology (multiprotocol). Benefits of MPLS include fast rerouting in case of failures and providing QoS support. One of the settings in which MPLS is used is in SD-WAN. This article provides a helpful contrast: What is the difference between SD-WAN and MPLS? These are the main applications of MPLS:

1. Traffic Engineering: allows network administrator to make the path deterministic (normal hop-by-hop routing is not). In other words, a frame forwarding policy can be used instead of relying on dynamic routing protocols.
2. QoS: the MPLS EXP bits are used for marking traffic per the labels.

This is quite the array of topics, especially for an introduction to networks course. I have a greater appreciation of how broad this space is.

Introduction to Networks

I’m taking an online introductory course on networks. I have been surprised by how much ground this course is covering. I didn’t expect to cover wireless (mobile) networks, for example. I looked for videos on some of the topics to learn more, e.g. 4g network architecture – YouTube. Networking is turning out to be much cooler and more interesting than I thought possible. This post is a compilation of all the key topics introduced in the course (in the general order they were introduced, but not particularly organized into a coherent story).

My main takeaway from this first video is that 4G networks are entirely packet switched (basic, but new to me).

The next video on how messages are transmitted to the cell phone tower is insightful as well. I appreciated the high-level discussion of antennas.

The concept of control plane and data plane came up as well. One advantage of this separation as per the overview below are independent evolution and development of each (e.g. control software can be upgraded without changing the hardware).

There are so many concepts in this space, most of them new to me, e.g. OAM, NMS, and EMS. Some places they are discussed include LTE Architecture Concepts, Differences Between an NMS and an EMS, and this video on Management Plane vs. Control Plane vs. Data Plane. We briefly got into the differences between 4G and 5G, one being the service-based architecture. Here’s a video I found introducing it:

Then of course there are the fundamental concepts of throughput, delay, and packet loss error. Jim Kurose’s book (and video below) covers these topics but it’s been a while since I read that book.

The professor also clarified the difference between bandwidth and throughput. The next video briefly touches on this distinction:

The course has also introduced me to the concept of spectral efficiency as part of understanding the difference between bandwidth and throughput. There is no shortage of concepts to learn about, from the different types of lines like T1 and T3 to bit robbing to the existence of network interface devices. The video below is good intro to T1.

There was also a discussion about cable networks, with an onslaught of concepts like Hybrid fiber-coaxial. This Cable 101 video is a helpful resource.

The HFC Cable Systems Introduction video below starts out with a comparison of coax and fiber then explains the flow of signals from the core network to the home.

I still need to learn more information about the Cable modem termination system (CMTS) and the next resource is perfect. It mentions CMTS vendors like Arris, Cisco, and Motorola, which inspires me to look up the Cisco CMTS.

I have never researched how most of these systems work so I am greatly appreciating this introduction to networks course! Here’s a video on how cable modems work, including their interactions with the CMTS.

The communication between the CMTS and the CMs is done via DOCSIS. Here is the reference I found with insight into DOCSIS.

Something I picked up is that CableLabs does a lot of the research for these systems. Other concepts to know include wavelength-division multiplexing (WDM), which was used in the traditional coax networks. The following explanation is an example of WDM in fiber.

The next technology described is DSL (Digital subscriber line). With DSL, the last mile is not shared (unlike cable networks). It evolved into ADSL and VDSL to support higher throughput. It’s interesting that it uses Asynchronous Transfer Mode (ATM) from back in the day. We also briefly introduce passive optical networks.

Next, we get into the 7-layer OSI model. The example given for the physical layer is SONET technology. Another foray into T1 technology reveals the fact that bipolar transmission is used for T1 since it is more power efficient.

Multiplexing is the next interesting topic introduced. I have included some videos below on the different types of multiplexing employed in communications.

1. FDM involves modulating message signals over carrier frequencies then using bandpass filters to extract the individual signals.
2. Time-division multiplexing: one variant is statistical TDM, which was a first step toward IP.
3. Wavelength-division multiplexing (WDM)

The course also addresses transmission fundamentals like the difference between bit rate and baud rate, the Shannon–Hartley theorem, the Nyquist–Shannon sampling theorem, modulation, modems, and codecs. I have compiled a few videos covering these topics below.

Here is an explanation of the Shannon–Hartley theorem:

The intuitition behind the Nyquist–Shannon sampling theorem is explained in the next video:

The concept of modulation comes next:

Other concepts introduced include the constellation diagram and Quadrature amplitude modulation (QAM). The following videos introduce these 2 concepts:

We then start getting into network addressing. One of the important concepts here is how the exhaustion of IPv4 addresses is handled: private IP addresses, DHCP, subnetting, and IPv6. One particularly interesting point was the difference between IPv4 and IPv6 headers:

The history of telecom is also worth knowing. More recent key events are the 1984 Modified Final Judgement and the Telecommunications Act of 1996. Verify that this playlist covers the 1984 Modified Final Judgement.

In a discussion of the impact of TCP on throughput, the professor called out TCP global synchronization as an issue that networks need to avoid. Here’s one video about it.

Avoiding packet reordering is another important aspect of TCP. The contrast with UDP is especially interesting when other protocols like Google’s QUIC are designed. The RTP protocol (a relative of UDP, informally speaking) is used for VoIP. This is a good description of RTP:

The Session Initiation Protocol (SIP) may be used to set up the RTP bearer streams. Here is a high level overview of SIP.

RTP Control Protocol is a related protocol used to provide feedback on the quality of service (QoS).